PCAT Exam: Biological Processes - Human Anatomy and Physiology

Anatomy may be defined as the structural makeup of an organism. The study of anatomy may be divided into microscopic/fine anatomy and macroscopic/gross anatomy. Fine anatomy concerns itself with viewing the features of the body with the aid of a microscope, while gross anatomy concerns itself with viewing the features of the body with the naked eye. Physiology refers to the functions of an organism and it examines the chemical or physical functions that help the body function appropriately.

Cells
All the parts of the human body are built of individual units called cells. Groups of similar cells are arranged into tissues, different tissues are arranged into organs, and organs working together form entire organ systems.

The human body has twelve organ systems that govern circulation, digestion, immunity, hormones, movement, support, coordination, urination and excretion, reproduction, respiration, and general protection.

Histology is the examination of specialized cells and cell groups that perform a specific function by working together. Although there are trillions of cells in the human body, there are only 200 different types of cells.

Groups of cells form biological tissues, and tissues combine to form organs, such as the heart and kidney.

Organs are structures that have many different functions that are vital to living creatures.

There are four primary types of tissue: epithelial, connective, muscle, and neural. Each tissue type has specific characteristics that enable organs and organ systems to function properly.

Mitosis and Meiosis
The human body is made of trillions of cells. Cell division and replication are responsible for growth of the body. There are many different types of cells in the body, and most undergo mitosis to divide and replicate. Mitosis also occurs to replenish damaged and dying cells. In mitosis, one cell replicates its own genetic material and splits into two identical daughter cells. It is an important part of both growth and the maintenance of homeostasis within the body.
Although the majority of cells undergo mitosis when replicating, gamete cells (reproductive cells) undergo a more complicated process, called meiosis.

In the human body, the gametes are the egg and the sperm. Human reproduction happens by sexual reproduction, which means one gamete fertilizes another gamete. The resulting cell is not identical to either of its parent cells, which is in contrast to daughter cells produced by mitosis. Eggs and sperm cells contain twenty-three chromosomes each. They combine to form a diploid cell that contains all forty-six chromosomes. The diploid cell replicates its chromosomes and splits into two cells, each containing forty-six chromosomes. Those two cells split again to create four cells, each containing twenty-three chromosomes. None of the daughter gamete cells are identical to either of the parent gamete cells. During the process of meiosis, the chromosomes from each parent get mixed together, then split into the daughter gamete cells randomly. This process explains why children are not exactly identical to their parents, but instead have characteristics from each of them.

Tissues
Human tissues can be grouped into four categories:

Epithelial Tissue
Epithelial tissue covers the external surfaces of organs and lines many of the body’s cavities. Epithelial tissue helps to protect the body from invasion by microbes (bacteria, viruses, parasites), fluid loss, and injury.

Epithelial cell shapes can be:
1. Squamous: cells with a flat shape
2. Cuboidal: cells with a cubed shape
3. Columnar: cells shaped like a column

Epithelial cells can be arranged in four patterns:
1. Simple: a type of epithelium composed solely from a single layer of cells
2. Stratified: a type of epithelium composed of multiple layers of cells
3. Pseudostratified: a type of epithelium that appears to be stratified but in reality, consists of only one layer of cells
4. Transitional: a type of epithelium noted for its ability to expand and contract

Connective Tissue
Connective tissue fills internal spaces and is not exposed to the outside of the body. It provides structural support for the body and stores energy. Fluid can be transported by connective tissue between different regions of the body. This type of tissue is also a protective barrier for delicate organs and for the body against microorganisms.

Muscle Tissue
Muscle tissue has characteristics that allow motion. Skeletal muscles are long fibers of actin and myosin that slide past each other to cause a contraction, which shortens the muscle. They are filled with mitochondria, because they expend a lot of energy. Smooth muscle tissue is structured differently, because its movement is in the form of peristalsis. For example, the esophagus moves food in a wave-like contraction, as opposed to a shortening of muscle to bring structures together. Smooth muscle tissues make up internal organs, with the exception of the heart, which is composed of thick, contracting muscle tissue called cardiac muscle.



Neural Tissue
Neural tissue conducts electrical impulses, which help send information and instructions throughout the body. Most of it is concentrated in the brain and spinal cord. The changes in frequency and patterns of the impulse are important distinctions in the messages being sent. Its structure is composed of message-receiving projections called dendrites and a long, myelinated axon. The myelin sheath surrounds the axon to conduct the action potential, ultimately causing neurotransmitter release at the synapse, or the boundary between the axon of one cell and the dendrite of another.

Membranes
Membranes are formed by the combination of epithelia and connective tissues. They consist of an underlying connective tissue layer covered by an epithelial sheet, and together they cover and protect structures and tissues in the body. There are four types of membranes: mucous, serous, cutaneous, and synovial.

Mucous membranes line cavities that connect with the exterior part of the body and form a barrier against pathogens. The epithelial layer is always moist with mucus or glandular secretions. The loose connective tissue layer is called the lamina propria, and it connects the epithelium to the underlying structures. Serous membranes consist of a mesothelium connected to loose connective tissue. These membranes line the subdivisions of the ventral body cavity. They are very thin and are firmly attached to both the body wall and the organs they are covering. Serous membranes are covered in fluid, called transudate, which helps minimize friction between the organs and the body wall and is the main function of the membrane. The cutaneous membrane is also known as skin. It consists of epithelium and an underlying layer of loose connective tissue that is reinforced by a layer of dense connective tissue and covers the entire surface of the body. It is a thick membrane and is usually dry. Synovial membranes consist of large areas of loose connective tissue bound by an incomplete layer of squamous or cuboidal cells. It is found between the joint capsule and joint cavity of synovial joints. Synovial fluid lubricates this area and distributes oxygen and nutrients. It also cushions impact and acts as a shock absorber at the joints.

Cartilage
Cartilage is a firm gel substance that contains complex polysaccharides, called chondroitin sulfates. It contains collagen fibers, which provide tensile strength, and chondrocytes, which are cartilage cells. It is an avascular material, because the chondrocytes do not allow blood vessels to form within the fibrous network.

There are three types of cartilage: hyaline cartilage, elastic cartilage, and fibrocartilage. Hyaline cartilage is the body’s most abundant cartilage, and is made of tightly-packed collagen fibers. It is tough and flexible, but is also the weakest type of cartilage. Elastic cartilage contains elastic fibers that make it very resilient and flexible. Fibrocartilage has densely woven collagen fibers that make it very durable and tough.

Organs

Glands
Glands are organs that synthesize and secrete chemical substances, such as hormones, for use inside the body or to be discharged outside the body. There are two types of glands: endocrine and exocrine.

Endocrine glands secrete hormones into the bloodstream and are important in maintaining homeostasis within the body. They do not have a duct system. Examples of endocrine glands are the pancreas, the pineal gland, the thymus gland, the pituitary gland, the thyroid gland, and the adrenal glands.

Exocrine glands have ducts that are used to secrete substances to the surface of the body and can be classified into three types: apocrine, holocrine, and merocrine. In apocrine glands, part of the cell’s body is lost during secretion; in holocrine glands, the whole cell body disintegrates during secretion; and in merocrine glands, cells use exocytosis to secrete fluids.

Exocytosis occurs when the chemical substance is carried in a vacuole across the cell membrane for release outside of the cell. The cells remain intact in merocrine glands.

Body Cavities
The body is partitioned into different hollow spaces that house organs. The human body contains the following cavities:
Cranial cavity: The cranial cavity is surrounded by the skull and contains organs such as the brain and pituitary gland.
Thoracic cavity: The thoracic cavity is encircled by the sternum (breastbone) and ribs. It contains organs such as the lungs, heart, trachea (windpipe), esophagus, and bronchial tubes.
Abdominal cavity: The abdominal cavity is separated from the thoracic cavity by the diaphragm. It contains organs such as the stomach, gallbladder, liver, small intestines, and large intestines. The abdominal organs are held in place by a membrane called the peritoneum.
Pelvic cavity: The pelvic cavity is enclosed by the pelvis, or bones of the hips. It contains organs such as the urinary bladder, urethra, ureters, anus, and rectum. In females, the pelvic cavity also contains the uterus.
Spinal cavity: The spinal cavity is surrounded by the vertebral column. The vertebral column has five regions: cervical, thoracic, lumbar, sacral, and coccygeal. The spinal cord runs through the middle of the spinal cavity.

Systems

Skeletal System
The skeletal system consists of the 206 bones that make up the skeleton, as well as the cartilage, ligaments, and other connective tissues that stabilize them. Bone is made of collagen fibers and mineral salts (mainly calcium and phosphorous). The mineral salts are strong but brittle, and the collagen fibers are weak but flexible, but the combination in the bony matrix makes bone resistant to shattering.

There are two types of bone: compact and spongy.

Compact bone has a basic functional unit, called the Haversian system. Osteocytes, or bone cells, are arranged in concentric circles around a central canal, called the Haversian canal, which contains blood vessels. While Haversian canals run parallel to the surface of the bone, perforating canals, also known as the canals of Volkmann, run perpendicularly between the central canal and the surface of the bone. The concentric circles of bone tissue that surround the central canal within the Haversian system are called lamellae. The spaces that are found between the lamellae are called lacunae. The Haversian system is a reservoir for calcium and phosphorus for blood. Compact bone is also called cortical bone.
Spongy bone, in contrast to compact bone, is lightweight and porous. It covers the outside of the bone and it gives it a shiny, white appearance. It has a branching network of parallel lamellae, called trabeculae. Although spongy bone forms an open framework around the compact bone, it is still quite strong. Different bones have different ratios of compact-to-spongy bone, depending on their functions. Spongy bone is also called cancellous bone.
 

The outside of the bone is covered by a periosteum, which has four major functions. It isolates and protects bones from the surrounding tissue; provides a place for attachment of the circulatory and nervous system structures; participates in growth and repair of the bone; and attaches the bone to the deep fascia. An endosteum is found inside the bone, covers the trabeculae of the spongy bone, and lines the inner surfaces of the central canals.
One major function of the skeletal system is to provide structural support for the entire body. It provides a framework for the soft tissues and organs to attach to. The skeletal system also provides a reserve of important nutrients, such as calcium and lipids. Normal concentrations of calcium and phosphate in body fluids are partly maintained by the mineral salts stored in bone. Lipids that are stored in yellow bone marrow can be used as a source of energy. Red bone marrow produces red blood cells, white blood cells, and platelets that circulate in the blood. Certain groups of bones form protective barriers around delicate organs. The ribs, for example, protect the heart and lungs, the skull encloses the brain, and the vertebrae cover the spinal cord.

Muscular System
The muscular system is responsible for movement. There are approximately 700 muscles in the human body that are attached to the bones of the skeletal system and that make up half of the body’s weight. Muscles are attached to the bones through tendons. Tendons are made up of dense bands of connective tissue and have collagen fibers that firmly attach to the bone on one side and the muscle on the other. Their fibers are actually woven into the coverings of the bone and muscle, so they can withstand the large forces that are put on them when muscles are moving.

There are three types of muscle tissue in the body. Skeletal muscle tissue pulls on the bones of the skeleton and causes body movement, cardiac muscle tissue helps pump blood through veins and arteries, and smooth muscle tissue helps move fluids and solids along the digestive tract and contributes to movement in other body systems.

All of these muscle tissues have four important properties in common. They are excitable, meaning they respond to stimuli; contractile, meaning they can shorten and pull on connective tissue; extensible, meaning they can be stretched repeatedly, but maintain the ability to contract; and elastic, meaning they rebound to their original length after a contraction.

Muscles begin at an origin and end at an insertion. Generally, the origin is proximal to the insertion and the origin remains stationary while the insertion moves. For example, when bending the elbow and moving the hand up toward the head, the part of the forearm that is closest to the wrist moves and the part closer to the elbow is stationary. Therefore, the muscle in the forearm has an origin at the elbow and an insertion at the wrist.
Body movements occur by muscle contraction. Each contraction causes a specific action.

Muscles can be classified into one of three muscle groups based on the action they perform. Primary movers, or agonists, produce a specific movement, such as flexion of the elbow. Synergists are in charge of helping the primary movers complete their specific movements. They can help stabilize the point of origin or provide extra pull near the insertion. Some synergists can aid an agonist in preventing movement at a joint. Antagonists are muscles whose actions are the opposite of that of the agonist. If an agonist is contracting during a specific movement, the antagonist is stretched. During flexion of the elbow, the biceps brachii muscle contracts and acts as an agonist, while the triceps brachii muscle on the opposite side of the upper arm acts as an antagonist and stretches.
Skeletal muscle tissue has several important functions. It causes movement of the skeleton by pulling on tendons and moving the bones. It maintains body posture through the contraction of specific muscles responsible for the stability of the skeleton. Skeletal muscles help support the weight of internal organs and protect these organs from external injury. They also help to regulate body temperature within a normal range. Muscle contractions require energy and produce heat, which heats the body when cold.

Nervous System
The human nervous system coordinates the body’s response to stimuli from inside and outside the body. There are two major types of nervous system cells: neurons and neuroglia. Neurons are the workhorses of the nervous system and form a complex communication network that transmits electrical impulses (termed action potentials), while neuroglia connect and support them.

Although some neurons monitor the senses, some control muscles, and some connect the brain to others, all neurons have four common characteristics:
1. Dendrites: These receive electrical signals from other neurons across small gaps called synapses.
2. Nerve cell body: This is the hub of processing and protein manufacture for the neuron.
3. Axon: This transmits the signal from the cell body to other neurons.
4. Terminals: These bridge the neuron to dendrites of other neurons and deliver the signal via chemical messengers called neurotransmitters.
Here an illustration of a basic neuron:


There are two major divisions of the nervous system: central and peripheral:



Central Nervous System
The central nervous system (CNS) consists of the brain and spinal cord. Three layers of membranes, called the meninges, cover and separate the CNS from the rest of the body.
 

The major divisions of the brain are the forebrain, the midbrain, and the hindbrain.
The forebrain consists of the cerebrum, the thalamus and hypothalamus, and the rest of the limbic system. The cerebrum is the largest part of the brain, and its most well-documented part is the outer cerebral cortex. The cerebrum is divided into right and left hemispheres, and each cerebral cortex hemisphere has four discrete areas, or lobes: frontal, temporal, parietal, and occipital.

The frontal lobe governs duties such as voluntary movement, judgment, problem-solving, and planning, while the other lobes have more of a sensory function. The temporal lobe integrates hearing and language comprehension, the parietal lobe processes sensory input from the skin, and the occipital lobe functions to process visual input from the eyes. For completeness, the other two senses, smell and taste, are processed via the olfactory bulbs. The thalamus helps organize and coordinate all of this sensory input in a meaningful way for the brain to interpret.


The hypothalamus controls the endocrine system and all of the hormones that govern long-term effects on the body. Each hemisphere of the limbic system includes a hippocampus (which plays a vital role in memory), an amygdala (which is involved with emotional responses like fear and anger), and other small bodies and nuclei associated with memory and pleasure.

The midbrain is in charge of alertness, sleep/wake cycles, and temperature regulation. It includes the substantia nigra, which produces melatonin to regulate sleep patterns. The notable components of the hindbrain include the medulla oblongata and cerebellum. The medulla oblongata is located just above the spinal cord and is responsible for crucial involuntary functions such as breathing, heart rate, swallowing, and the regulation of blood pressure. Together with other parts of the hindbrain, the midbrain and medulla oblongata form the brain stem. The brain stem connects the spinal cord to the rest of the brain. To the rear of the brain stem sits the cerebellum, which plays a key role in posture, balance, and muscular coordination. The spinal cord, encapsulated by its protective bony spinal column, carries sensory information to the brain and motor information to the body.

Peripheral Nervous System
The peripheral nervous system (PNS) includes all nervous tissue besides the brain and spinal cord. The PNS consists of the sets of cranial and spinal nerves and relays information between the CNS and the rest of the body. The PNS has two divisions: the autonomic nervous system and the somatic nervous system.



Autonomic Nervous System
The autonomic nervous system (ANS) governs involuntary, or reflexive, body functions. Ultimately, the autonomic nervous system controls functions such as breathing, heart rate, digestion, body temperature, and blood pressure.
The ANS is split between parasympathetic nerves and sympathetic nerves. These two nerve types are antagonistic, and have opposite effects on the body. Parasympathetic nerves typically are useful when resting or during safe conditions. They decrease heart rate and breath rate, prepare digestion, and allow urination and excretion. Sympathetic nerves, on the other hand, become active when a person is under stress or excited, and they increase heart rate and respiration rate, and inhibit digestion, urination, and excretion.

Somatic Nervous System and the Reflex Arc
The somatic nervous system (SNS) governs the conscious, or voluntary, control of skeletal muscles and their corresponding body movements. The SNS contains afferent, or sensory, neurons and efferent, or motor, neurons.

fferent neurons carry sensory messages from the skeletal muscles, skin, or sensory organs to the CNS. Efferent neurons relay motor messages from the CNS to skeletal muscles, skin, or sensory organs.
The SNS also has a role in involuntary movements called reflexes. A reflex is defined as an involuntary response to a stimulus. They are transmitted via what is termed a reflex arc, where a stimulus is sensed by an affector and its afferent neuron, interpreted and rerouted by an interneuron, and delivered to effector muscles by an efferent neuron, where the response to the initial stimulus is carried out. A reflex is able to bypass the brain by being rerouted through the spinal cord; the interneuron decides the proper course of action rather than the brain. The reflex arc results in an instantaneous, involuntary response. For example, a physician tapping on the knee produces an involuntary knee jerk referred to as the patellar tendon reflex.

Circulatory System
The circulatory system is a network of organs and tubes that transport blood, hormones, nutrients, oxygen, and other gases to cells and tissues throughout the body. It is also known as the cardiovascular system. The major components of the circulatory system are the blood vessels, blood, and heart.

Blood Vessels
In the circulatory system, blood vessels are responsible for transporting blood throughout the body. The three major types of blood vessels in the circulatory system are arteries, veins, and capillaries.

Arteries carry blood from the heart to the rest of the body. Veins carry blood from the body to the heart. Capillaries connect arteries to veins and form networks that exchange materials between the blood and the cells.
In general, arteries are stronger and thicker than veins, as they need to withstand the high pressures exerted by the blood as the heart pumps it through the body. Arteries control blood flow through either vasoconstriction (narrowing of the blood vessel’s diameter) or vasodilation (widening of the blood vessel’s diameter). The blood in veins is under much lower pressures, so veins have valves to prevent the backflow of blood.
Most of the exchange between the blood and tissues takes place through the capillaries. There are three types of capillaries: continuous, fenestrated, and sinusoidal.
Continuous capillaries are made up of epithelial cells tightly connected together. As a result, they limit the types of substances that pass into and out of the blood. Continuous capillaries are the most common type of capillary. Fenestrated capillaries have openings that allow materials to be freely exchanged between the blood and tissues. They are commonly found in the digestive, endocrine, and urinary systems. Sinusoidal capillaries have larger openings and allow proteins and blood cells through. They are found primarily in the liver, bone marrow, and spleen.

Blood
Blood is vital to the human body. It is a liquid connective tissue that serves as a transport system for supplying cells with nutrients and carrying away their wastes. The average adult human has five to six quarts of blood circulating through their body. Approximately 55% of blood is plasma (the fluid portion), and the remaining 45% is composed of solid cells and cell parts. There are three major types of blood cells:
1. Red blood cells transport oxygen throughout the body. They contain a protein called hemoglobin that allows them to carry oxygen. The iron in the hemoglobin gives the cells and the blood their red colors.
2. White blood cells are responsible for fighting infectious diseases and maintaining the immune system. There are five types of white blood cells: neutrophils, lymphocytes, eosinophils, monocytes, and basophils.
3. Platelets are cell fragments which play a central role in the blood clotting process.
All blood cells in adults are produced in the bone marrow—red blood cells from red marrow and white blood cells from yellow marrow.

Heart
The heart is a two-part, muscular pump that forcefully pushes blood throughout the human body. The human heart has four chambers—two upper atria and two lower ventricles, a pair on the left and a pair on the right. Anatomically, left and right correspond to the sides of the body that the patient themselves would refer to as left and right.
Four valves help to section off the chambers from one another. Between the right atrium and ventricle, the three flaps of the tricuspid valve keep blood from backflowing from the ventricle to the atrium, similar to how the two flaps of the mitral valve work between the left atrium and ventricle. As these two valves lie between an atrium and a ventricle, they are referred to as atrioventricular (AV) valves. The other two valves are semilunar (SL), and they control blood flow into the two great arteries leaving the ventricles. The pulmonary valve connects the right ventricle to the pulmonary artery while the aortic valve connects the left ventricle to the aorta.


 

Cardiac Cycle
A cardiac cycle is one complete sequence of cardiac activity. The cardiac cycle represents the relaxation and contraction of the heart and can be divided into two phases: diastole and systole.
Diastole is the phase during which the heart relaxes and fills with blood. It gives rise to the diastolic blood pressure (DBP), which is the bottom number of a blood pressure reading. Systole is the phase during which the heart contracts and discharges blood. It gives rise to the systolic blood pressure (SBP), which is the top number of a blood pressure reading. The heart’s electrical conduction system coordinates the cardiac cycle.

Types of Circulation
Five major blood vessels manage blood flow to and from the heart: the superior and inferior venae cavae, the aorta, the pulmonary artery, and the pulmonary vein.
The superior vena cava is a large vein that drains blood from the head and upper body. The inferior vena cava is a large vein that drains blood from the lower body. The aorta is the largest artery in the human body. It carries blood from the heart to body tissues. The pulmonary arteries carry blood from the heart to the lungs. The pulmonary veins transport blood from the lungs to the heart.

In the human body, there are two types of circulation: pulmonary circulation and systemic circulation.

Pulmonary circulation supplies blood to the lungs. Deoxygenated blood enters the right atrium of the heart and is routed through the tricuspid valve into the right ventricle. Deoxygenated blood then travels from the right ventricle of the heart through the pulmonary valve and into the pulmonary arteries. The pulmonary arteries carry the deoxygenated blood to the lungs. In the lungs, oxygen is absorbed, and carbon dioxide is released. The pulmonary veins carry oxygenated blood to the left atrium of the heart.
Systemic circulation supplies blood to all other parts of the body, except the lungs. Oxygenated blood flows from the left atrium of the heart through the mitral, or bicuspid, valve into the left ventricle of the heart. Oxygenated blood is then routed from the left ventricle of the heart through the aortic valve and into the aorta. The aorta delivers blood to the systemic arteries, which supply the body tissues. In the tissues, oxygen and nutrients are exchanged for carbon dioxide and other wastes. The deoxygenated blood, along with carbon dioxide and waste products, enter the systemic veins, where they are returned to the right atrium of the heart via the superior and inferior vena cava.

Respiratory System
The respiratory system mediates the exchange of gas between the air and the blood, mainly by the act of breathing. This system is divided into the upper respiratory system and the lower respiratory system.

The upper system includes the nose, the nasal cavity and sinuses, and the pharynx. The lower respiratory system includes the larynx (voice box), the trachea (windpipe), the small passageways leading to the lungs, and the lungs. The upper respiratory system is responsible for filtering, warming, and humidifying the air that gets passed to the lower respiratory system, protecting the lower respiratory system’s more delicate tissue surfaces. The process of breathing in is referred to as inspiration while the process of breathing out is referred to as expiration.

The Lungs
Bronchi are tubes that lead from the trachea to each lung, and are lined with cilia and mucus that collect dust and germs along the way. The bronchi, which carry air into the lungs, branch into bronchioles and continue to divide into smaller and smaller passageways, until they become alveoli, which are the smallest passages. Most of the gas exchange in the lungs occurs between the blood-filled pulmonary capillaries and the air-filled alveoli. Within the lungs, oxygen and carbon dioxide are exchanged between the air in the alveoli and the blood in the pulmonary capillaries. Oxygen-rich blood returns to the heart and is pumped through the systemic circuit. Carbon dioxide-rich air is exhaled from the body. Together, the lungs contain approximately 1,500 miles of airway passages, and this extremely large amount is due to the enormous amount of branching.



Breathing
When a breath of air is inhaled, oxygen enters the nose or mouth, and passes into the sinuses, where the temperature and humidity of the air are regulated. The air then passes into the trachea and is filtered. From there, the air travels into the bronchi and reaches the lungs. Bronchi are tubes that lead from the trachea to each lung, and are lined with cilia and mucus that collect dust and germs along the way. Within the lungs, oxygen and carbon dioxide are exchanged between the air in the alveoli and the blood in the pulmonary capillaries. Oxygen-rich blood returns to the heart and is pumped through the systemic circuit. Carbon dioxide-rich air is exhaled from the body.
Breathing is possible due to the muscular diaphragm contracting downward, which expands the space in the thoracic cavity. This allows the lungs to inhale, increasing their volume and decreasing their pressure. Air flows from the external high-pressure system to the low-pressure system inside the lungs. When breathing out, the diaphragm releases its pressure difference, decreases the lung volume, and forces the stale air back out.

Functions of the Respiratory System
The respiratory system has many functions. Most importantly, it provides a large area for gas exchange between the air and the circulating blood. It protects the delicate respiratory surfaces from environmental variations and defends them against pathogens. It is responsible for producing the sounds that the body makes for speaking and singing, as well as for non-verbal communication. It also helps regulate blood volume and blood pressure by releasing vasopressin, and it is a regulator of blood pH due to its control over carbon dioxide release, as the aqueous form of carbon dioxide is the chief buffering agent in blood.

Endocrine System
The endocrine system is made of the ductless tissues and glands that secrete hormones into the interstitial fluids of the body. Interstitial fluid is the solution that surrounds tissue cells within the body. This system works closely with the nervous system to regulate the physiological activities of the other systems of the body to maintain homeostasis. While the nervous system provides quick, short-term responses to stimuli, the endocrine system acts by releasing hormones into the bloodstream that get distributed to the whole body. The response is slow but long-lasting, ranging from a few hours to a few weeks.
Hormones are chemical substances that change the metabolic activity of tissues and organs. While regular metabolic reactions are controlled by enzymes, hormones can change the type, activity, or quantity of the enzymes involved in the reaction. They bind to specific cells and start a biochemical chain of events that changes the enzymatic activity. Hormones can regulate development and growth, digestive metabolism, mood, and body temperature, among other things. Often small amounts of hormone will lead to large changes in the body.

The following are the major endocrine glands:
Hypothalamus: A part of the brain, the hypothalamus connects the nervous system to the endocrine system via the pituitary gland. Although it is considered part of the nervous system, it plays a dual role in regulating endocrine organs.
Pituitary Gland: A pea-sized gland found at the bottom of the hypothalamus. It has two lobes, called the anterior and posterior lobes. It plays an important role in regulating the function of other endocrine glands. The hormones released control growth, blood pressure, certain functions of the sex organs, salt concentration of the kidneys, internal temperature regulation, and pain relief.
Thyroid Gland: This gland releases hormones, such as thyroxine, that are important for metabolism, growth and development, temperature regulation, and brain development during infancy and childhood. Thyroid hormones also monitor the amount of circulating calcium in the body.
Parathyroid Glands: These are four pea-sized glands located on the posterior surface of the thyroid. The main hormone secreted is called parathyroid hormone (PTH), which helps with the thyroid’s regulation of calcium in the body.
Thymus Gland: The thymus is located in the chest cavity, embedded in connective tissue. It produces several hormones important for development and maintenance of normal immunological defenses. One hormone promotes the development and maturation of lymphocytes, which strengthens the immune system.
Adrenal Gland: One adrenal gland is attached to the top of each kidney. It produces adrenaline and is responsible for the “fight or flight” reactions in the face of danger or stress. The hormones epinephrine and norepinephrine cooperate to regulate states of arousal.
Pancreas: The pancreas is an organ that has both endocrine and exocrine functions. The endocrine functions are controlled by the pancreatic islets of Langerhans, which are groups of beta cells scattered throughout the gland that secrete insulin to lower blood sugar levels in the body. Neighboring alpha cells secrete glucagon to raise blood sugar.
Pineal Gland: The pineal gland secretes melatonin, a hormone derived from the neurotransmitter serotonin. Melatonin can slow the maturation of sperm, oocytes, and reproductive organs. It also regulates the body’s circadian rhythm, which is the natural awake/asleep cycle. It also serves an important role in protecting the CNS tissues from neural toxins.
Testes and Ovaries: These glands secrete testosterone and estrogen, respectively, and are responsible for secondary sex characteristics, as well as reproduction.

Digestive System
The human body relies completely on the digestive system to meet its nutritional needs. After food and drink are ingested, the digestive system breaks them down into their component nutrients and absorbs them so that the circulatory system can transport them to other cells to use for growth, energy, and cell repair. These nutrients may be classified as proteins, lipids, carbohydrates, vitamins, and minerals.
 

The digestive system is thought of chiefly in two parts: the digestive tract (also called the alimentary tract or gastrointestinal tract) and the accessory digestive organs.

The digestive tract is the pathway in which food is ingested, digested, absorbed, and excreted. It is composed of the mouth, pharynx, esophagus, stomach, small and large intestines, rectum, and anus. Peristalsis, or wave-like contractions of smooth muscle, moves food and wastes through the digestive tract. The accessory digestive organs are the salivary glands, liver, gallbladder, and pancreas.



Mouth and Stomach
The mouth is the entrance to the digestive system. Here, the mechanical and chemical digestion of the ingested food begins. The food is chewed mechanically by the teeth and shaped into a bolus by the tongue so that it can be more easily swallowed by the esophagus. The food also becomes more watery and pliable with the addition of saliva secreted from the salivary glands, the largest of which are the parotid glands. The glands also secrete amylase in the saliva, an enzyme that begins chemical digestion and breakdown of the carbohydrates and sugars in the food.
The food then moves through the pharynx and down the muscular esophagus to the stomach.
The stomach is a large, muscular sac-like organ at the distal end of the esophagus. Here, the bolus is subjected to more mechanical and chemical digestion. As it passes through the stomach, it is physically squeezed and crushed while additional secretions turn it into a watery nutrient-filled liquid that exits into the small intestine as chyme.
The stomach secretes many substances into the lumen of the digestive tract. Some cells produce gastrin, a hormone that prompts other cells in the stomach to secrete a gastric acid composed mostly of hydrochloric acid (HCl). The HCl is at such a high concentration and low pH that it denatures most proteins and degrades a lot of organic matter. The stomach also secretes mucous to form a protective film that keeps the corrosive acid from dissolving its own cells. Gaps in this mucous layer can lead to peptic ulcers. Finally, the stomach also uses digestive enzymes like proteases and lipases to break down proteins and fats; although there are some gastric lipases here, the stomach is most known for breaking down proteins.

Small Intestine
The chyme from the stomach enters the first part of the small intestine, the duodenum, through the pyloric sphincter, and its extreme acidity is partly neutralized by sodium bicarbonate secreted along with mucous. The presence of chyme in the duodenum triggers the secretion of the hormones secretin and cholecystokinin (CCK). Secretin acts on the pancreas to dump more sodium bicarbonate into the small intestine so that the pH is kept at an appropriate level, while CCK acts on the gallbladder to release the bile that it has been storing. Bile is a substance produced by the liver and stored in the gallbladder which helps to emulsify, or dissolve, fats and lipids.
Because of the bile, which aids in lipid absorption, and the secreted lipases, which break down fats, the duodenum is the chief site of fat digestion in the body. The duodenum also represents the last major site of chemical digestion in the digestive tract, as the other two sections of the small intestine (the jejunum and ileum) are instead heavily involved in absorption of nutrients.
The small intestine reaches 40 feet in length, and its cells are arranged in small finger-like projections called villi. This is due to its key role in the absorption of nearly all nutrients from the ingested and digested food, effectively transferring them from the lumen of the GI tract to the bloodstream, where they travel to the cells that need them. These nutrients include simple sugars like glucose from carbohydrates, amino acids from proteins, emulsified fats, electrolytes like sodium and potassium, minerals like iron and zinc, and vitamins like D and B12. Although the absorption of vitamin B12 takes place in the intestines, it is actually aided by intrinsic factor that was released into the chyme back in the stomach.

Large Intestine
The leftover parts of food that remain unabsorbed or undigested in the lumen of the small intestine next travel through the large intestine, which may also be referred to as the large bowel or colon. The large intestine is mainly responsible for water absorption. As the chyme at this stage no longer has anything useful that can be absorbed by the body, it is now referred to as waste, and it is stored in the large intestine until it can be excreted from the body. Removing the liquid from the waste transforms it from liquid to solid stool, or feces.
This waste first passes from the small intestine to the cecum, a pouch which forms the first part of the large intestine. In herbivores, it provides a place for bacteria to digest cellulose, but in humans most of it is vestigial and is known as the appendix. From the cecum, waste next travels up the ascending colon, across the transverse colon, down the descending colon, and through the sigmoid colon to the rectum. The rectum is responsible for the final storage of waste before being expelled through the anus. The anal canal is a small portion of the rectum leading through to the anus and the outside of the body.

Pancreas
The pancreas has endocrine and exocrine functions. The endocrine function involves releasing the hormones insulin, which decreases blood glucose levels, and glucagon, which increases blood glucose levels, directly into the bloodstream. Both hormones are produced in the islets of Langerhans—insulin in the beta cells and glucagon in the alpha cells.
The major part of the gland has an exocrine function, which consists of acinar cells secreting inactive digestive enzymes (zymogens) into the main pancreatic duct. The main pancreatic duct joins the common bile duct, which empties into the small intestine (specifically the duodenum). The digestive enzymes are then activated and take part in the digestion of carbohydrates, proteins, and fats within chyme (the mixture of partially digested food and digestive juices).

Integumentary System (Skin)
Skin consists of three layers: epidermis, dermis, and the hypodermis. There are four types of cells that make up the keratinized stratified squamous epithelium in the epidermis. They are keratinocytes, melanocytes, Merkel cells, and Langerhans cells. Skin is composed of many layers, starting with a basement membrane. On top of that sits the stratum germinativum, the stratum spinosum, the stratum granulosum, the stratum lucidum, and then the stratum corneum at the outer surface. Skin can be classified as thick or thin. These descriptions refer to the epidermis layer. Most of the body is covered with thin skin, but areas such as the palms are covered with thick skin. The dermis consists of a superficial papillary layer and a deeper reticular layer. The papillary layer is made of loose connective tissue, containing capillaries and the axons of sensory neurons. The reticular layer is a meshwork of tightly packed irregular connective tissue, containing blood vessels, hair follicles, nerves, sweat glands, and sebaceous glands. The hypodermis is a loose layer of fat and connective tissue. Since it is the third layer, if a burn reaches this third degree, it has caused serious damage.
Sweat glands and sebaceous glands are important exocrine glands found in the skin. Sweat glands regulate temperature, and remove bodily waste by secreting water, nitrogenous waste, and sodium salts to the surface of the body. Some sweat glands are classified as apocrine glands. Sebaceous glands are holocrine glands that secrete sebum, which is an oily mixture of lipids and proteins. Sebum protects the skin from water loss, as well as bacterial and fungal infections.
The three major functions of skin are protection, regulation, and sensation. Skin acts as a barrier and protects the body from mechanical impacts, variations in temperature, microorganisms, and chemicals. It regulates body temperature, peripheral circulation, and fluid balance by secreting sweat. It also contains a large network of nerve cells that relay changes in the external environment to the body.

Immune System
The immune system is the body’s defense against invading microorganisms (bacteria, viruses, fungi, and parasites) and other harmful, foreign substances. It is capable of limiting or preventing infection.
As mentioned, there are two general types of immunity: innate immunity and acquired immunity. Innate immunity uses physical and chemical barriers to block the entry of microorganisms into the body. Some of these barriers include the skin, mucous membranes, saliva, tears, stomach acid, and certain white blood cells.
Acquired immunity, which consists of a primary and secondary response, refers to a specific set of events used by the body to fight a particular infection after previously encountering it before.

Urinary System
The urinary system is made up of the kidneys, ureters, urinary bladder, and the urethra. It is the main system responsible for getting rid of the organic waste products, excess water and electrolytes are generated by the body’s other systems. The kidneys are responsible for producing urine, which is a fluid waste product containing water, ions, and small soluble compounds. The urinary system has many important functions related to waste excretion. It regulates the concentrations of sodium, potassium, chloride, calcium, and other ions in the plasma by controlling the amount of each excreted in urine. This also contributes to the maintenance of blood pH. The urinary system also regulates blood volume and pressure by controlling the amount of water lost in the urine, and releasing erythropoietin and renin. It eliminates toxic substances, drugs, and organic waste products, such as urea and uric acid. It also synthesizes calcitriol, which is a hormone derivative of vitamin D3 that aids in calcium ion absorption by the intestinal epithelium.

The Kidneys
Under normal circumstances, humans have two functioning kidneys. They are the main organs that are responsible for filtering waste products out of the blood and transferring them to urine. Every day, the kidneys filter approximately 120 to 150 quarts of blood and produce one to two quarts of urine. Kidneys are made of millions of tiny filtering units, called nephrons. Nephrons have two parts: a glomerulus, which is the filter, and a tubule. As blood enters the kidneys, the glomerulus allows fluid and waste products to pass through it and enter the tubule. Blood cells and large molecules, such as proteins, do not pass through and remain in the blood. The filtered fluid and waste then pass through the tubule, where any final essential minerals are sent back to the bloodstream. The final product at the end of the tubule is called urine.

Waste Excretion
Once urine accumulates, it leaves the kidneys. The urine travels through the ureters into the urinary bladder, a muscular organ that is hollow and elastic. As more urine enters the urinary bladder, its walls stretch and become thinner so there is no significant difference in internal pressure. The urinary bladder stores the urine until the body is ready for urination, at which time, the muscles contract and force the urine through the urethra and out of the body.

Reproductive System 
The reproductive system is responsible for producing, storing, nourishing, and transporting functional reproductive cells, or gametes, in the human body. It includes the reproductive organs, also known as gonads, the reproductive tract, the accessory glands and organs that secrete fluids into the reproductive tract, and the perineal structures, which are the external genitalia.

The Male System
The male gonads are called testes. The testes secrete androgens, mainly testosterone, and produce and store 500 million spermatocytes, which are the male gametes, each day. An androgen is a steroid hormone that controls the development and maintenance of male characteristics. Once the sperm are mature, they move through a duct system, where they mix with additional fluids secreted by accessory glands, forming a mixture called semen.

The Female System
The female gonads are the ovaries. Ovaries generally produce one immature gamete, or oocyte, per month. They are also responsible for secreting the hormones estrogen and progesterone. When the oocyte is released from the ovary, it travels along the uterine tubes, or Fallopian tubes, and then into the uterus. The uterus opens into the vagina. When sperm cells enter the vagina, they swim through the uterus and may fertilize the oocyte in the Fallopian tubes. The resulting zygote travels down the tube and implants into the uterine wall. The uterus protects and nourishes the developing embryo for nine months until it is ready for the outside environment. If the oocyte is not fertilized, it is released in the uterine, or menstrual, cycle. The menstrual cycle occurs monthly and involves the shedding of the functional part of the uterine lining.
Mammary glands are a specialized accessory organ of the female reproductive system. The mammary glands are located in the breast tissue, and during pregnancy, they begin to grow, and the cells proliferate in preparation for lactation. After pregnancy, the cells begin to secrete nutrient-filled milk, which is transferred into a duct system and out through the nipple for nourishment of the baby.

Human Reproduction
Humans procreate through sexual reproduction. Sexual reproduction involves the fusion of gametes, one from each parent, through a process called fertilization. Gametes are created by the human reproductive systems. In women, the ovaries produce on average one mature egg per month, which is referred to as the menstrual cycle. The release of an egg from the ovaries is termed ovulation. The female menstrual cycle is under the control of hormones such as luteinizing hormone (LH), follicle stimulating hormone (FSH), estrogen, and progesterone. In men, the testes produce sperm, the male gamete, and they produce millions of sperm at a time. The hormones LH and testosterone regulate the production of sperm in the testes. Leydig cells in the testes produce testosterone, while sperm is manufactured in the seminiferous tubules of the testes.
The fusion of the gametes (egg and sperm) is termed fertilization, and the resulting fusion creates a zygote. The zygote takes approximately seven days to travel through the fallopian tube and implant itself into the uterus. Upon implantation, it has developed into a blastocyst and will next grow into a gastrula. It is during this stage that the embryological germ layers are formed. The three germ layers are the ectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer). All of the human body systems develop from one or more of the germ layers. The gastrula further develops into an embryo, which then matures into a fetus. The entire process takes approximately nine months and culminates in labor and birth.