Human Body Systems Outline Essay

The Human Body Systems By Yash Dhayal, Mathew Nemet, and Tom Battaglini Table Of Contents Overview * Skeletal system – Functions and Parts * Muscular System – Types of Muscles * Integumentary System – Functions * Circulatory System – Parts of the Circulatory System * Immune System – Organs and Cells of he Immune System * Respiratory System – Parts of the Respiratory System * Digestive System * Urinary System * Endocrine System * Nervous System Overview * The human body itself is a complex system—many sets of interacting parts that work to keep the human machine running.

On any single day, we can estimate that your heart beats 103,689 times, your blood travels 168,000,000 miles, your digestive system processes 7. 8 pounds of waste, and your lungs take in 438 cubic feet of air. These are only a few of the multitude of functions the human body performs. And while the least little mishap could cause a glitch in the system, amazingly, day in and day out over most of our lifetime, our bodies operate almost flawlessly. Skeletal System Skeletal system is made up of your bones, ligaments, and tendons. It etermines the shape and symmetry of the body; acts as a protective device for your organs; acts as a firm base for the attachments of muscles (without bones, your muscles would not function properly); and the marrow tissues in the cavity of the bones produces red cells and some white cells, required in your blood. Skeletal System Functions * Its 206 bones form a rigid framework to which the softer tissues and organs of the body are attached. * Vital organs are protected by the skeletal system. The brain is protected by the surrounding skull as the heart and lungs are encased by the sternum and rib cage. Bodily movement is carried out by the interaction of the muscular and skeletal systems. For this reason, they are often grouped together as the musculo-skeletal system. Muscles are connected to bones by tendons. Bones are connected to each other by ligaments. Where bones meet one another is typically called a joint. Muscles which cause movement of a joint are connected to two different bones and contract to pull them together. An example would be the contraction of the biceps and a relaxation of the triceps. This produces a bend at the elbow.

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The contraction of the triceps and relaxation of the biceps produces the effect of straightening the arm. * Blood cells are produced by the marrow located in some bones. An average of 2. 6 million red blood cells are produced each second by the bone marrow to replace those worn out and destroyed by the liver. * Bones serve as a storage area for minerals such as calcium and phosphorus. When an excess is present in the blood, buildup will occur within the bones. When the supply of these minerals within the blood is low, it will be withdrawn from the bones to replenish the supply.

Parts of the Skeletal Sysytem There are 2 parts of the Skeletal system: * The axial skeleton – consists of bones that form the axis of the body and support and protect the organs of the head, neck, and trunk. * The appendicular skeleton – composed of bones that anchor the appendages to the axial skeleton. The Axial Skeleton The axial skeleton contains: * The Skull -Is the bony framework of the head. It is comprised of the eight cranial and fourteen facial bones. * The Sternum -A flat, dagger shaped bone located in the middle of the chest.

Along with the ribs, the sternum forms the rib cage that protects the heart, lungs, and major blood vessels from damage. * The Ribs -thin, flat, curved bones that form a protective cage around the organs in the upper body. They are comprised 24 bones arranged in 12 pairs. * The Vertebral Column (also called the backbone, spine, or spinal column) -consists of a series of 33 irregularly shaped bones, called vertebrae. These 33 bones are divided into five categories depending on where they are located in the backbone. The Appendicular Skeleton The appendicular skeleton contains: The upper extremity – consists of three parts: the arm, the forearm, and the hand. * The lower extremity – composed of the bones of the thigh, leg, foot, and the patella (commonly known as the kneecap). * The Shoulder Girdle (also called the Pectoral Girdle) – composed of four bones: two clavicles (collarbones) and two scapulae (shoulder blades). * The Pelvic Girdle (also called the hip girdle) – composed to two coxal (hip) bones. Muscular System Over 600 skeletal muscles function for body movement through contraction and relaxation of voluntary, striated muscle fibers.

These muscles are attached to bones, and are typically under conscious control for locomotion, facial expressions, posture, and other body movements. Muscles account for approximately 40 percent of body weight. The metabolism that occurs in this large mass-produces heat essential for the maintenance of body temperature Types of Muscles The types of muscles are: * Cardiac muscle * Smooth muscle * Skeletal muscle Cardiac Muscle * Cardiac muscle is only in the heart and makes up the atria and ventricles (heart walls). Like skeletal muscle, cardiac muscle contains striated fibers.

Cardiac muscle is called involuntary muscle because conscious thought does not control its contractions. Specialized cardiac muscle cells maintain a consistent heart rate. Smooth muscle * Smooth muscle is throughout the body, including in visceral (internal) organs, blood vessels, and glands. Like cardiac muscle, smooth muscle is involuntary. Unlike skeletal and cardiac muscle, smooth muscle is nonstriated (not banded). Smooth muscle, which is extensively within the walls of digestive tract organs, causes peristalsis (wave-like contractions) that aids in food digestion and transport. Except the heart, any action that the body performs without conscious thought is done by smooth muscle contractions. This includes diverse activities such as constricting (closing) the bronchioles (air passages) of the lungs or pupils of the eye or causing goosebumps in cold conditions. Skeletal muscle * A skeletal muscle has regular, ordered groups of fascicles, muscle fibers, myofibrils, and myofilaments. Epimysium (thick connective tissue) binds groups of fascicles together. A fascicle has muscle fibers; perimysium (connective tissue) envelops the fascicle. Endomysium (connective tissue) surrounds the muscle fibers. A muscle fiber divides into even smaller parts. Within each fiber are strands of myofibrils. These long cylindrical structures appear striped due to strands of tiny myofilaments. Myofilaments have two types of protein: actin (thin myofilaments) and myosin (thick myofilaments). * The actin and myosin myofilaments align evenly, producing dark and light bands on the myofibril. Each dark band depicts an area where the myofilaments overlap, causing the striated appearance of skeletal muscle. * All dark and light bands of the myofilaments have names. At the Z-line, actin strands interweave.

The region between two Z-lines is a sarcomere, the functional unit of skeletal muscle. Muscle contraction occurs when overlapping actin and myosin myofilaments overlap further and shorten the muscle cell. The myofilaments keep their length. The overlapping of myofilaments is the basis for the sliding filament theory of contraction. * Skeletal muscle is a system of pairs that relax and contract to move a joint. For example, when front leg muscles contract, the knee extends (straightens) while back leg muscles relax. Conversely, to flex (bend) the knee, back leg muscles contract while front leg muscles relax.

Some muscles are named for their ability to extend or flex a joint; for example, extensor carpiradialis longus muscle and flexor digitorum brevis muscle. * Tendons attach most skeletal muscles to bones. Tendons are strong sheets of connective tissue that are identical with ligaments. Tendons and ligaments differ in function only: tendons attach muscle to bone and ligaments attach bone to bone. Physical exercise strengthens the attachment of tendons to bones. * Skeletal muscles have muscle tone (remain partly contracted), which helps maintain body posture.

Ongoing signals from the nervous system to the muscle cells help maintain tone and readiness for physical activity. * Skeletal muscle aids in heat generation. During muscle contractions, muscle cells expend much energy, most of which is converted to heat. To prevent overheating, glands in the skin produce sweat to cool the skin; and, the body radiates heat from the blood and tissues through the skin. When the body is chilly, shivering causes quick muscle contractions that generate heat. Integumentary System * The integumentary system is the largest organ system. In humans, this system accounts for about 16 percent of total body weight.

It distinguishes, separates, protects and informs the animal with regard to its surroundings. The Integumentary system is the organ system that protects the body from damage, comprising the skin and its appendages (including hair, scales, feathers, and nails). The Integumentary system has a variety of functions; it may serve to waterproof, cushion and protect the deeper tissues, excrete wastes, regulate temperature and is the attachment site for sensory receptors to detect pain, sensation, pressure and temperature. In humans the integumentary system additionally provides vitamin D synthesis.

Functions of Integumentary System The functions of the Integumentary System are: * Protect the body’s internal living tissues and organs * Protect against invasion by infectious organisms * Protect the body from dehydration * Protect the body against abrupt changes in temperature * Help excrete waste materials through perspiration * Act as a receptor for touch, pressure, pain, heat, and cold * Protect the body against sunburns * Generate vitamin D through exposure to ultraviolet light * Store water, fat, glucose, and vitamin D * Participate in temperature regulation Circulatory System The Circulatory System is responsible for transporting materials throughout the entire body. It transports nutrients, water, and oxygen to your billions of body cells and carries away wastes such as carbon dioxide that body cells produce. It is an amazing highway that travels through your entire body connecting all your body cells. Parts of the Circulatory System The circulatory System is divided into three major parts: * The Heart * The Blood * The Blood Vessels The Heart The heart beats about 3 BILLION times during an average lifetime. It is a muscle about the size of your fist.

The heart is located in the center of your chest slightly to the left. It’s job is to pump your blood and keep the blood moving throughout your body. The Blood * Blood is pumped by your heart. * Blood travels through thousands of miles of blood vessels right within your own body. * Blood carries nutrients, water, oxygen and waste products to and from your body cells. * A young person has about a gallon of blood. An adult has about 5 quarts. * Blood is not just a red liquid but rather is made up of liquids, solids and small amounts of oxygen and carbon dioxide. * Blood contains blood cells. Blood Cells Where are the blood cells made?

The Red Blood Cells, White Blood Cells and Platelets are made by the bone marrow. Bone marrow is a soft tissue inside of our bones that produces blood cells. * Red Blood Cells are responsible for carrying oxygen and carbon dioxide. Red Blood Cells pick up oxygen in the lungs and transport it to all the body cells. After delivering the oxygen to the cells it gathers up the carbon dioxide(a waste gas produced as our cells are working) and transports carbon dioxide back to the lungs where it is removed from the body when we exhale(breath out). There are about 5,000,000 Red Blood Cells in ONE drop of blood. White Blood Cells help the body fight off germs. White Blood Cells attack and destroy germs when they enter the body. When you have an infection your body will produce more White Blood Cells to help fight an infection. Sometimes our White Blood Cells need a little help and the Doctor will prescribe an antibiotic to help our White Blood Cells fight a large scale infection. * Platelets are blood cells that help stop bleeding. When we cut ourselves we have broken a blood vessel and the blood leaks out. In order to plug up the holes where the blood is leaking from the platelets start to stick to the opening of the damaged blood vessels.

As the platelets stick to the opening of the damaged vessel they attract more platelets, fibers and other blood cells to help form a plug to seal the broken blood vessel. When the platelet plug is completely formed the wound stops bleeding. We call our platelet plugs scabs. The Blood Vessels * In class we talked about three types of blood vessels: * Arteries * Capillaries * Veins Arteries, Capillaries, and Veins * Arteries Arteries are blood vessels that carry oxygen rich blood away from the heart. * Capillaries Capillaries are tiny blood vessels as thin or thinner than the hairs on your head.

Capillaries connect arteries to veins. Food substances(nutrients), oxygen and wastes pass in and out of your blood through the capillary walls. * Veins Veins carry blood back toward your heart. Immune System The Immune System is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism’s own healthy cells and tissues in order to function properly. Organs of the Immune System Bone Marrow – All the cells of the immune system are initially derived from the bone marrow. They form through a process called hematopoiesis. During hematopoiesis, bone marrow-derived stem cells differentiate into either mature cells of the immune system or into precursors of cells that migrate out of the bone marrow to continue their maturation elsewhere. The bone marrow produces B cells, natural killer cells, granulocytes and immature thymocytes, in addition to red blood cells and platelets. * Thymus – The function of the thymus is to produce mature T cells.

Immature thymocytes, also known as prothymocytes, leave the bone marrow and migrate into the thymus. Through a remarkable maturation process sometimes referred to as thymic education, T cells that are beneficial to the immune system are spared, while those T cells that might evoke a detrimental autoimmune response are eliminated. The mature T cells are then released into the bloodstream. * Spleen – The spleen is an immunologic filter of the blood. It is made up of B cells, T cells, macrophages, dendritic cells, natural killer cells and red blood cells.

In addition to capturing foreign materials (antigens) from the blood that passes through the spleen, migratory macrophages and dendritic cells bring antigens to the spleen via the bloodstream. An immune response is initiated when the macrophage or dendritic cells present the antigen to the appropriate B or T cells. This organ can be thought of as an immunological conference center. In the spleen, B cells become activated and produce large amounts of antibody. Also, old red blood cells are destroyed in the spleen. * Lymph Nodes – The lymph nodes function as an immunologic filter for the bodily fluid known as lymph.

Lymph nodes are found throughout the body. Composed mostly of T cells, B cells, dendritic cells and macrophages, the nodes drain fluid from most of our tissues. Antigens are filtered out of the lymph in the lymph node before returning the lymph to the circulation. In a similar fashion as the spleen, the macrophages and dendritic cells that capture antigens present these foreign materials to T and B cells, consequently initiating an immune response. Cells of the Immune System * T-Cells – T lymphocytes are usually divided into two major subsets that are functionally and phenotypically (identifiably) different.

The T helper subset, also called the CD4+ T cell, is a pertinent coordinator of immune regulation. The main function of the T helper cell is to augment or potentiate immune responses by the secretion of specialized factors that activate other white blood cells to fight off infection. Another important type of T cell is called the T killer/suppressor subset or CD8+ T cell. These cells are important in directly killing certain tumor cells, viral-infected cells and sometimes parasites. The CD8+ T cells are also important in down-regulation of immune responses.

Both types of T cells can be found throughout the body. They often depend on the secondary lymphoid organs (the lymph nodes and spleen) as sites where activation occurs, but they are also found in other tissues of the body, most conspicuously the liver, lung, blood, and intestinal and reproductive tracts. * Natural Killer Cells – Natural killer cells, often referred to as NK cells, are similar to the killer T cell subset (CD8+ T cells). They function as effector cells that directly kill certain tumors such as melanomas, lymphomas and viral-infected cells, most notably herpes and cytomegalovirus-infected cells.

NK cells, unlike the CD8+ (killer) T cells, kill their targets without a prior “conference” in the lymphoid organs. However, NK cells that have been activated by secretions from CD4+ T cells will kill their tumor or viral-infected targets more effectively. * B Cells – The major function of B lymphocytes is the production of antibodies in response to foreign proteins of bacteria, viruses, and tumor cells. Antibodies are specialized proteins that specifically recognize and bind to one particular protein that specifically recognize and bind to one particular protein.

Antibody production and binding to a foreign substance or antigen, often is critical as a means of signaling other cells to engulf, kill or remove that substance from the body. * Granulocytes or Polymorphonuclear (PMN) Leukocytes – Another group of white blood cells is collectively referred to as granulocytes or polymorphonuclear leukocytes (PMNs). Granulocytes are composed of three cell types identified as neutrophils, eosinophils and basophils, based on their staining characteristics with certain dyes. These cells are predominantly important in the removal of bacteria and parasites from the body.

They engulf these foreign bodies and degrade them using their powerful enzymes. * Macrophages – Macrophages are important in the regulation of immune responses. They are often referred to as scavengers or antigen-presenting cells (APC) because they pick up and ingest foreign materials and present these antigens to other cells of the immune system such as T cells and B cells. This is one of the important first steps in the initiation of an immune response. Stimulated macrophages exhibit increased levels of phagocytosis and are also secretory. Dendritic Cells – Another cell type, addressed only recently, is the dendritic cell. Dendritic cells, which also originate in the bone marrow, function as antigen presenting cells (APC). In fact, the dendritic cells are more efficient apcs than macrophages. These cells are usually found in the structural compartment of the lymphoid organs such as the thymus, lymph nodes and spleen. However, they are also found in the bloodstream and other tissues of the body. It is believed that they capture antigen or bring it to the lymphoid organs where an immune response is initiated.

Unfortunately, one reason we know so little about dendritic cells is that they are extremely hard to isolate, which is often a prerequisite for the study of the functional qualities of specific cell types. Of particular issue here is the recent finding that dendritic cells bind high amount of HIV, and may be a reservoir of virus that is transmitted to CD4+ T cells during an activation event. Respiratory System The respiratory system is a group of organs and tissues that help you breathe. The main parts of this system are: * The Airways * The Lungs and Blood Vessels The Muscles used for breathing The Airways The airways are pipes that carry oxygen-rich air to the lungs and carbon dioxide, a waste gas, out of the lungs. The airways include: * Nose and linked air passages called nasal cavities * Mouth * Larynx , or voice box * Trachea, or windpipe * Tubes called bronchial tubes or bronchi, and their branches Air first enters the body through the nose or mouth, which wets and warms the air. (Cold, dry air can irritate the lungs. ) The air then travels through the voice box and down the windpipe. The windpipe splits into two bronchi that enter the lungs.

A thin flap of tissue called the epiglottis covers the windpipe when a person swallows. This prevents food or drink from entering the air passages that lead to the lungs. Except for the mouth and some parts of the nose, all of the airways have special hairs called cilia that are coated with sticky mucus. The cilia trap germs and other foreign particles that enter the airways when a person breathes in air. These fine hairs then sweep the particles up to the nose or mouth. There, they’re swallowed, coughed, or sneezed out of the body. Nose hairs and mouth saliva also trap particles and germs. The Lungs and Blood Vessels

The lungs and linked blood vessels deliver oxygen to a person’s body and remove carbon dioxide. The lungs lie on either side of a person’s breastbone and fill the inside of his/her chest cavity. The left lung is slightly smaller than the right lung to allow room for the heart. Within the lungs, the bronchi branch into thousands of smaller, thinner tubes called bronchioles. These tubes end in bunches of tiny round air sacs called alveoli. Each of these air sacs is covered in a mesh of tiny blood vessels called capillaries. The capillaries connect to a network of arteries and veins that move blood through the body.

The pulmonary artery and its branches deliver blood rich in carbon dioxide (and lacking in oxygen) to the capillaries that surround the air sacs. Inside the air sacs, carbon dioxide moves from the blood into the air. Oxygen moves from the air into the blood in the lungs. The oxygen-rich blood then travels to the heart through the pulmonary vein and its branches. The heart pumps the oxygen-rich blood out to the body. The lungs are divided into five main sections called lobes. Some people need to have a diseased lung lobe removed. However, they can still breathe well using the rest of their lung lobes.

Muscles Used for Breathing Muscles near the lungs help expand and contract the lungs to allow breathing. These muscles include the: * Diaphragm -A dome-shaped muscle located below your lungs. It separates the chest cavity from the abdominal cavity. The diaphragm is the main muscle used for breathing. * Intercostal muscles -Located between the ribs. They also play a major role in helping people breathe. * Abdominal muscles -Beneath the diaphragm. These help a person breathe out when they’re breathing fast (for example, during physical activity). Muscles in the neck and collarbone area -Help a person breathe in when other muscles involved in breathing don’t work properly, or when lung disease impairs the person’s breathing. Muscles Used for Breathing Muscles near the lungs help expand and contract the lungs to allow breathing. These muscles include the: * Diaphragm -A dome-shaped muscle located below your lungs. It separates the chest cavity from the abdominal cavity. The diaphragm is the main muscle used for breathing. * Intercostal muscles -Located between the ribs. They also play a major role in helping people breathe. Abdominal muscles -Beneath the diaphragm. These help a person breathe out when they’re breathing fast (for example, during physical activity). * Muscles in the neck and collarbone area -Help a person breathe in when other muscles involved in breathing don’t work properly, or when lung disease impairs the person’s breathing. Digestive System The human digestive system is a complex series of organs and glands that processes food. In order to use the food we eat, our body has to break the food down into smaller molecules that it can process; it also has to excrete waste.

Most of the digestive organs (like the stomach and intestines) are tube-like and contain the food as it makes its way through the body. The digestive system is essentially a long, twisting tube that runs from the mouth to the anus, plus a few other organs (like the liver and pancreas) that produce or store digestive chemicals. The Process of Digestion The start of the process – the mouth: The digestive process begins in the mouth. Food is partly broken down by the process of chewing and by the chemical action of salivary enzymes (these enzymes are produced by the salivary glands and break down starches into smaller molecules).

On the way to the stomach: the esophagus – After being chewed and swallowed, the food enters the esophagus. The esophagus is a long tube that runs from the mouth to the stomach. It uses rhythmic, wave-like muscle movements (called peristalsis) to force food from the throat into the stomach. This muscle movement gives us the ability to eat or drink even when we’re upside-down. In the stomach – The stomach is a large, sack-like organ that churns the food and bathes it in a very strong acid (gastric acid). Food in the stomach that is partly digested and mixed with stomach acids is called chyme.

In the small intestine – After being in the stomach, food enters the duodenum, the first part of the small intestine. It then enters the jejunum and then the ileum (the final part of the small intestine). In the small intestine, bile (produced in the liver and stored in the gall bladder), pancreatic enzymes, and other digestive enzymes produced by the inner wall of the small intestine help in the breakdown of food. In the large intestine – After passing through the small intestine, food passes into the large intestine. In the large intestine, some of the water and electrolytes (chemicals like sodium) are removed from the food.

Many microbes (bacteria like Bacteroides, Lactobacillus acidophilus, Escherichia coli, and Klebsiella) in the large intestine help in the digestion process. The first part of the large intestine is called the cecum (the appendix is connected to the cecum). Food then travels upward in the ascending colon. The food travels across the abdomen in the transverse colon, goes back down the other side of the body in the descending colon, and then through the sigmoid colon. The end of the process – Solid waste is then stored in the rectum until it is excreted via the anus. Urinary System

Your body takes nutrients from food and uses them to maintain all bodily functions including energy and self-repair. After your body has taken what it needs from the food, waste products are left behind in the blood and in the bowel. The urinary system works with the lungs, skin, and intestines—all of which also excrete wastes—to keep the chemicals and water in your body balanced. Adults eliminate about a quart and a half of urine each day. The amount depends on many factors, especially the amounts of fluid and food a person consumes and how much fluid is lost through sweat and breathing.

Certain types of medications can also affect the amount of urine eliminated. The urinary system removes a type of waste called urea from your blood. Urea is produced when foods containing protein, such as meat, poultry, and certain vegetables, are broken down in the body. Urea is carried in the bloodstream to the kidneys. Process of the Urinary System The kidneys are bean-shaped organs about the size of your fists. They are near the middle of the back, just below the rib cage. The kidneys remove urea from the blood through tiny filtering units called nephrons. Each ephron consists of a ball formed of small blood capillaries, called a glomerulus, and a small tube called a renal tubule. Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney. From the kidneys, urine travels down two thin tubes called ureters to the bladder. The ureters are about 8 to 10 inches long. Muscles in the ureter walls constantly tighten and relax to force urine downward away from the kidneys. If urine is allowed to stand still, or back up, a kidney infection can develop.

Small amounts of urine are emptied into the bladder from the ureters about every 10 to 15 seconds. The bladder is a hollow muscular organ shaped like a balloon. It sits in your pelvis and is held in place by ligaments attached to other organs and the pelvic bones. The bladder stores urine until you are ready to go to the bathroom to empty it. It swells into a round shape when it is full and gets smaller when empty. If the urinary system is healthy, the bladder can hold up to 16 ounces (2 cups) of urine comfortably for 2 to 5 hours. Circular muscles called sphincters help keep urine from leaking.

The sphincter muscles close tightly like a rubber band around the opening of the bladder into the urethra, the tube that allows urine to pass outside the body. Nerves in the bladder tell you when it is time to urinate, or empty your bladder. As the bladder first fills with urine, you may notice a feeling that you need to urinate. The sensation to urinate becomes stronger as the bladder continues to fill and reaches its limit. At that point, nerves from the bladder send a message to the brain that the bladder is full, and your urge to empty your bladder intensifies.

When you urinate, the brain signals the bladder muscles to tighten, squeezing urine out of the bladder. At the same time, the brain signals the sphincter muscles to relax. As these muscles relax, urine exits the bladder through the urethra. When all the signals occur in the correct order, normal urination occurs. Endocrine System In general, the endocrine system is in charge of body processes that happen slowly, such as cell growth. Faster processes like breathing and body movement are controlled by the nervous system.

But even though the nervous system and endocrine system are separate systems, they often work together to help the body function properly. Parts of the Endocrine System * hypothalamus The hypothalamus is located in the brain, near the optic chiasm. It secretes hormones that stimulate or suppress the release of hormones in the pituitary gland, in addition to controlling water balance, sleep, temperature, appetite, and blood pressure. * pineal body The pineal body is located below the corpus callosum, a part of the brain. It produces the hormone melatonin. * pituitary The pituitary gland is located at the base of the brain.

No larger than a pea, the gland controls many functions of the other endocrine glands. * thyroid and parathyroids The thyroid gland and parathyroid glands are located in front of the neck, below the larynx (voice box). The thyroid plays an important role in the body’s metabolism. Both the thyroid and parathyroid glands also play a role in the regulation of the body’s calcium balance. * thymus The thymus is located in the upper part of the chest and produces T-lymphocytes (white blood cells that fight infections and destroy abnormal cells). * adrenal gland The pair of adrenal glands are located on top of both kidneys.

Adrenal glands work hand-in-hand with the hypothalamus and pituitary gland. * kidney The pair of kidneys are located near the middle of the back, just below the rib cage. The kidneys process the blood to sift out waste products and extra water. This waste and extra water becomes urine, which is stored in the bladder. * pancreas The pancreas is located across the back of the abdomen, behind the stomach. The pancreas plays a role in digestion, as well as hormone production. * ovary A female’s ovaries are located on both sides of the uterus, below the opening of the fallopian tubes (tubes that extend from the uterus to the ovaries).

In addition to containing the egg cells necessary for reproduction, the ovaries also produce estrogen and progesterone. * testis A male’s testes are located in a pouch that hangs suspended outside his body. The testes produce testosterone and sperm. The Nervous System As the most complex system, the nervous system serves as the body control center and communications electrical-chemical wiring network. As a key homeostatic regulatory and coordinating system, it detects, interprets, and responds to changes in internal and external conditions.

The nervous system integrates countless bits of information and generates appropriate reactions by sending electrochemical impulses through nerves to effector organs such as muscles and glands. The brain and spinal cord are the central nervous system (CNS); the connecting nerve processes to effectors and receptors serve as the peripheral nervous system (PNS). Special sense receptors provide for taste, smell, sight, hearing, and balance. Nerves carry all messages exchanged between the CNS and the rest of the body. central nervous system: Neurons, Brain, and Spinal Cord Neurons The neuron transmits electric signals like an electric wire.

The perikaryon (cell body) is the neuron central part. Dendrites, short branches, extend from the neuron. These input channels receive information from other neurons or sensory cells (cells that receive information from the environment). A long branch, the axon, extends from the neuron as its output channel. The neuron sends messages along the axon to other neurons or directly to muscles or glands. Neurons must be linked to each other in order to transmit signals. The connection between two neurons is a synapse. When a nerve impulse (electrical signal) travels across a neuron to the synapse, it causes the release of neurotransmitters.

These chemicals carry the nerve signal across the synapse to another neuron. Nerve impulses are propagated (transmitted) along the entire length of an axon in a process called continuous conduction. To transmit nerve impulses faster, some axons are partially coated with myelin sheaths. These sheaths are composed of cell membranes from Schwann cells, a type of supporting cell outside the CNS. Nodes of Ranvier (short intervals of exposed axon) occur between myelin sheaths. Impulses moving along myelinated axons jump from node to node. This method of nerve impulse transmission is saltatory conduction.

Brain The brain has billions of neurons that receive, analyze, and store information about internal and external conditions. It is also the source of conscious and unconscious thoughts, moods, and emotions. Four major brain divisions govern its main functions: the cerebrum, the diencephalon, the cerebellum, and the brain stem. The cerebrum is the large rounded area that divides into left and right hemispheres (halves) at a fissure (deep groove). The hemispheres communicate with each other through the corpus callosum (bundle of fibers between the hemispheres).

Surprisingly, each hemisphere controls muscles and glands on the opposite side of the body. Comprising 85 percent of total brain weight, the cerebrum controls language, conscious thought, hearing, somatosensory functions (sense of touch), memory, personality development, and vision. Gray matter (unmyelinated nerve cell bodies) composes the cerebral cortex (outer portion of the cerebrum). Beneath the cortex lies the white matter (myelinated axons). During embryonic development, the cortex folds upon itself to form gyri (folds) and sulci (shallow grooves) so that more gray matter can reside within the skull cavity.

The diencephalon forms the central part of the brain. It consists of three bilaterally symmetrical structures: the hypothalamus, thalamus, and epithalamus. The hypothalamus ‘master switchboard’ resides in the brain stem upper end. It controls many body activities that affect homeostasis (maintenance of a stable internal environment in the body). The hypothalamus is the main neural control center (brain part that controls endocrine glands). The pituitary gland lies just below the hypothalamus. The pituitary gland is a small endocrine gland that secretes a variety of hormones (organic chemicals that regulate the body’s physiological processes).

When the hypothalamus detects certain body changes, it releases regulating factors (chemicals that stimulate or inhibit the pituitary gland). The pituitary gland then releases or blocks various hormones. Because of this close association between the nervous and endocrine systems, together they are called the neuroendocrine system. The hypothalamus also regulates visceral (organ-related) activities, food and fluid intake, sleep and wake patterns, sex drive, emotional states, and production of antidiuretic hormone (ADH) and oxytocin. The pituitary gland produces both these ormones. The thalamus is a relay and preprocessing station for the many nerve impulses that pass through it. Impulses carrying similar messages are grouped in the thalamus, then relayed to the appropriate brain areas. The epithalamus is the most dorsal (posterior) portion of the diencephalon. It contains a vascular network involved in cerebrospinal fluid production. Extending from the epithalamus posteriorly is the pineal body, or pineal gland. Its function is not yet fully understood; it is thought to control body rhythms. At the rear of the brain is the cerebellum.

The cerebellum is similar to the cerebrum: each has hemispheres that control the opposite side of the body and are covered by gray matter and surface folds. In the cerebellum, the folds are called folia; in the cerebrum, sulci. The vermis (central constricted area) connects the hemispheres. The cerebellum controls balance, posture, and coordination. The brain stem connects the cerebrum and cerebellum to the spinal cord. Its superior portion, the midbrain, is the center for visual and auditory reflexes; examples of these include blinking and adjusting the ear to sound volume.

The middle section, the pons, bridges the cerebellum hemispheres and higher brain centers with the spinal cord. Below the pons lies the medulla oblongata; it contains the control centers for swallowing, breathing, digestion, and heartbeat. The reticular formation extends throughout the midbrain. This network of nerves has widespread connections in the brain and is essential for consciousness, awareness, and sleep. It also filters sensory input, which allows a person to ignore repetitive noises such as traffic, yet awaken instantly to a baby’s cry. The spinal cord is a continuation of the brain stem.

It is long, cylindrical, and passes through a tunnel in the vertebrae called the vertebral canal. The spinal cord has many spinal segments, which are spinal cord regions from which pairs (one per segment) of spinal nerves arise. Like the cerebrum and cerebellum, the spinal cord has gray and white matter, although here the white matter is on the outside. The spinal cord carries messages between the CNS and the rest of the body, and mediates numerous spinal reflexes such as the knee-jerk reflex. Meninges, three connective tissue layers, protect the brain and spinal cord.

The outermost dura layer forms partitions in the skull that prevents excessive brain movement. The arachnoid middle layer forms a loose covering beneath the dura. The innermost pia layer clings to the brain and spinal cord; it contains many tiny blood vessels that supply these organs. Another protective substance, cerebrospinal fluid, surrounds the brain and spinal cord. The brain floats within the cerebrospinal fluid, which prevents against crushing under its own weight and cushions against shocks from walking, jumping, and running. eripheral nervous system: somatic (voluntary) nervous system, autonomic (involuntary) nervous system The peripheral nervous system includes sensory receptors, sensory neurons, and motor neurons. Sensory receptors are activated by a stimulus (change in the internal or external environment). The stimulus is converted to an electronic signal and transmitted to a sensory neuron. Sensory neurons connect sensory receptors to the CNS. The CNS processes the signal, and transmits a message back to an effector organ (an organ that responds to a nerve impulse from the CNS) through a motor neuron.

The PNS has two parts: the somatic nervous system and the autonomic nervous system. The somatic nervous system, or voluntary nervous system, enables humans to react consciously to environmental changes. It includes 31 pairs of spinal nerves and 12 pairs of cranial nerves. This system controls movements of skeletal (voluntary) muscles. Thirty-one pairs of spinal nerves emerge from various segments of the spinal cord. Each spinal nerve has a dorsal root and a ventral root. The dorsal root contains afferent (sensory) fibers that transmit information to the spinal cord from the sensory receptors.

The ventral root contains efferent (motor) fibers that carry messages from the spinal cord to the effectors. Cell bodies of the efferent fibers reside in the spinal cord gray matter. These roots become nerves that transmit nerve impulses to muscles and organs throughout the body. Twelve pairs of cranial nerves transmit from special sensory receptors information on the senses of balance, smell, sight, taste, and hearing. Cranial nerves also carry information from general sensory receptors in the body, mostly from the head region.

This information is processed in the CNS; the resulting orders travel back through the cranial nerves to the skeletal muscles that control movements in the face and throat, such as for smiling and swallowing. In addition, some cranial nerves contain somatic and autonomic motor fibers. The involuntary nervous system (autonomic nervous system) maintains homeostasis. As its name implies, this system works automatically and without voluntary input. Its parts include receptors within viscera (internal organs), the afferent nerves that relay the information to the

CNS, and the efferent nerves that relay the action back to the effectors. The effectors in this system are smooth muscle, cardiac muscle and glands, all structures that function without conscious control. An example of autonomic control is movement of food through the digestive tract during sleep. The efferent portion of the autonomic system is divided into sympathetic and parasympathetic systems. The sympathetic nerves mobilize energy for the ‘Fight or Flight’ reaction during stress, causing increased blood pressure, breathing rate, and bloodflow to muscles.

Conversely, the parasympathetic nerves have a calming effect; they slow the heartbeat and breathing rate, and promote digestion and elimination. This example of intimate interaction with the endocrine system is one of many that explain why the two systems are called the neuroendocrine system. The relationship between sensory and motor neurons can be seen in a reflex (rapid motor response to a stimulus). Reflexes are quick because they involve few neurons. Reflexes are either somatic (resulting in contraction of skeletal muscle) or autonomic (activation of smooth and cardiac muscle).

All reflex arcs have five basic elements: a receptor, sensory neuron, integration center (CNS), motor neuron, and effector. Spinal reflexes are somatic reflexes mediated by the spinal cord. These can involve higher brain centers. In a spinal reflex, the message is simultaneously sent to the spinal cord and brain. The reflex triggers the response without waiting for brain analysis. If a finger touches something hot, the finger jerks away from the danger. The burning sensation becomes an impulse in the sensory neurons. These neurons synapse in the spinal cord with motor neurons that cause the burned finger to pull away.

This spinal reflex is a flexor, or withdrawal reflex. The stretch reflex occurs when a muscle or its tendon is struck. The jolt causes the muscle to contract and inhibits antagonist muscle contraction. A familiar example is the patellar reflex, or knee-jerk reflex, that occurs when the patellar tendon is struck. The impulse travels via afferent neurons to the spinal cord where the message is interpreted. Two messages are sent back, one causing the quadriceps muscles to contract and the other inhibiting the antagonist hamstring muscles from contracting. The contraction of the quadriceps and inhibition of hamstrings cause the lower leg to kick.