MUSCULAR SYSTEM

HOME

Muscular System

Muscle, tissue or organ of the animal body characterized by the ability to contract, usually in response to a stimulus from the nervous system. The basic unit of all muscle is the myofibril, a minute, threadlike structure composed of complex proteins. Each muscle cell, or fiber, contains several myofibrils, which are composed of regularly arranged myofilaments of two types, thick and thin. Each thick myofilament contains several hundred molecules of the protein myosin. Thin filaments contain two strands of the protein actin. The myofibrils are made up of alternating rows of thick and thin myofilaments with their ends interleaved. During muscular contractions, these interdigitated rows of filaments slide along each other by means of cross bridges that act as ratchets. The energy for this motion is generated by densely packed mitochondria that surround the myofibrils.

Three types of muscular tissue are recognized: smooth, skeletal, and cardiac.

Anterior Muscles

This illustration shows the body’s complex network of skeletal muscles. Skeletal muscles attach to bones of the skeleton and control all voluntary movements. A skeletal muscle crosses a joint between two bones and is attached to each bone either directly or by means of a tendon or a fibrous sheet or band called a fascia. Bones move when muscles contract, or shorten, across the joint.

The size of a muscle depends on the function it performs. Where dexterity is required, as in the fingers, the muscles are typically very small. Where strength is needed, as in the thigh, the muscles are large.

Posterior Muscles

This illustration shows the body’s complex network of skeletal muscles. Skeletal muscles attach to bones of the skeleton and control all voluntary movements. A skeletal muscle crosses a joint between two bones and is attached to each bone either directly or by means of a tendon or a fibrous sheet or band called a fascia. Bones move when muscles contract, or shorten, across the joint.

The size of a muscle depends on the function it performs. Where dexterity is required, as in the fingers, the muscles are typically very small. Where strength is needed, as in the thigh, the muscles are large.

IMMUNE SYSTEM

HOME 

Immune system

The immune system defends the body from invading organisms that may cause disease. One part of the immune system uses barriers to protect the body from foreign substances. These barriers include the skin and the mucous membranes, which line all body cavities; and protective chemicals, such as enzymes in saliva and tears that destroy bacteria.

Another part of the immune system uses lymphocytes, specialized white blood cells that respond to specific types of foreign invaders. B lymphocytes produce proteins called antibodies, which circulate in the blood and attack specific disease-causing organisms. T lymphocytes attack invading organisms directly.

Tonsils

Tonsils are masses of lymphoid tissue forming a ring around the walls of the pharynx, or throat. The lymphoid cells in the tonsils help protect the pharynx from invasion by disease-producing bacteria.

Thymus gland

Some lymphocytes form in the bone marrow and then travel to the thymus gland, where they mature into T lymphocytes.

Lymph nodes

Lymph nodes are masses of tissue that attract lymphocytes and deploy them to areas of the body under attack by infectious agents.

Spleen

The spleen is one of the lymphoid organs. Mature lymphocytes constantly travel through the blood to the lymphoid organs and then back to the blood again. This recirculation ensures that the body is continuously monitored for invading substances. Among its many functions, the spleen produces antibodies against various disease organisms and removes worn-out red blood cells from the bloodstream.

Bone marrow

The bone marrow is the soft substance found in the center of some bones. All lymphocytes originate in the bone marrow. Those that mature in the bone marrow develop into B lymphocytes.

Lymphatic vessels

Lymphocytes travel throughout the body in the blood, but they often migrate into lymphatic vessels, which are found in all parts of the body except the brain. Lymphocytes travel within these vessels in a pale,

fat-laden liquid known as lymph.

SKELETAL SYSTEM

HOME

Skeletal System

The human skeleton is a strong, flexible framework of 206 bones that supports the body and protects internal organs. In addition, the bones of the skeleton store calcium, a mineral essential for the activity of nerve and muscle cells. The soft core of bone, the bone marrow, is where red blood cells, certain white blood cells, and blood platelets form.

Bones come in different shapes and sizes, each adapted to perform specific functions. The breastbone, for example, is a flat plate of bone that helps to protect the heart and lungs in the chest. The fused bones of the skull safely encase the brain. The short, delicate bones in the wrist and hand enhance dexterity, providing flexibility for small, precise motions. The long, heavy femur bone in the leg acts as a strong lever for powerful or speedy movement. Cartilage is flexible connective tissue that provides support to skeletal bones and allows joints to move without rubbing against each other.

Exoskeleton

A form of exoskeleton is the shell of calcium or silica secreted by certain protozoans known as foraminiferans. Commercial sponges have an exoskeleton consisting of spongin, which is a tough, elastic substance. Cnidarians secrete a wide variety of exoskeletal substances, ranging from the elastic covering of the jellyfish to the stony material deposited by coral. The familiar shells of most mollusks are composed of calcium carbonate and an organic ground substance known as conchiolin. Among insects, each of the three principal divisions of the body—the head, the thorax, and the abdomen—is enclosed in a framework of horny plates. The plates of each primary division are separated from those of the next division by elastic tissue that permits flexibility of motion. The appendages are enclosed by sheaths projecting from the exoskeleton; elastic tissue similar to that between the plates joins the segments of the appendages and attaches them to the body.

Endoskeleton

Vertebrates have a more or less rigid group of structures composed of cartilage or bone or of a combination of these two connective tissues. The most primitive of these structures is the notochord, which is a backbone of cartilage occurring in fishes. Animals higher on the evolutionary scale have an axial skeleton, consisting of the skull, spinal column, and ribs, and an appendicular skeleton, made up of the pelvic and pectoral girdles and the appendages.

In higher animals, the skeleton formed in the embryo is initially cartilaginous; bone and calcium are deposited as the organism matures. In humans, the process of bone hardening, or ossification, is completed at about the age of 25. The last bone to ossify is the breastbone.

The total number of bones in any animal varies with its age; many bones fuse together during the ossification process. The average number of distinct skeletal structures in a young human is 200, exclusive of the 6 ossicles found in the ears. The human skeleton is subject to a number of pathological conditions, most important of which are fracture and a deficiency disease that is known as rickets.

NERVOUS SYSTEM

HOME

Nervous System

The human nervous system oversees the activity of all other body systems. The central nervous system is composed of the brain and spinal cord. The peripheral nervous system is made up of a network of nerves connecting the brain and spinal cord with the rest of the body. Together, the central and peripheral nervous systems control both voluntary and involuntary functions in the body. 

Brain

The center of the nervous system is the brain, which is composed of the brainstem, the cerebellum, and the cerebrum. The brainstem controls basic functions such as breathing rate and heartbeat and it contains nerve centers involved in hearing and vision.

The cerebellum controls balance, equilibrium, and locomotion. The cerebrum regulates such mental functions as understanding, remembering, speaking, learning, reasoning, and feeling emotions.

Spinal cord

The spinal cord extends from the brain into the spine through a canal encased in bone. The spinal cord is a collection of nerve fibers and cells that carry sensory impulses from the outer parts of the body up to the brain. The brain responds by sending impulses down through other nerve fibers in the spinal cord to stimulate muscles that are under voluntary control.

 Autonomic Nervous System

Among the motor fibers may be found groups that carry impulses to viscera. These fibers are designated by the special name of autonomic nervous system. That system consists of two divisions, more or less antagonistic in function, that emerge from the central nervous system at different points of origin. One division, the sympathetic, arises from the middle portion of the spinal cord, joins the sympathetic ganglionated chain, courses through the spinal nerves, and is widely distributed throughout the body. The other division, the parasympathetic, arises both above and below the sympathetic, that is, from the brain and from the lower part of the spinal cord. These two divisions control the functions of the respiratory, circulatory, digestive, and urogenital systems.

Nerves cell

Each nerve cell consists of a central portion containing the nucleus, known as the cell body, and one or more structures referred to as axons and dendrites. The dendrites are rather short extensions of the cell body and are involved in the reception of stimuli. The axon, by contrast, is usually a single elongated extension; it is especially important in the transmission of nerve impulses from the region of the cell body to other cells. See Neuro physiology.

Urinary System

HOME

Urinary system

The urinary system eliminates waste products from the body and helps regulate the body’s water and chemical balance. The organs of the urinary system include two oval-shaped kidneys, each with a long, slender tube, called the ureter, that connects to the bladder. Another tubular vessel, the urethra, extends from the bladder to an opening out of the body.

Kidney

The kidneys are the major excretory organs of the body. As the body’s blood flows through the kidneys, microscopic tubules called nephrons remove urea, salts, and other poisonous waste products formed during metabolism. The kidneys produce urine to carry away these waste products.

Nephrons also selectively absorb proteins, salts, sugar, calcium, and other nutrients. The kidneys return these life-sustaining substances to the bloodstream to maintain a healthy blood composition. The kidneys also secrete hormones that regulate body functions such as blood pressure and the production of red blood cells.

Bladder

Urine is produced in the kidneys and travels through the ureters to the bladder, a muscular sac that stores the urine. When the bladder is full, nerves signal the bladder to contract and expel urine from the body through the urethra.

Endocrine system

HOME

Endocrine system

The endocrine system is made of many glands—groups of cells that release chemical substances called hormones into the bloodstream. Hormones influence and regulate diverse activities such as metabolism, growth, mental development, and emotional behaviour.

Pituitary gland

The pituitary gland is a pea-sized gland located in the centre of the skull. The pituitary gland acts as a master controlling gland, releasing a number of hormones that activate other glands.

Parathyroid gland

The parathyroid glands are four small glands located in the neck behind the thyroid gland. These glands secrete a hormone that regulates the body’s use of calcium and phosphorus to maintain healthy bones. Parathyroid hormone also affects muscle contraction and the conduction of nerve impulses.

Thyroid gland

The thyroid gland, located in the neck, secretes the hormone thyroxine. Thyroxine increases body metabolism, in which food is broken down and converted into heat and energy. Too little thyroxine in the blood produces lethargy and fatigue, while too much thyroxine results in overactivity, nervousness, and weight loss.

Adrenal gland

The adrenals are two small glands, each located on the top of one kidney. These two glands consist of an inner core, called the adrenal medulla, and an outer area, known as the adrenal cortex. The adrenal medulla releases the hormone epinephrine, which speeds up heart rate and increases blood pressure to help the body cope with emergencies. The adrenal cortex releases hormones that control the level of salts and water in the blood and help regulate the use of sugar. It also secretes small amounts of male sex hormones, or androgens, in both males and females. Of the two parts of the adrenal glands, only the adrenal cortex is under the control of the pituitary gland.

Pancreas

The pancreas is a long, narrow gland located in the abdomen behind the stomach and beneath the liver. The pancreas secretes insulin, a hormone that regulates the body’s use of sugar. When too little insulin is produced, the body tissues cannot use or store sugar, and a disease known as diabetes mellitus develops.

Ovaries

Females have sex glands called ovaries that release hormones called estrogens. These hormones contribute to the development of female sexual characteristics, including skin, hair, and breast development. Estrogens work with certain pituitary hormones to control the menstrual cycle.

Testes

Males have sex glands called testes that secrete androgens, male sex hormones. In addition to contributing to male sexual characteristics, androgens contribute to the production of sperm and the development of the prostate gland.

CIRCULATORY SYSTEM

HOME

Circulatory System

Cardio vascular system, in humans, the combined function of the heart, blood, and blood vessels to transport oxygen and nutrients to organs and tissues throughout the body and carry away waste products. Among its vital functions, the circulatory system increases the flow of blood to meet increased energy demands during exercise and regulates body temperature. In addition, when foreign substances or organisms invade the body, the circulatory system swiftly conveys disease-fighting elements of the immune system, such as white blood cells and antibodies, to regions under attack. Also, in the case of injury or bleeding, the circulatory system sends clotting cells and proteins to the affected site, which quickly stop bleeding and promote healing.

Blood consists of three types of cells: oxygen-bearing red blood cells, disease-fighting white blood cells, and blood-clotting platelets, all of which are carried through blood vessels in a liquid called plasma. Plasma is yellowish and consists of water, salts, proteins, vitamins, minerals, hormones, dissolved gases, and fats.

Three types of blood vessels form a complex network of tubes throughout the body. Arteries carry blood away from the heart, and veins carry it toward the heart. Capillaries are the tiny links between the arteries and the veins where oxygen and nutrients diffuse to body tissues. The inner layer of blood vessels is lined with endothelial cells that create a smooth passage for the transit of blood. This inner layer is surrounded by connective tissue and smooth muscle that enable the blood vessel to expand or contract. Blood vessels expand during exercise to meet the increased demand for blood and to cool the body. Blood vessels contract after an injury to reduce bleeding and also to conserve body heat.

Heart

The human heart is a hollow, pear-shaped organ about the size of a fist. The heart is made of muscle that rhythmically contracts, or beats, pumping blood throughout the body. Oxygen-poor blood from the body enters the heart from two large blood vessels, the inferior vena cava and the superior vena cava, and collects in the right atrium. When the atrium fills, it contracts, and the blood passes through the tricuspid valve into the right ventricle. When the ventricle becomes full, it starts to contract, and the tricuspid valve closes to prevent blood from moving back into the atrium. As the right ventricle contracts, it forces blood into the pulmonary artery, which carries blood to the lungs to pick up fresh oxygen. When blood exits the right ventricle, the ventricle relaxes and the pulmonary valve shuts, preventing blood from passing back into the ventricle. 

Heart Valve

The human heart is a hollow, pear-shaped organ about the size of a fist. The heart is made of muscle that rhythmically contracts, or beats, pumping blood throughout the body. Oxygen-poor blood from the body enters the heart from two large blood vessels, the inferior vena cava and the superior vena cava, and collects in the right atrium. When the atrium fills, it contracts, and the blood passes through the tricuspid valve into the right ventricle. When the ventricle becomes full, it starts to contract, and the tricuspid valve closes to prevent blood from moving back into the atrium. As the right ventricle contracts, it forces blood into the pulmonary artery, which carries blood to the lungs to pick up fresh oxygen. When blood exits the right ventricle, the ventricle relaxes and the pulmonary valve shuts, preventing blood from passing back into the ventricle. 

Respiratory System

HOME

RESPIRATORY SYSTEM

The respiratory system consists of the lungs, a pair of elastic organs housed in the chest cavity, and the air passages leading to them. The air inhaled into the lungs provides oxygen to cells throughout the body. Air forced out of the lungs removes carbon dioxide from the body.

Air enters the respiratory system through the nose or mouth. It then travels through the larynx (voice box) and into the trachea (windpipe). At about the middle of the chest, the trachea divides into two tubes, the right and left bronchi. The right bronchus carries air to the three lobes of the right lung. The left bronchus supplies air to the two lobes of the left lung.

LUNGS

Inside the lungs the bronchi divide into smaller branches called bronchioles, which eventually empty into thousands of minute sacs called alveoli. The alveoli are surrounded by thin- walled capillaries. The air in the alveoli passes through to the blood cells within the capillaries. At the same time, carbon dioxide from these blood cells passes into the alveoli. From the alveoli, air containing carbon dioxide travels out of the lungs and is exhaled through the nose or mouth.