THE HUMAN BODY SYSTEMS
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Introduction 
           Anatomy (Greek anatome, "dissection"), is a branch of natural science dealing with the structural organization of living things. It is an old science, having its beginnings in prehistoric times. For centuries, anatomical knowledge consisted largely of observations of dissected plants and animals. The proper understanding of structure, however, implies a knowledge of function in the living organism. Anatomy is therefore almost inseparable from physiology, which is sometimes called functional anatomy. As one of the basic life sciences, anatomy is closely related to medicine and to other branches of biology. 
  
Here is a list of different systems in the human body: 

The Musculoskeletal 
The Circulatory System 
The Respiratory System 
The Endocrine System 
Skin 
The Digestive and Excretory System 
The Immune System 
The Brain System 

 
The human body operates through systems that are summarized below. 

The Musculoskeletal System 
The human skeleton consists of more than 200 bones bound together by tough and relatively inelastic connective tissues called ligaments. The different parts of the body vary greatly in their degree of movement. Thus, the arm at the shoulder is freely movable, whereas the knee joint is definitely limited to a hinge like action. The movements of individual vertebrae are extremely limited; the bones composing the skull are immovable. Movements of the bones of the skeleton are effected by contractions of the skeletal muscles, to which the bones are attached by tendons. These muscular contractions are controlled by the nervous system. 

The Nervous System

The nervous system has two divisions: 
the somatic, which allows voluntary control over skeletal muscle, and 
the autonomic, which is involuntary and controls cardiac and smooth muscle and glands.
 
The autonomic nervous system has two divisions: the sympathetic and the parasympathetic. Many, but not all, of the muscles and glands that distribute nerve impulses to the larger interior organs possess a double nerve supply; in such cases the two divisions may exert opposing effects. Thus, the sympathetic system increases heartbeat, and the parasympathetic system decreases heartbeat. 
The two nervous systems are not always antagonistic, however. For example, both nerve supplies to the salivary glands excite the cells of secretion.  The sweat glands, the muscles that cause involuntary erection or bristling of the hair, the smooth muscle of the spleen, and the blood vessels of the skin and skeletal muscle are actuated only by the sympathetic division. 

Voluntary movement of head, limbs, and body is caused by nerve impulses arising in the motor area of the cortex of the brain and carried by cranial nerves or by those that emerge from the spinal cord to reach skeletal muscles. The reaction involves both excitation of nerve cells stimulating the muscles involved and inhibition of the cells that stimulate opposing muscles. A nerve impulse is an electrical change within a nerve cell or fiber; measured in millivolts, it lasts a few milliseconds and can be recorded by electrodes. Movement may occur also in direct response to an outside stimulus; thus, a tap on the knee causes a jerk, and a light shone into the eye makes the pupil contract. These involuntary responses are called reflexes. 

Various nerve terminals called receptors constantly send impulses into the central nervous system. These are of three classes: 

exteroceptors, which are sensitive to pain, temperature, touch, and pressure; 
interoceptors, which react to changes in the internal environment; and 
proprioceptors, which respond to variations in movement, position, and tension. 
These impulses terminate in special areas of the brain, as do those of special receptors concerned with sight, hearing, smell, and taste. 
  
Circulatory System

In passing through the system, blood pumped by the heart follows a winding course through the right chambers of the heart, through the lungs, where it picks up oxygen, and back into the left chambers of the heart. From there it is pumped into the main artery, the aorta, which branches into increasingly smaller arteries until it passes through the smallest arteries, known as arterioles. Beyond the arterioles, the blood passes through a vast amount of tiny, thin-walled structures called capillaries. Here, the blood gives up its oxygen and its nutrients to the tissues and absorbs from them carbon dioxide and other waste products of metabolism. The blood completes its circuit by passing through small veins that join to form increasingly larger vessels until it reaches the largest veins, the inferior and superior venae cavae, which return it to the right side of the heart. 

Blood is propelled mainly by contractions of the heart; contractions of skeletal muscle also contribute to circulation. Valves in the heart and in the veins ensure its flow in one direction.
  
Immune System

The body defends itself against foreign proteins and infectious microorganisms by means of a complex dual system that depends on recognizing a portion of the surface pattern of the invader. 
The two parts of the system are termed cellular immunity, in which lymphocytes are the effective agent, and humoral immunity, based on the action of antibody molecules. 

When lymphocytes recognize a foreign molecular pattern (termed an antigen), some release antibodies in great numbers; others store the memory of the pattern for future release of antibodies should the molecule reappear. Antibodies attach themselves to the antigen and in that way mark them for destruction by other substances in the body's defense arsenal. These are primarily complement, a complex of enzymes that make holes in foreign cells, and phagocytes, cells that engulf and digest foreign matter. They are drawn to the area by chemical substances released by activated lymphocytes. 

Lymphocytes, which resemble blood plasma in composition, are manufactured in the bone marrow and multiply in the thymus and spleen. They circulate in the bloodstream, penetrating the walls of the blood capillaries to reach the cells of the tissues. From there they migrate to an independent network of capillaries that is comparable to and almost as extensive as that of the blood's circulatory system. The capillaries join to form larger and larger vessels that eventually link up with the bloodstream through the jugular and subclavian veins; valves in the lymphatic vessels ensure flow in one direction. Nodes at various points in the lymphatic network act as stations for the collection and manufacture of lymphocytes; they may become enlarged during an infectious disease. 

In anatomy, the network of lymphatic vessels and the lymph nodes are together called the lymphatic system; its function as the vehicle of the immune system was not recognized until the 1960s. 
  
Respiratory System

Respiration is carried on by the expansion and contraction of the lungs. The process and the rate at which it proceeds are controlled by a nervous center in the brain.
 
In the lungs, oxygen enters tiny capillaries, where it combines with haemoglobin in the red blood cells and is carried to the tissues. Simultaneously, carbon dioxide, which entered the blood in its passages through the tissues, passes through capillaries into the air contained within the lungs. Inhaling draws into the lungs air that is higher in oxygen and lower in carbon dioxide; exhaling forces from the lungs air that is high in carbon dioxide and low in oxygen. Changes in the size and gross capacity of the chest are controlled by contractions of the diaphragm and of the muscles between the ribs. 

Digestive and Excretory Systems

The energy required for maintenance and proper functioning of the human body is supplied by food. 
After it is broken into fragments by chewing and mixed with saliva, digestion begins.
 
The food passes down the gullet into the stomach, where the process is continued by the gastric and intestinal juices. 
Thereafter, the mixture of food and secretions, called chyme, is pushed down the alimentary canal by peristalsis. Peristalsis are rhythmic contractions of the smooth muscle of the gastrointestinal system. The contractions are initiated by the parasympathetic nervous system.
 
Absorption of nutrients from chyme occurs mainly in the small intestine. 
Unabsorbed food and secretions and waste substances from the liver pass to the large intestines and are expelled as faeces. 
Water and water-soluble substances pass in the blood to the kidneys, where all the constituents of the blood plasma except protein pass through the thin membranes of the blood capillaries into the kidney tubules. The remaining water, salts, and waste products pass down the kidney tubes, which return most of the water and salts to the body and secrete other salts and waste products from the blood. 
Urine, the fluid that remains, is stored in the urinary bladder until it is expelled.

The Endocrine System

In addition to the integrative action of the nervous system, control of various body functions is exerted by the endocrine glands. An important part of this system, the pituitary, lies at the base of the brain. This master gland secretes a variety of hormones, including the following: (1) a hormone that stimulates the thyroid gland and controls its secretion of thyroxine, which dictates the rate at which all cells utilize oxygen; (2) a hormone that controls the secretion in the adrenal gland of hormones that influence the metabolism of carbohydrates, sodium, and potassium and control the rate at which substances are exchanged between blood and tissue fluid; (3) substances that control the secretion in the ovaries of estrogen and progesterone and the creation in the testicles of testosterone; (4) the somatotropic, or growth, hormone, which controls the rate of development of the skeleton and large interior organs through its effect on the metabolism of proteins and carbohydrates; and (5) an insulin inhibitor-a lack of insulin causes diabetes mellitus. 
The posterior lobe of the pituitary secretes vasopressin, which acts on the kidney to control the volume of urine; a lack of vasopressin causes diabetes insipidus, which results in the passing of large volumes of urine. Other glands in the endocrine system are the pancreas, which secretes insulin, and the parathyroid, which secretes a hormone that regulates the quantity of calcium and phosphorus in the blood.
  
Skin

The skin is an organ of double-layered tissue stretched over the surface of the body and protecting it from drying or losing fluid, from harmful external substances, and from extremes of temperature. 
The inner layer, called the dermis, contains sweat glands, blood vessels, nerve endings , and the bases of hair and nails. 
The outer layer, the epidermis, is only a few cells thick; it contains pigments, pores, and ducts, and its surface is made of dead cells that it sheds from the body.
 
The sweat glands excrete waste and cool the body through evaporation of fluid droplets; the blood vessels of the dermis supplement temperature regulation by contracting to preserve body heat and expanding to dissipate it. Separate kinds of receptors convey pressure, temperature, and pain. Fat cells in the dermis insulate the body, and oil glands lubricate the epidermis.