Acharya Mahaprajna
Though everyone lives with one's own body each moment of one's life, one knows little about the structure of its vital organs and less about how they function. First of all we must acquire knowledge of the working of various body systems. Only then we shall be familiar with our vital organs such as heart, lungs and liver. We shall then be equipped to stop misusing them and take better care of them.
Cell: the Unit of Life
The human body and its parts are made up of (i) trillions of microscopic structures called cells, (ii) inter-cellular material that the cells produce, and (iii) body fluids. If we compare our body to a building, the cells are the bricks i.e. they are the smallest living units of the body.
There are 60 bilbon[1] cells in a human body. Nearly all of them need a high magnification microscope to be seen and a super microscope to peep inside its body. The smallest cells (certain brain cells) are about 1/200 mm. and the largest ones (ova) are about 1/4 mm. in diameter.
For its operation the cell required a lot of energy which is generated in super-minute power station in each cell In almost every tissue, cells wear out and must be replaced New cell formation occurs through cell-division. Deoxyribonucleic acid (DN A) and its duplication in the nucleus of the cell is the essential event for cell-division.
Each living cell contains thousands of different kinds of chemicals. These chemicals are not an inert mixture but are constantly interacting with one another. The blueprints of heredity are encoded in chemical form. The structures of the various organs of the body are built up from chemical constituents and differences in chemical composition distinguish one type from another. Cells participate in every foundation of the body from birth to death.
Tissues, Organs and System
Groups of cells with a similar structure together with the non-living material (inter-cellular substance) between them form many types of tissues such as:
- Epithelial or covering tissue
- Bone and cartilage
- Muscle tissue
- Nerve tissue etc.
Tissues performing a common function or functions are grouped into organs, e.g. heart is one of the vital organs of the body. Team work is an essential ingredient for survival in the living body. A system is a group of organs working together to perform a series of functions.
The skeletal system, the muscular system, the skin etc. are some of these. The other important systems of the body which merit more detailed treatment are:
- Nervous system
- Circulatory system
- Respiratory system
- Digestive system
- Endocrine system
- Urinary system
1. Nervous System
The nervous system is the most complex system in the human body. It co-ordinates and regulates the work of other systems (of the body) and through them controls the functions of the body as a whole. That is why it is considered to be most important system of the body. Its failure will result in total cessation of its activities, paralysis of all the organs and ultimate stoppage of all vital processes. One would be unable to use one's muscles, unable to move one's hands, blink one's eyes, to sit or stand and even to breathe.
It has two divisions: (i) Central Nervous System (CNS) and (ii) Peripheral Nervous System (PNS).
The Central Nervous System is composed of:
- The brain, and
- The spinal cord.
They merge smoothly into one another without any obvious demarcation; the portion above the foramen magnum (the opening in the base of the skull) is considered as the brain, while the portion below it is the spinal cord.
The Peripheral Nervous System consists of 12 cranial nerves and 31 spinal nerves which branch and rebranch, after leaving the brain and spinal cord respectively, to form an intricate network throughout the body. The PNS also contains the autonomic nerves which regulate the activities of the internal organs. The action of these nerves is not generally under voluntary control.
More than 10 billion neurons comprise the human nervous system. Through the intricate network run impulses that are the foundation of creative thoughts as well as desires. Innumerable bits of data continually flood the body's sensory organs and pass from the receptors along the peripheral system to the central nervous system. The latter system screens and evaluates the incoming impulses; stores important data for future retrieval; formulates decisions and initiates action via impulses sent along the peripheral nerves to the muscular system.
Thus the nervous system has two basic functions - (i) the detection and processing of information from within and outside the body, and (ii) production and regulation of movement by muscle-action. Some parts of the brain are also responsible for the control of emotions and the storage of information and are also concerned with personality factors and intellect.
The Brain has three major divisions: (i) the fore-brain (ii) the mid brain and (iii) the hind brain.
The fore-brain and mid brain together make up the cerebrum. It constitutes the bulk of the brain. It is composed of an outer grey layer beneath which is a mass of white tissues. Its wrinkled surface allows a very large number of brain cells to be packed into a limited space.
Buried deep within the brain are various important structures including the hypothalamus. It contains numerous specialised control centres. It is intimately connected with the pituitary gland, thus interlocking the endocrine system. Medulla oblongata joins the brain to the spinal cord.
The Spinal Cord extends from the foramen magnum down to the level of the second lumbar vertebral. The cord itself is suspended rather loosely in the vertebral canal. Its length is about 45 centimeters. Each pair of nerves is composed of (i) sensory pathways which carry the message of sensation to the brain, and (ii) motor pathways which carry the orders for movement from the brain to the muscles in the trunk and the limbs. The spinal cord also contains 'reflex centres' for immediate response to certain incoming stimuli without involvement of the brain.
Sense-organs
The Human body is equipped with a versatile assortment of sensory outposts. Our eyes, ears, nose, mouth and the entire body surface are endowed with a vast variety of receptors for collection of useful information from outside. Sensations are the conscious results of the sequence: stimulus→receptor→conducting pathway→sensory area in the brain. Though hundreds of thousands of sensory messages are received by the brain, only those which are important are perceived while the rest are ignored. An intricate network of nerve-cells, called Reticular Activising System (RAS) acts as a filter and allows only strong or novel signals to pass up to higher brain for conscious perception. The process by which the mind converts raw sensations into perception is complicated. The RAS action permits us to concentrate on a particular thought or activity.
2. Circulatory System
Every cell of the body needs a continued supply of nutrients and oxygen and a continual removal of wastes. Disease-fighters, chemical messengers and other vital substances also need a transporting medium to distribute them through the body to keep the cells active and alive. The blood-circulatory system with its intricately branching and interconnecting tubes provides these services.
The main organs of this system are: heart, aorta, arteries, veins and capillaries. The propulsive force that keeps the blood moving is the steady beating of a powerful pump - the heart. The contractions of the heart drive blood into arteries which branch and rebranch into smaller tubes—arterioles—ultimately leading into tiny capillaries, that penetrate the tissues. Here, the delivery of the nutrients and chemicals and exchange of gases takes place. In time, the blood, reloaded with wastes, begins the return-trip draining it into successively larger veins and ultimately reaches the heart. The very name of the circulatory system implies that whatever blood goes out from the heart comes back to it, completing the circuit. The general sequence of circulation is -
Heart→ artery → capillary → vein→ heart.
The human heart is a miracle pump. Every day it pumps and circulates blood through about 1,00,000 km of blood-vessels which form a completely closed system.
The heart is a hollow muscular organ which has four chambers, two each in the right and the left side. While there is no connection between the two sides, the upper and lower chambers on each side are connected by one-way valves. It is actually a dual action pump—one to send the blood into the lungs and the other to push it out into the body. The first action is done by the right side and the second one by the left side.
Muscular walls of the heart contract and relax rhythmically. This rhythm is self-initiated requiring no nervous systole while that of relaxation is called diastole. A full cardiac cycle-systole and diastole—takes less than a second, normal heart-rate being 70/72 beats per minute. But in the space of just one minute, a volume equivalent to all the blood, about 5 litres in the body, passes through the heart. Heart-rate steadily slows down as one gets older; it is 140 before birth, 90 as a child and 70/72 as an adult
Blood-vessels
Multitudinous branches of blood-vessels of various sizes form a complex inter-communicating network that reaches practically every cell in the body. The main types of blood-vessels are arteries, veins and capillaries. The blood vessels which carry the blood from the heart to the body are commonly known as arteries, while those which carry the blood in the opposite direction, i.e. towards the heart are called veins.
Capillaries are so small that the red blood cells have to pass through them in a single file. They permeate the tissues and service the body-cell directly. Oxygen, nutrients and hormones transported in the blood are transferred to the cells, while cell-produced products for export (carbon dioxide etc.) diffuse into the blood. This exchange is so rapid that each given unit of blood spends only one to three seconds in a particular capillary.
The Blood-stream
Besides the oxygen, a variety of other substances must be delivered to the cells and tissues to keep them alive and vigorous. Glucose, fat, amino-acids and water are some of the more common, while copper and cobalt, iodine and phosphorous are a few of the uncommon requirements of the cells. Salts, minerals, vitamins, and hormones are also carried by the blood-stream for delivery to appropriate places. Thus blood is ever-changing, yet its overall composition remains surprisingly constant. The main components of the blood are:
- Blood-plasma—A clear straw-coloured fluid in which are dissolved salts, proteins,fats, sugar, hormones,vitamins,as well as waste products such as urea and lactic acid.
- Red Blood Cells—These are small flattened biconcave discs. 3000 of them would be needed to measure an inch. There are five million red cells per cubic millimeter. Every second, three million cells die and equal number of them are born.
Hemoglobin—The red pigment of the red cells is an iron containing protein. Total amount of iron in the hemoglobin of all the red cells is about 3 grams. But it is priceless as one cannot live without it. - White Blood Cells—They are larger and fewer than red cells. They are colourless and not uniform in size, shape and appearance They are the soldiers of the body—defenders against foreign invaders, ready to fight to death.
- Platelets—They go into action when a blood vessel is cut or pierced and blood is being lost A substance called platelet factor initiates a chain of chemical reactions leading to the formation of a clot which effectively plugs the cut or hole.
3. Respiratory System
The body needs a continual supply of oxygen. In addition, the body also needs some means of disposing of the waste carbon dioxide, produced by the body cells. Respiratory system fulfils both these requirements.
This system includes passage-ways and tubes through which the air passes: the nose, trachea, bronchi and bronchioles arranged in a sequence that branches and rebranches and looks like an inverted tree. The tubes end in tiny air sacs called alveoli in which the exchange of gases takes place. The bronchioles and alveoli constitute the lungs. Human lungs contain about 300 to 500 million alveoli covering a total surface area of more than 90 sq mtrs enough to carpet a tennis court Each alveolus is a roughly globular structure about 100 microns in diameter. It has an extremely thin wall and is surrounded by a network of equally thin-walled capillaries. An exchange of the two gases occurs in the lungs. Oxygen passes out through the thin walls of the alveoli and in through those of the capillaries that surround them. At the same time there is a net movement of carbon dioxide in the opposite direction. Lungs themselves are devoid of muscular action but the system includes a bellow's arrangement-the rib cage—operated by muscle and controlled by nerves.
To a physiologist breathing is divided into:
- external respiration i.e. passage of oxygen from the lungs into the blood and the passage of carbon dioxide from the blood to the alveoli, and
- internal respiration in which the body cells exchange carbon dioxide with fresh oxygen carried by the blood. The final aspect of respiration is cellular respiration i.e. chemical reactions of oxidation within the cells.
Three sets of muscles participate in the process of breathing-(i) diaphragm, (a dome-shaped muscle between the chest and the abdomen) (ii) intercostal muscle (attached to the ribs) and (iii) clavicular muscles (attached to the collar bone).
The average adult, at rest and not emotionally excited, breathes about 14 to 20 times a minute. Emotional stimulation, pain, temperature, carbon dioxide level and age cause variation from this basic level. Like the heart-beat, the respiration rate tends to decrease from the birth to adulthood and increases in old age.
4. Digestive System
Energy is essential to maintain such vital functions and processes as breathing, blood circulation and brain function. It is derived from foodstuffs but food cannot be utilised by the tissues until they have been broken down to smaller simpler components through the process of digestion.
The digestive system is composed of the alimentary tract and accessory organs which contribute their secretions to the tract. The digestion begins in the mouth and ends in the bowel. The passage from the mouth to the rectum is about 9 metres in length.
Mouth, Salivary Glands, Aesophagus
The first station on the alimentary canal is mouth. Saliva from three pairs of salivary glands enters the mouth and mixes with the food. The tongue pushes the food between the teeth for mastication, shapes it into a glob or bolus and shoves it to the aesophagus—a muscular tube about 2.5 cms in diameter and 25 cms long, leading down through the chest and diaphragm to the stomach. As the food enters the aesophagus, it is propelled down to the stomach by alternate waves of contraction and relaxation of the muscular tube.
The Stomach
The food has now reached the second processing station on the tract. Stomach is a muscular collapsible bag about 22 cms in length. It is tucked up in the abdomen at the lower rib-line under the diaphragm. Its wide end is at the top and towards the left while the narrow bottom end is towards the right. It retains food for 3 to 5 hours during which time partial digestion of protein takes place. The inside lining of the stomach contains 35 million gastric glands which secrete 2 to 3 litres of gastric juices containing mucin, hydrochloric acid and enzyme pepsin. Mucin lines the stomach and protects it from the acid and prevents it from being digested.
Duodenum, Small Intestine, Large Intestine (Colon)
The lower end of the stomach which becomes considerably narrower is connected to the small intestine through pyloric valve. The first portion of the small intestine called duodenum is about 16 to 17 cms long—a C-shaped tube. It passes behind the liver and encircles the head of pancreas. It is the third processing station and an important organ of the digestive tract. The second and third portions of the small intestine are called jejunum and ileum respectively. The major part of food-digestion and absorption takes place here. Peristalsis, the slow automatic movement along the whole length of the tract, propels the contents continuously onwards.
Next to the small intestine comes the large intestine or bowel. It is much wider—6 to 8 cms in diameter, and about 2 metres in length. First it passes upwards. It bends when it reaches the bottom of the liver and remains horizontal up to the spleen. Then it bends downwards: its last part is in the pelvic cavity and is called the rectum. Finally, it ends at the external opening, the anus. Its outside wall is puckered rather than tube-like. Its lining secretes mucus but no digestive enzymes.
Accessory Organs of the Digestive System Liver and Biliary System
The liver is the most important and an extraordinary organ which contributes to the process of digestion. It is reddish brown in colour and weighs about 1.5 to 2 kg. It is not only the largest but the most versatile single organ in the body. It lies in the upper part of the abdomen on the right side, beneath (and loosely attached to) the diaphragm. The bile secreted by the liver goes to the gallbladder through bile-ducts, where it is stored and concentrated. Liver is the largest chemical factory in the body, and has at least five hundred known functions.
Pancreas and the Islets of Langerhans
Pancreas is an oblong, rather flattened, boneless, fatless and fleshy gland about 15 to 18 cms long and 4 cms wide. It has a head, a body and a tail; its head rests in the curves of the duodenum and its tail touches the spleen. It is a versatile organ, the second largest gland in the body (liver being the largest). It is connected to the duodenum through the pancreatic duct which extends throughout its length. 700 mis. of pancreatic juice containing several digestive enzymes is produced and poured into the duodenum.
This gland functions both as a digestive organ (exocrine gland), and as an endocrine gland. In fact, it is two unrelated organs into one. Scattered throughout the pancreas are many pin-head- sized clusters of special cells. These are the Islets of Langerhans which carry out the endocrine function by secreting insulin of the body's carbohydrate metabolism by keeping the blood-sugar level within a relatively narrow range.
5. Endocrine System
The endocrine glands are ductless and their production passes directly into the blood-stream, circulates all over the body and acts at places far from where it originated. These glands are scattered through the body like islands. Nevertheless, they are unified into a finely coordinated system, function in a marvellously harmonious fashion and control and coordinate the activities of the body. The main endocrines are: the pineal, pituitary, thyroid, parathyroid, thymus, adrenals, Islets of Langerhans and gonads (sexglands). All these glands are comparatively small. They have access to very rich vascular supply. The products of these glands are organic chemical compounds called hormones which are effective in very small quantities.
The Pineal Gland
This gland lies near the centre of the brain. It is very small, not much larger than a grain of wheat. As its name implies, it is a cone-shaped body, reddish in colour and weighs about 1/2 gm. It is hidden away at the base of brain in a tiny cave behind and above the pituitary gland.
The Pituitary Gland
This gland is about the size of a pea, situated almost exactly in the centre of the head at the base of thebrain and just behind the nose. It hangs suspended from the underside of the brain in a little cup or cradle. It has a greyish yellow colour. It increases in size until about 30th year, and in the adult male weighs about 600 mgs. (slightly more in women). No part of the body is exempt from its influence.
Thyroid Gland
This gland consists of two maroon-coloured masses astride the upper end of the trachea, close to the larynx. The two parts are connected with a narrow strip of the same tissue just below the Adam's apple. Each lobe is about 5 cms in length, 3 cms in width and about 2 mms thick. The normal thyroid of an adult weighs 25 to 40 gms but it size fluctuates with age, habitation and diet It is heavier in the female than in the male and becomes enlarged during sex-excitement, menstruation and pregnancy. The vascular supply to the gland is exceptionally rich, e.g. it receives four times as much blood as do the kidneys.
The Parathyroid Glands
They are four minute yellowish brown ovoid bodies about 6 mms long and 3 mms wide, embedded in the lobes of the thyroid gland (two in each lobe). Sometimes the lower two are found much farther down end in the chest
The Thymus Gland
This gland is the source of one or more hormone-like factors. A lymphoid - two lobed structure—this gland is situated in the chest between the two lungs and extends up into the neck. A brownish mass - it reaches its largest size at the beginning of puberty, when it is about 5 cms long, 3.5 cms wide and 6 mms thick.
The Adrenal Glands
They are a pair of three-cornered hat-shaped glands capping the upper end of the kidneys. Each adrenal is a double gland, composed of a cortex—an outer layer and a medulla—an inner layer. The cortex makes up the bulk of the gland, which is bright yellow outside and reddish brown inside. The medulla is much thinner and grey in colour. The tremendous importance of these glands is better understood when it is known that death occurs very quickly if they are removed. More hormones - more than three dozens—are produced by the adrenal cortex than by any other endocrine gland. Number of these hormones is essential for life
The Gonads (Sex-glands)
The term "Gonad" literally means "seed" and the male and female sex organs - the gonads - produce the seeds of the new generation. The gonads also double as potent endocrine glands, secreting hormones that condition the functional state and influence the psycho-biological phenomena involved in the sexual act Thus, besides producing the ovum, the ovaries also produce hormones that vitalize a woman and make her feminine. Similarly, secretion of male sex-glands are the male energising forces and what makes him really male. These hormones have profound influences not only on the sexual lives but also a number of body organs and functions of an individual
6. Urinary System
The Kidneys
The major excretory organs for the elimination of nitrogenous wastes are the kidneys. A pair of bean-shaped organs—they are among the hardest working organs in the body. The normal adult kidney is about 10 to 12 cms long, 5 to 6 cms. wide and about 3 to 4 cm thick. They are located under the diaphragm, just above the waist line, one on each side of the spine against the posterior body wall. They are embedded in a heavy cushion of a fat which both protects and supports them.
The Ureters
The urine that is formed continuously in each kidney trickles out through the ureters. Each ureter is a long muscular tube about 25 to 30 cms long and 4 to 5 mms in diameter.
The Bladder
It in an expandable baglike structure, situated mainly in the pelvic cavity below the navel As it fills up, it expands upward into the abdomen. The two ureters enter the bladder and travel for a few centimeters under the bladder wall.
The Urethra
In both sexes, urine is emptied from the bladder through a pencil-size tube, the urethra, which emerges at the exterior surface of the body in an opening. In females, the urethra is a short tube about 2.5 to 3 cms long. The male urethra takes a much longer, tortuous course. It goes through the prostate gland and has a total length of about 20 cms. It serves the double function of carrying urine as well as seminal fluid. Enlargement of prostate gland (in older men) can restrict the flow of urine making urination difficult and painful.
Besides cleaning and filtering the blood, kidneys encourage production of red blood cells. They regulate in proportion of sodium and potassium salt, water and other substance in the blood. A little too much or a shade too little of any of them can be fatal. They control vital water balance. They also keep blood neither too acidic nor too alkaline.
