First-Aid: Structure and Function of Human Body

The human body is an amazing combination of different systems that are well coordinated for smooth functioning as a unit. All the systems are equally important for health, and no particular system can be called as more important that other systems, a first aider needs to have knowledge of the structure and function of various systems so as to be able to clearly understand sickness and effects of injury on the body.


We all know that the smallest functioning unit of every living being is a cell. A group of cells together form a tissue. Various tissues together form an organ. There may be a number of organs in a system that serve a specific purpose.


The basic unit of the nervous system is a nerve cell (neuron). Each nerve cell has a body and processes which connect it to adjacent or distant nerve cells. Smaller branched processes called dendrites carry impulses to the nerve eel! body, while longer unbranched processes called axons carry impulses away from the body. The sites of contact are called synapses. Neurotransmitters like acetylcholine and noradrenaline help transmit impulses across synapses.

Midline – It divides the body into right and left halves with a vertical line.
Lateral – Anything away from the midline is said to be lateral.
Superior – Towards the head end.
Inferior – Towards the foot end.
Posterior – Back of the body.
Anterior – Front of the body.
Proximal – Towards the root of a limb.
Distal – Towards the end of a limb.


1. Central nervous system : brain and spinal cord. The nervous system consists of the brain, spinal cord, and nerves. It is divided into central and peripheral. The brain is situated in the hollow of the cranial bones. It comprises of two hemispheres. Each hemisphere has gray matter outside and white matter inside. The gray matter is responsible for storage, appreciation of sensations and generation of motor impulses. There are twelve cranial nerves coming out of various foramens in base of the skull. Brain is connected to rest of the body through spinal cord.

2. Peripheral nervous system : cranial and peripheral nerves.

The nerve fibres may be sensory, motor, mixed, or autonomous (sympathetic and parasympathetic). They carry nervous impulses, motor fibers from the central nervous system to the muscles, sensory fibers from sense organs to the central nervous system, and autonomic fibers from autonomic centers to various systems.

Injuries to the head and spine can cause damage to the central nervous system. The brain takes care of thinking, receiving sensory input and giving motor output to bring about movements. It also controls vital functions like heart activity and respiration. The spinal cord is responsible for transmitting sensory impulses from skin and joints to the brain, and also for reflex actions of muscles. Injury to the skull could injure the brain, causing failure of vital body functions. Injury to the spine could cause paralysis of lower and/or upper limbs. If the damage is severe enough, the lost function may never recover.


It consists of heart, arteries, veins, and blood. The heart is a hollow muscular organ, made of special type of muscle. It is situated between the two lungs in the thoracic cavity more towards left side of the chest. It measures 12 cm in length, 9 cm in breadth, and 6 cm in thickness. It weighs 280 g. It has 4 chambers, upper two are known as right and left atria, while the lower two are known as right arid left ventricles. The heart contracts and relaxes continuously to work as pump. Its primary function is to purify and circulate the blood in the body and to help in distributing the nutrients and oxygen to the body, and waste material away from the sites of production to the organs of excretion.

Our Blood vessels are of three types. They are :

1. Arteries : They are the strongest of the blood vessels, owing to the presence of elastic tissue in their walls. They are red in colour and carry pure blood away from the heart. They branch to form arterioles and finally capillaries.

2. Capillaries : They are the result of the final branching of the arterioles. They are made of a thin layer of endothelial cells through which fluids and gases can pass to and from the tissue cells of the body.

3. Veins : They are not as strong as arteries due to less of elastic tissue in their walls. They are formed by joining of capillaries. They are bluish in colour. They carry impure blood back to heart. Blood coming from the digestive system also contains nutrients obtained by digestion of food.

Blood is circulated in a continuously repeated cycle by the contraction of the heart. Heart rate in a normal adult at rest is 72 times per minute. Each time the heart muscle contracts, blood is forced out of the right ventricle into the pulmonary arteries for perfusion of the lungs, and from the left ventricle into the aorta to perfuse the various parts of the body. During relaxation phase of the heart, deoxygenated blood collects in the right atrium from the inferior and superior vena cava, and oxygenated blood in the left atrium from the pulmonary veins.

Then it passes to the ventricles of the respective sides. Biueness (cyanosis) arises when the blood is low in oxygen. Normal human body contains about five litres of blood. Blood cells are formed by liver in fetal life and by bone marrow thereafter. There are red blood cells, white blood cells, and platelets. There are 5-8 millions red blood cells/mm3. They contain hemoglobin, with the help of which they carry oxygen from lungs to various parts of the body and carbon dioxide from various parts of the body to the lungs. There are 5000-8000 white blood cells/ mm3. They serve to protect the body from infections. Platelets help in clotting of blood. Biood also contains some clotting factors that are responsible for clotting of blood.


The respiratory system is concerned with breathing for exchange of carbon dioxide from the body with oxygen in the air. Air is a mixture of gases containing 21% oxygen. Oxygen is essential for life. The aim of breathing is to transfer oxygen from the air to the lungs where it is exchanged for carbon dioxide in blood. The oxygen is then circulated to the body, while the carbon dioxide is expelled out by expiration. Breathing is an automatic function. It is divided into three phases.

1. Inspiration : breathing in.
2. Expiration : breathing out.
3. Pause.

Structure – Features

Nose – Outermost primary way for air to enter and leave the system.

Mouth – Outermost secondary way for air to enter and leave the system.

Larynx – Connects the upper airway to the trachea.

Trachea – A cartilaginous tube made of many incomplete rings. It is a passage way for air flowing from larynx to the bronchi.

Bronchi – They form by branching of the trachea. Final small branches are bronchioles, to which alveoli are attached.

Lungs – Elastic organs, one on each side of the heart, containing microscopic air sacs called alveoli, where exchange of oxygen and carbon dioxide takes place between blood in alveolar capillaries and air in the alveoli.

There are muscles between the ribs that form the thoracic cage. There is a strong muscular partition between the chest cavity and the abdominal cavity called diaphragm. When one breathes in the chest muscles pull the ribs upwards while the diaphragm moves downwards. Thus the thoracic cavity expands, the pressure within it decreases, and air is drawn into the lungs. When these muscles relax, the ribs descend, the diaphragm rises to its resting level, and the thoracic cavity reduces in size. Then the air in the lungs is expelled. A short pause follows before the cycle starts all over again. Some residual air is left in the lungs so that circulating blood always has some oxygen available. Respiratory centre in the brain stem determines the rate and depth of breathing. Respiratory rate in an adults is 16-18/minute, while that in infants and children is 20-30/minutes. The rate increases during stress, exercise, injury or illness.


It consists of two kidneys, two ureters, a urinary bladder and a urethra. Kidneys form urine by filtration of waste products and other harmful substances not required in the body. The ureters conduct the urine to the urinary bladder, which is an expansible muscular bag for collecting urine. When it fills to about 200 to 250 ml, one gets a sensation of full bladder, and then expels the urine by voluntary contraction of the bladder muscle.


The skeletal system consists of 206 bones joined together by ligaments, cartilages and muscles. Its various parts are as follows.

1. Skull : It is made up of many flat bones jointed together so that no movement is allowed in between the bones. It holds the brain. Eyes are located in bony cavities on the front of skull. The nose is made up of small bones attached to skull.

2. Spine : It consists of 33 bones. There are seven in neck (cervical), twelve in chest back (thoracic), five in lumbar region, five in sacrum, and four in the coccyx. These are small bones with central cavities which joined end to end form a canal that contains the spinal cord.

3. Thorax : It is made of the thoracic vertebrae behind, sternum in front, and twelve ribs on the sides. It protects the heart and lungs.

4. Scapulae : These are two flat triangular bones on the back that connect the upper limbs to the thorax.

5. Upper Limbs : Each upper limb consists of a long bone called humerus in arm, two long bones called radius and ulna in the forearm, and many small bones in the wrist and hands.

6. Lower limbs : Each lower limb consists of a long bone called femur in thigh, two long bones called tibia and fibula in the leg, and many small bones in the ankle and foot.

7. Pelvis : It is made of the sacrum behind, and one inominate bone on either side. The lower limbs are attached to the pelvis. The pelvis contains urinary bladder, terminal parts of large intestines and rectum, the prostate in male and uterus in female.

The main functions of the skeletal system are as follows.

1. Supporting framework for soft tissues of the body.
2. Protecting vital organs like brain, heart, lungs, and abdominal organs.
3. Permitting movements by functioning as levers at the joints.
4. Formation of blood cells.


There are more than 600 muscles in the human body. All muscles are divided into three types.

1. Striated or skeletal muscles : These are attached to some part of skeleton across joints between bones. Their contraction and relaxation produce voluntary movements.

2. Smooth muscles : These are small and delicate. They are found in the walls of bowel, respiratory tract and blood vessels. They are known as involuntary because one does not have direct control over their activity.

3. Cardiac muscle : Its fibers show some striations under a microscope, but it is involuntary in nature.


Our digestive system consists of the digestive tract and various glands that secrete digestive juices into the tract. The tract includes mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum and anus. The accessory glands include the salivary glands, liver, gall bladder, and pancreas. Teeth are used to tear the food to pieces and chew (masticate) it. Saliva is produced by salivary glands. Dry food is mixed with saliva to moisten it. Ptylin in the saliva digests some carbohydrates.

Food reaches the stomach through the esophagus. There it mixes with rennin, pepsin, and hydrochloric acid, which are digestive juices. Then it passes into the small intestine, where it mixes with pancreatic juice and bile, which digest the food further. Products of digestion are absorbed in the small intestine. Undigested food passes into the large intestine. Water is absorbed from it there, and the residue is thrown out of the rectum and anus as feces.


The function of the excretory system is excretion of waste products present in the body. The following organs perform this function.

1. Skin : perspiration.
2. Lungs : through respiration.
3. The urinary system : as discussed before.


Testes. They are situated in a sac of skin between the legs, called the scrotum. They produce the spermatozoa, or male sex cells, from the time of puberty. They also produce a hormone called testosterone, which maintains male sexual function.

Vasa deferentia. They carry sperm from testes to the seminal vesicles.

Seminal vesicles. They function as reservoir for sperm, and also produce a secretion required for survival of sperm.

Prostate. It produces a secretion required for survival of sperm.

Ejaculatory ducts. They carry semen into the urethra. Semen is a mixture of sperm, and secretions of seminal vesicles and prostate.

Penis. It is the external genital organ for copulation. The urethra passes through it to open at its tip.

Female Reproductive System

Fallopian tubes. They collect ova and transmit them to the uterus. They also permit transfer of sperm to the ova for fertilization.

Ovary. It produces ova, the female sex cells.

Uterus. It is lined by a special epithelium called endometrium, which produces menstruation every month if the woman does not get pregnant in that cycle, and permits implantation of the fertilized ovum and its growth if she gets pregnant.

Vagina. It is the lower end of the female genital tract. It is the organ of copulation.

Vulva. It is the external genitals of a woman. It includes the clitoris, labia minora, and labia majora.


Endocrine glands are ductless glands. Their secretions called hormones are poured directly into the blood stream. The blood carries them to distant sites for action. The hormones regulate a number of physiologic and metabolic functions of the body. Various endocrine glands and their secretions are as follows.

Gland – Hormones

Pituitary gland – Growth hormone, thyroid stimulating hormone, adrenocorticotropic hormone, prolactin, follicle stimulating hormone, luteinizing hormone, antidiuretic hormone and oxytocin.

Thyroid gland – Thyroxine, triiodothyronine.

Parathyroid glands – Parathormone.

Adrenal glands – Glucocorticoids, mineralocorticoids, sex steroids, adrenaline, noradrenaline.

Pancreas – Insulin, glucagons.

Testes – Testosterone.

Ovaries – Estrogen, progesterone.

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