Human Body Systems “Nervous system”, “Respiratory system”, “Excretory System” and their Functioning

Structure and Function of Human Body

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

What is the basic structure of the human body?

Smallest functioning unit of every living being is a cell. A group of cells together form a tissue. Various tissues form an organ. There may be a number of organs in a system that serves a specific purpose.

Term                                                 Meaning

Midline                                            It divides the body into right and left                                                                                      halves with a vertical line.

Lateral                                             Anything away from the midline is                                                                                          said to be a 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.


Nervous system

The basic unit of the nervous system is a nerve cell (neuron). Each nave 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 cell 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


  1. Central nervous system: brain and spinal cord. The system consists of the brain, spinal cord, and 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 gay 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.


  1. Peripheral nervous system: cranial and peripheral nerves. The nave fibers may be sensory, motor, mixed, 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

Circulatory system

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.

Blood Vessels are of three types

  1. Arteries: they are the strongest of the blood vessels, owing to the presence of elastic tissue in their walls.. They are red in color. niey 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 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 color. 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 an 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 a 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. Blueness (cyanosis) arises when the blood is low in oxygen. Normal human body contains about five liters 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. Blood also contains some clotting factors that are responsible for clotting of blood.

Respiratory system

The respiratory system is concerned with breathing for exchange of carbon dioxide from the body with oxygen in the air.

Structure                                                    Features

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

Mouth        Outermost secondary way for air to 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 from 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

Alveoli      Exchange of oxygen and carbon dioxide takes place between blood in alveolar capillaries and air in the alveoli.


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.

l .Inspiration: breathing in.

  1. Expiration: breathing out
  2. Pause.

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 diaphragrn. 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 diaphragrn 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 adult is 16-18/minute, while that in infants and children is 20-30/minute. It rate increases during stress, exercise, injury or illness.

Urinary System

It consists of two kidneys, two ureters, a urinary bladder and a urethra. Kidney& 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 by ligaments, cartilages and muscles. Its various parts are as follows.

  1. Skull: it is made up of many flat bones joined 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. %ese 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.

l . Supporting framework for soft tissues of the body.

  1. Protecting vital organs like brain, heart, lungs, and abdominal organs.
  2. Permitting movements by functioning as levers at the joints.
  3. Formation of blood cells.

The muscular system

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.

The Digestive System

The 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. The 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 Excretory System

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

2.Skin: perspiration.

3.Lungs: through respiration

4.The urinary system: as discussed before.

The Reproductive System

Reproductive System


Tastes         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                     They carry sperm form testes to the seminal vesicles.

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

Vesicles     secretion required for survival of sperm.

Prostate      it produces a secretion required for survival of sperm

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

Ducts 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.


Fallopian tubes-           they collect ova and transmit them to the uterus. They also permit transfer of sperm of 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 the she gets pregnant.

Vagina                          it is the lower end 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 system

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 secrtions are as follows.

Gland                                                              Hormones

Pituitary gland         growth hormone, thyroid stimulating hormone, adrenocorticotropic hormone, prolactin, follicle stimulating hormone, luteinzing hormone, antidiuretic hormone, and oxytocin.

Thyroid gland          Thyroxine, triiodothyronine.

Parathyroid              Parathormone.


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

Pancreas                   Insulin, glucagons.

Testes                       Testosterone.

Ovaries                     Estrogen, progesterone.


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