Important points on human body for science section of competitive exams


Blood contained in blood-vessels is a connective tissue in the form of a red viscous fluid.

Quantity: On an average, a healthy man has about 5 litres of blood in the body, while a woman has about 500 ml less than man. Total volume of blood is said to be 60-80m1 per kg of body weight.

Constituents: There are two main constituents of blood, viz., solid or cellular part called blood cells (35%) and fluid or humoral part called plasma (65%). The blood cells called corpuscles are of three types.

Red Blood Corpuscles (RBC): The RBCs, which are called Erythrocytes are disc- shaped cells concave in the middle and visible under a microscope. They have no nucleus and contain a pigment called Haemoglobin which gives blood its red colour.

RBC’s are produced in the spleen and the bone marrow at the rate of 1.2 million corpuscles per second. In a lifetime the bone marrow creates about half a tones of RBC. Life of the RBC is about 100-120 days.

Function: Haemoglobin in the RBC picks up oxygen in the lung tissues by forming a chemical compound with it. The oxygen is carried to the tissues where it is used in chemical reactions with the products of digestion in order to produce energy. It then combines with carbon dioxide which is produced in these reactions and returns to the lungs via the heart where the cycle starts again.

White Blood Corpuscles (WBC): The WBCs are the ‘soldiers’ of the body’s defence system. They are round, semi-transparent cells contain a nucleus and visible only under a microscope. They are a little larger than RBCs.

Functions: Broadly the WBCs act as a defence system in the body.

Blood Platelets (Thrombocytes): are tiny, circular or oval colourless cells, a quarter of the size of WBC, which clump together to release thromboplasm. This is a substance which sets off blood clotting, a mechanism that seals tears in blood vessels. Blood platelets live only for a few hours.


Plasma- Plasma is the watery part of the blood making up to 50-60% of the total. It is a clear, yellow fluid which transports blood cells. Plasma contains about 90% water, protein and inorganic salts.

Rh factor- A blood antigen discovered in 1940 by Landsteiner and A.S wiener. It is another blood grouping which has important bearing on blood transfusion along with the A, AB and 0 blood grouping. The Rh factor is an agglutinate found in RBC of most people and is called Re. It was initially found in the rhesus monkey and later on found in man. People who do not have this antigen in their blood are called Rh-. The Rh- blood does not carry anti-Rh antibodies naturally but could synthesize them if sensitized through blood transfusion of Rh+ blood.


The Skeletal System (Bones)


  • There are 206 bones in the skeletal system of an adult. Bones of hands and feet alone constitute 50% of the total bones in the human body.
  • Bones account for about 1/5th of the body’s total weight.
  • A new born baby has 300 bones. Out of which 94 bones fuse together as it grows.
  • The largest bone of human body is the femur in the thigh which constitutes about 27.5% of a person’s stature. The average length of this bone is about 50 cm.
  • The shortest bone in the human body is stapes or stirrup bone in the middle ear; the hammer, the anvil and the stirrup.


  • They are connective tissues in the body
  • They stiffen the body
  • They provide levers upon which muscles work
  • They give shape to the body.
  • They protect internal organs
  • The bone marrow produces blood cells (see details on ‘Blood’).


Muscular System


Muscles are tissues attached to bones; are composed of fibres; are capable of contracting and relaxing to effect body movement.


Total number of muscles: There are about 630 important muscles in the human body which normally account for 40% of the body weight. There are about 100 joints and about 10,000 km of blood vessels in the human body. Water constitutes about 70% of human body tissues (45 litres).


Largest muscle: The bulkiest muscle in the body is Gluteus maximum or buttock muscles.


Longest muscle: Sartorius (Tailor’s muscles) is the longest muscle in the body. It has origin in the upper part of the hip bone, crosses obliquely over the upper portion of the thigh, passes behind the femur and is attached to the tibia bone of the leg.


Smallest muscle: The smallest muscle in the human body is the Stapedius which controls the stapes, an auditory ossicle in the middle ear. It is less than 1/20 of an inch and 0.127 cm long. The ear also contains one of the few tissues which have no blood supply.


Main Organs


Heart: The heart is situated on the left side beneath the left nipple and is enclosed in a tough fibrous wrapping called the pericardium. Its average weight is about 340 grams in men and 255 grams in women. The heart consists of two halves, the left half and the right half divided by a wall called septum. Each half, in turn, is divided into an upper chamber called auricle and a lower chamber called ventricle. The upper half (auricle) receives blood from veins and lower half (ventricle) pumps blood into the arteries.

Heartbeats In a normal, healthy adult contraction and relaxation mechanism of the ventricles results in a heart beat of about 70-72/ min in males and 78/82/min in females. Contraction of ventricles is called systole and relaxation is called diastole. The hearts beats more than 2,000 million times and pumps more than 500 million litres of blood during the lifespan of a person.


Lungs: A pair of spongy organs consisting of elastic tissues situated in the chest cavity and separated from each other by the heart and other contents of the mediastinum.

The right lung is larger than the left lung. The right lung weighs approximately 620g while the left lung weighs about 570g and together they weigh between 1.18-1.19 kg, in a healthy adult.



Structure: The liver, situated on the right side of the stomach is the largest gland in the human body. It is dark brown in colour and divided into two lobes varying in weight from 1.359 g to 1.812 g. The gall bladder is attached to the liver and stores bile produced by it. The gall bladder has the storage capacity of 30-60 ml of bile.

Functions: The liver secretes bile, forms and stores glycogen and plays as important part in metabolism of protein and fats. The liver is responsible for:

  • The metabolism of the products of digestion
  • The storage and release of substances (principally glucose) so as to maintain constant level in the blood
  • The synthesis, conjugation and transformation of substances (e.g., formation of proteins, detoxification of poisonous substances, production of carbohydrates from proteins, etc.)


Kidneys: The two kidneys are situated in the upper posterior abdominal cavity, one on each side of the vertebral column. A kidney is approximately 10 cm long, 5 cm wide and 2.5 cm thick. From each kidney emerges a long channel called the ureter by which the urine passes into the urinary bladder. Each kidney has thousands of minute glands in the form of canals known as uriniferous tubules.

Functions: The filter nitrogenous waste of the body from the blood and throw them out in the form of urine. Kidneys are responsible for the removal of excess water, salts and waste products from the blood and maintaining its pH level (pH value is a number used to express degrees of acidity and alkalinity in a solution).


Human Brain- Human brain consists of two parts, viz., the brain lodged in the brain case (skull) and the spinal cord lodged in the vertebral column.

The weight of the average human brain triples between birth and adulthood.

The final weight of the brain in an adult male is about 1.4kg, and 1.3 kg in the case of a woman; which averages about 3% of body weight of a normal person.

In both sexes the brain makes up a similar proportion of the total body weight.

• The brain uses about 20% of the oxygen a man breathes, 20% of calories a man takes in and about 15 % of body blood.

• The average brain contains about 10,000 million neurons- microscopic nerves cells.


Central nervous system: The brain and spinal chord along with nerves constitute the nervous system. The brain consists of:

  • Cerebrum- The largest part of the brain consists of two hemispheres separated by corpus callosum, a deep median furrow. It controls voluntary actions and is the seat of intelligence, memory association, imagination and will.
  • Cerebellum- The large mass having ridges and furrows, situated above and behind the medulla and attached to cerebrum. It regulates muscular movement of locomotion.
  • Medulla oblongata — It is the lowermost part of the brain which continues as the spinal chord in the vertebral column. It controls involuntary action.
  • Spinal chord is the elongated, nearly cylindrical continuation of the medulla. It is enclosed in vertebrae and runs down the back. The length of the spinal cord in an average man is about one metre (3.3 ft).
  • Nerves are whitish cords consisting of large numbers if exceedingly fine filaments (nerve fibres) of variable diameter, bound together in bundles by fibrous tissues.


Digestive system


The process of converting food into energy-giving substances is carried out by the digestive. The digestive system comprises the alimentary canal and the associated digestive glands like liver and pancreas.

Alimentary canal: The entire tube-like structure starting from mouth to the anus is called alimentary canal. It includes various digestive organs, e.g., mouth, gullet, oesophagus, stomach, small and large intestine, rectum and anus.


Digestive organs: Teeth The teeth cut and grind the food with the help of saliva secreted by the mouth. Milk teeth- Appear by the age 7 months and are 20 in number.

Adult teeth- Milk teeth drop and 32 permanent teeth begin to replace them from the 6’h year onwards. There are four chisel-shaped incisors, two conical premolars and six molars for crushing, and grinding the food. The two wisdom teeth, the rearmost, develop at a later stage and are largely non-functional.

In the mouth salivary glands secrete saliva which moistens the food and ptyalin ferment contained in it acts on carbohydrates.


Gullet- Also called oesophagus, is a 25 cm long duct system covering mouth and the food pipe pharynx. The food is carded to the stomach thereon by peristalsis movement (contraction of wall of the pipe).


Stomach: A warehouse where food can be stored to await the main process of digestion. Gastric juices produced in the stomach help in digestion of food.


Small intestine- Measuring 6-7 m long and 2 cm in diameter is arranged in coils. Here the food from the stomach is mixed with bile and pancreatic juice and moves forward through peristalsis movement. Part of the intestine near the stomach is called the duodenum where bile and pancreatic juice digest the food.


Large intestine- A 1.4m long tube called colon. It receives undigested material from the small intestine and absorbs water. The remaining waste material is sent to the rectum and discharged from the anus.

Process of digestion- Chewed food is converted into food pulp called chyme. After being processed by bile and other secretions it becomes an emulsion called chyle. The end product is absorbed through the walls of the small intestines and taken into the blood. The undigested waste part is excreted as faeces.



The Reproductive Organs

A human reproductive organs or sex organ, or primary sexual characteristic, narrowly defined, is any of those parts of the body (which are not always bodily organs according to the strict definition) which are involved in sexual reproduction and constitute the reproductive system in an complex organism; namely:

Female- Vulva (notably the clitoris), vagina (notably the cervix), labia, uterus, Fallopian tubes, ovaries, Skene’s glands, Bartholin’s glands.

Male- Penis (notably the glans penis and foreskin), testicles, scrotum, prostate, seminal vesicles, epididymis, Cowper’s glands.

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