Human Body Systems Notes for Competitive Exams

The human body is a wonderful machine in which many organ systems work together to keep us alive and healthy. Questions on human body systems, organs, hormones and blood are asked in SSC, railways, banking, defence, state-level examinations and the UPSC Prelims, as part of the biology section of general science. This note explains the major systems: digestive, respiratory, circulatory, excretory, nervous, endocrine, skeletal and muscular, with the important organs, enzymes, hormones and facts in simple language.

Digestive System

Digestion is the breakdown of complex food into simple, absorbable substances. The alimentary canal consists of the mouth, pharynx, oesophagus, stomach, small intestine, large intestine and anus. In the mouth, teeth chew food and the saliva (from salivary glands) contains the enzyme salivary amylase (ptyalin), which starts the digestion of starch into maltose. In the stomach, gastric glands release hydrochloric acid (which kills germs and makes the medium acidic), pepsin (which digests proteins) and mucus (which protects the stomach wall). In the small intestine, which is about 6 to 7 metres long, the digestion is completed with the help of bile, pancreatic juice and intestinal juice, and the digested food is absorbed through finger-like projections called villi. The large intestine absorbs water and forms faeces.

Carbohydrates are digested into glucose, proteins into amino acids, and fats into fatty acids and glycerol. The liver is the largest gland and the second largest organ after the skin (the skin is the largest organ). The liver can regenerate itself after damage. Jaundice affects the liver, and gallstones form in the gall bladder.

Respiratory System

Respiration provides oxygen to the body and removes carbon dioxide. Air enters through the nose, passes through the pharynx, larynx (voice box), trachea (windpipe), bronchi and bronchioles and reaches the tiny air sacs called alveoli in the lungs, where gases are exchanged with the blood. The diaphragm, a muscle below the lungs, and the rib muscles help in breathing: when we inhale, the diaphragm moves down and the ribs move up and out; when we exhale, the reverse happens. The normal breathing rate of an adult at rest is about 12 to 20 breaths per minute. The right lung is slightly larger than the left, which has a notch for the heart. Oxygen is carried by haemoglobin in the red blood cells. Cellular respiration takes place in the mitochondria, where glucose is broken down to release energy in the form of ATP; the mitochondria are called the powerhouse of the cell.

Circulatory System

The circulatory system carries oxygen, nutrients, hormones and wastes. It consists of the heart, blood vessels and blood. The human heart has four chambers: two upper atria and two lower ventricles. The right atrium receives deoxygenated blood from the body, which passes to the right ventricle and goes to the lungs through the pulmonary artery. The left atrium receives oxygenated blood from the lungs via the pulmonary veins, which passes to the left ventricle and is pumped to the whole body through the aorta, the largest artery. This is called double circulation. Arteries carry blood away from the heart (mostly oxygenated, except the pulmonary artery), veins carry blood towards the heart (mostly deoxygenated, except the pulmonary veins), and capillaries are tiny vessels where exchange takes place. The normal heart rate is about 72 beats per minute, and normal blood pressure is about 120/80 mm Hg, measured with a sphygmomanometer. The heart's natural pacemaker is the sinoatrial (SA) node.

Blood and Blood Groups

Blood is a fluid connective tissue. It has plasma (about 55 percent, mostly water), red blood cells (RBCs or erythrocytes) that carry oxygen using haemoglobin and live for about 120 days, white blood cells (WBCs or leucocytes) that fight infection, and platelets (thrombocytes) that help the blood to clot. RBCs are made in the bone marrow (and in the liver and spleen before birth) and destroyed in the spleen, which is called the graveyard of RBCs. Anaemia is a deficiency of haemoglobin, usually due to lack of iron. The ABO blood group system was discovered by Karl Landsteiner in 1901. The groups are A, B, AB and O, and each can be Rh positive or negative. Group O negative is the universal donor, and group AB positive is the universal recipient. Blood pressure can be high (hypertension) or low (hypotension). An average adult has about 5 to 6 litres of blood.

Excretory System

The excretory system removes waste products like urea. The main organs are two kidneys, two ureters, the urinary bladder and the urethra. The functional unit of the kidney is the nephron, in which blood is filtered in the glomerulus and useful substances are reabsorbed. Urine is mostly water with urea, uric acid and salts, and an adult passes about 1 to 1.5 litres a day. When kidneys fail, dialysis (artificial filtering of the blood) or a kidney transplant is needed. The skin removes salts and water in sweat, the lungs remove carbon dioxide, and the liver forms urea. Kidney stones are deposits of calcium oxalate and other salts.

Nervous System

The nervous system controls and coordinates the body. The neuron is the basic unit, with a cell body, dendrites (receive signals) and an axon (carries signals). The central nervous system (CNS) is the brain and spinal cord; the peripheral nervous system includes 12 pairs of cranial nerves and 31 pairs of spinal nerves. The adult brain weighs about 1.3 to 1.4 kg. Its parts are the cerebrum (thinking, memory, voluntary actions; the largest part), the cerebellum (balance and coordination), the medulla oblongata (involuntary actions like breathing, heartbeat and digestion) and the hypothalamus (body temperature, hunger, thirst). A reflex action is a quick, automatic response, like moving the hand away from a hot object, controlled by the spinal cord through the reflex arc. The autonomic nervous system controls involuntary activities and has sympathetic and parasympathetic divisions.

Endocrine System: Glands and Hormones

Skeletal and Muscular Systems

An adult human skeleton has 206 bones (a baby is born with about 270 that fuse later). The skull has 22 bones, the vertebral column has 33 vertebrae in childhood (26 in adults after fusion), and there are 12 pairs of ribs. The femur (thigh bone) is the longest and strongest bone, and the stapes in the middle ear is the smallest. Bones are joined by ligaments, and muscles are attached to bones by tendons. Joints are of several types: ball and socket (shoulder and hip), hinge (elbow and knee), pivot (neck), gliding (wrist) and fixed (skull). The human body has more than 600 muscles. Muscles are of three kinds: skeletal (voluntary, striped), smooth (involuntary, in the walls of organs) and cardiac (involuntary, in the heart). The gluteus maximus is the largest muscle and the muscle of the middle ear is the smallest. Calcium and vitamin D are needed for strong bones.

Sense Organs and Immunity

The eye sees, with the retina having rods (dim light) and cones (colour). The ear has three parts, outer, middle and inner, and the inner ear (the cochlea and the semicircular canals) helps in hearing and balance. The nose senses smell, the tongue senses taste (sweet, sour, salty, bitter and umami), and the skin senses touch, pressure, heat and pain. The immune system defends the body against germs through white blood cells, antibodies and organs like the thymus, spleen and lymph nodes. Vaccines train the immune system. The normal body temperature is about 37 degrees Celsius (98.6 degrees Fahrenheit).

Quick Revision

• Liver is the largest gland; skin is the largest organ; femur is the longest bone; stapes is the smallest bone.
• Heart has four chambers; aorta is the largest artery; O negative is the universal donor; AB positive is the universal recipient.
• Insulin is made by the pancreas; thyroxine by the thyroid; adrenaline by the adrenal; pituitary is the master gland.
• Adult skeleton has 206 bones; the brain has the cerebrum, cerebellum and medulla.

Make a chart of organs, functions and disorders, and revise it every week. Use your own body as an example while learning, and the facts will remain in your memory.

Related Notes to Read Next

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Frequently Asked Questions (FAQ)

Which is the largest gland in the human body?

The liver is the largest gland in the human body.

Which is the largest organ of the human body?

The skin is the largest organ of the human body.

How many bones are there in an adult human body?

An adult human body has 206 bones.

Which is the longest bone in the human body?

The femur (thigh bone) is the longest and strongest bone, and the stapes in the middle ear is the smallest.

What are the four chambers of the heart?

The heart has two upper chambers called atria and two lower chambers called ventricles.

Which blood group is the universal donor?

Blood group O negative is considered the universal donor, and AB positive is the universal recipient.

Who discovered the ABO blood groups?

Karl Landsteiner discovered the ABO blood groups in 1901.

Which hormone is produced by the pancreas to control blood sugar?

The pancreas produces insulin, which lowers blood sugar, and glucagon, which raises it.

Which gland is called the master gland?

The pituitary gland is called the master gland because it controls the activities of many other endocrine glands.

What is the functional unit of the kidney?

The nephron is the structural and functional unit of the kidney.

What does the cerebellum control?

The cerebellum controls balance and coordination of muscle movements.

Which enzyme in saliva begins the digestion of starch?

Salivary amylase (ptyalin) in saliva begins the digestion of starch into maltose.