Human Body Important Facts for Competitive Exams

Every general science paper carries two or three questions on the human body, and almost all of them are facts rather than reasoning: the smallest bone, the largest gland, the life of a red blood cell, the part of the brain that keeps the body balanced. The facts do not change, which is what makes this chapter worth memorising once and for all. This note gathers them system by system, with the numbers and the superlatives placed where they are easy to revise the night before a paper.

Bones, muscles and teeth

An adult human skeleton has 206 bones, while a newborn has about 300; many of them fuse as the child grows. The femur or thigh bone is the longest and strongest bone, and the stapes in the middle ear is the smallest. The skull is made of 22 bones, the vertebral column of 33 vertebrae of which the lower ones fuse to leave 26 in an adult, and the chest of 12 pairs of ribs. The hyoid bone in the neck is the only bone that is not joined to any other bone. The skeleton carries about 640 skeletal muscles; the gluteus maximus of the hip is the largest muscle, the stapedius of the middle ear the smallest, the sartorius of the thigh the longest and the masseter of the jaw the strongest by the force it exerts. An adult has 32 permanent teeth and a child 20 milk teeth, and enamel is the hardest substance in the human body.

Blood and the circulatory system

Blood group O negative is the universal donor and AB positive the universal recipient. The pulmonary artery is the only artery carrying deoxygenated blood and the pulmonary vein the only vein carrying oxygenated blood. William Harvey described the circulation of blood, and blood pressure is measured with a sphygmomanometer while the heart's electrical activity is recorded by an electrocardiogram.

Respiration and digestion

The right lung has three lobes and the left only two, because the heart occupies space on the left. Exchange of gases takes place in the alveoli, the air sacs of the lungs, and breathing is driven by the diaphragm and the intercostal muscles; a resting adult breathes about twelve to sixteen times a minute. The alimentary canal is roughly nine metres long, of which the small intestine is the longest part at about six metres, and absorption takes place through its villi. Digestion of starch begins in the mouth with salivary amylase, also called ptyalin; the stomach secretes hydrochloric acid and the enzyme pepsin for proteins; the pancreas, which is both an exocrine and an endocrine gland, sends trypsin, amylase and lipase into the small intestine. The liver is the largest gland and the largest internal organ of the body; it secretes bile, which contains no digestive enzyme but emulsifies fat, and bile is stored in the gall bladder. The vermiform appendix is a vestigial organ.

The nervous system and the sense organs

The human brain weighs roughly 1,300 to 1,400 grams. The cerebrum is its largest part and the seat of thought, memory and voluntary action; the cerebellum maintains balance and posture; the medulla oblongata controls the involuntary actions of heartbeat, breathing, swallowing and vomiting; and the hypothalamus regulates body temperature, hunger, thirst and sleep. There are 12 pairs of cranial nerves and 31 pairs of spinal nerves; the sciatic nerve is the longest nerve and the nerve cell or neuron the longest cell in the body. In the eye the cornea does most of the bending of light and the lens focuses it on the retina, where the rods respond in dim light with the help of the pigment rhodopsin and the cones give colour vision. In the ear the cochlea is the organ of hearing and the semicircular canals the organ of balance; the three ear ossicles are the malleus, the incus and the stapes. The skin is the largest organ of the body and its pigment is melanin.

Glands, hormones and excretion

• The pituitary, a pea-sized gland at the base of the brain, is called the master gland because its secretions control other endocrine glands.
• The thyroid is the largest endocrine gland; it secretes thyroxine, for which iodine is needed, and its deficiency causes goitre in adults and cretinism in children.
• The parathyroid glands regulate calcium and phosphorus in the blood.
• The adrenal glands secrete adrenaline, called the emergency hormone or the fight-or-flight hormone, and cortisol.
• Insulin comes from the beta cells and glucagon from the alpha cells of the islets of Langerhans in the pancreas; a lack of insulin causes diabetes mellitus, while a lack of the antidiuretic hormone causes diabetes insipidus.
• The nephron is the structural and functional unit of the kidney, and each kidney has about a million of them. Urea is formed in the liver, not in the kidney, and urine is about 95 per cent water.

Numbers and superlatives worth memorising

• Normal body temperature is 37 degrees Celsius or 98.6 degrees Fahrenheit, and a human cell carries 46 chromosomes in 23 pairs.
• Largest organ: skin; largest gland and largest internal organ: liver; largest endocrine gland: thyroid.
• Largest cell: the ovum; smallest cell: the sperm; longest cell: the neuron.
• Longest bone: femur; smallest bone: stapes; hardest substance: enamel; only unjoined bone: hyoid.