The Human Body: Organ Systems, Glands and Bones

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Digestive system

An adult has 32 teeth (8 incisors, 4 canines, 8 premolars and 12 molars) and a child 20 milk teeth; tooth enamel is the hardest substance in the body. In the mouth, salivary amylase (ptyalin) begins turning starch into sugar. In the stomach, hydrochloric acid kills germs and provides the acidic medium for pepsin, which digests proteins, while mucus protects the stomach lining. The liver, the largest gland, makes bile, which is stored in the gall bladder; bile has no enzyme but emulsifies fats. Pancreatic juice adds trypsin for proteins and lipase for fats. Digestion is completed in the small intestine, the longest part of the alimentary canal, whose villi absorb the digested food; the large intestine absorbs water. Humans cannot digest cellulose.

Respiratory system

Air passes through the nose, pharynx, larynx (voice box) and trachea (windpipe, held open by rings of cartilage) to the bronchi, bronchioles and alveoli, tiny air sacs where oxygen enters the blood and carbon dioxide leaves it. When the diaphragm contracts and flattens, the chest expands and air rushes in. An adult at rest breathes about 15 to 18 times a minute. In the mitochondria of cells, aerobic respiration breaks glucose into carbon dioxide and water and releases energy; muscles short of oxygen during hard exercise respire anaerobically and form lactic acid, which NCERT links to muscle cramps.

Heart and blood

William Harvey described the circulation of blood in 1628. The heart has 4 chambers, two atria and two ventricles; the left ventricle has the thickest wall because it pumps blood to the whole body, and blood passes through the heart twice in one round (double circulation). Arteries carry blood away from the heart and veins towards it; the exceptions are the pulmonary artery, which carries deoxygenated blood, and the pulmonary vein, which carries oxygenated blood. The aorta is the largest artery. The heart beats about 72 times a minute, normal blood pressure is about 120/80 mm Hg (measured with a sphygmomanometer) and an adult has about 5 to 6 litres of blood. Red blood cells carry oxygen with iron-containing haemoglobin, are made in the bone marrow, live about 120 days and are destroyed in the spleen; white blood cells fight infection; platelets help blood to clot.

Blood groups

Karl Landsteiner discovered the ABO blood groups in 1900. Group A has antigen A on its red cells and anti-B antibodies in its plasma, group B the reverse, group AB both antigens and no antibodies (universal recipient) and group O no antigens but both antibodies (universal donor). The Rh factor, named after the rhesus monkey, was discovered by Landsteiner and Wiener in 1940; an Rh-negative mother can form antibodies that harm a later Rh-positive baby.

Excretory system

The kidneys filter the blood and remove urea, which the liver makes from excess amino acids. The nephron is the structural and functional unit of the kidney; filtration happens at the glomerulus inside Bowman's capsule. Urine passes through the ureters to the urinary bladder and out through the urethra. When the kidneys fail, blood is cleaned by dialysis. The skin and lungs also help in excretion.

Nervous system

The neuron, the longest cell in the body, is the unit of the nervous system. The brain and spinal cord form the central nervous system; 12 pairs of cranial nerves and 31 pairs of spinal nerves form the peripheral system. The cerebrum, the largest part of the brain, controls thinking, memory and voluntary actions; the hypothalamus regulates body temperature, hunger and thirst; the cerebellum maintains balance and posture; the medulla oblongata controls involuntary actions such as heartbeat, breathing and blood pressure. Reflex actions, such as pulling the hand away from a flame or the knee jerk, are controlled by the spinal cord.

Endocrine glands and hormones

Endocrine glands are ductless and release hormones straight into the blood. The pituitary, controlled by the hypothalamus, is the master gland; excess growth hormone causes gigantism and its deficiency dwarfism. The thyroid, the largest endocrine gland, makes thyroxine, which needs iodine and controls metabolism; iodine deficiency causes goitre. The parathyroids regulate blood calcium. The adrenal glands, on top of the kidneys, release adrenaline, the fight-or-flight hormone. The pancreas is both an exocrine and an endocrine gland: its islets of Langerhans release insulin, which lowers blood sugar (its lack causes diabetes; Banting and Best isolated it in 1921), and glucagon, which raises it. The thymus aids immunity and the pineal gland makes melatonin.

Skeleton and classic one-liners

An adult has 206 bones, 80 in the axial skeleton and 126 in the appendicular skeleton; a newborn has more, which fuse during growth. The femur (thigh bone) is the longest and strongest bone and the stapes in the ear the smallest. The neck has 7 cervical vertebrae, as in a giraffe. Ball-and-socket joints are at the shoulder and hip, hinge joints at the knee and elbow, and a pivot joint where the neck joins the head. The skin is the largest organ, the liver the largest internal organ and the ovum the largest human cell; the cornea has no blood supply and is the part transplanted after eye donation.

Key facts

Teeth: adult 32 (incisors 8, canines 4, premolars 8, molars 12), milk teeth 20; enamel is the hardest substance in the body.
Digestion: ptyalin (starch) in the mouth; pepsin and HCl in the stomach; bile (no enzyme, emulsifies fat) from the liver; trypsin and lipase from the pancreas; villi absorb food.
Gas exchange happens in the alveoli; the larynx is the voice box; an adult at rest breathes 15-18 times a minute; anaerobic respiration in muscles makes lactic acid, which NCERT links to cramps.
Heart: 4 chambers, thickest wall in the left ventricle; the pulmonary artery carries deoxygenated blood and the pulmonary vein oxygenated blood; normal BP 120/80 mm Hg.
RBCs carry oxygen (haemoglobin, about 120 days, made in bone marrow, destroyed in the spleen), WBCs fight infection, platelets clot blood; ABO groups by Landsteiner (1900), Rh factor 1940.
Universal donor: O (no antigens); universal recipient: AB (no antibodies).
The nephron is the unit of the kidney; urea is made in the liver and removed by the kidneys; dialysis cleans blood when kidneys fail.
Cerebrum: thinking and memory; cerebellum: balance; medulla: heartbeat and breathing; hypothalamus: temperature, hunger and thirst; reflexes run through the spinal cord.
Pituitary: master gland (growth hormone); thyroid: thyroxine, iodine, goitre; adrenal: adrenaline (fight or flight); pancreas: insulin lowers blood sugar (Banting and Best, 1921).
206 bones in adults (80 axial + 126 appendicular); femur longest, stapes smallest; skin largest organ, liver largest internal organ and gland, thyroid largest endocrine gland.

Quick revision

32 adult teeth; enamel hardest; ptyalin starch, pepsin protein, bile emulsifies fat; liver largest gland; villi absorb; alveoli exchange gases; breathing 15-18 per minute; heart 4 chambers, 72 beats per minute, BP 120/80; RBC 120 days, spleen graveyard; Landsteiner ABO 1900; O universal donor, AB universal recipient; nephron unit of kidney; 12 cranial, 31 spinal nerve pairs; cerebellum balance, medulla heartbeat; pituitary master gland; insulin 1921; 206 bones; femur longest, stapes smallest.

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