Homeostasis: Class 10 Biology

Key points

Homeostasis is the maintenance of a steady internal environment in the body despite changes outside it. Body temperature, water content, salt level, blood glucose and pH are kept within narrow limits because enzymes and cells work properly only under such conditions. Most systems work by negative feedback: a change is detected, and the body responds in a way that reverses the change.

Osmoregulation is the control of water and salt balance in the body fluids. The water potential of body fluids must stay steady, otherwise cells shrink or swell. The kidneys, skin and lungs all help to control the balance between water gained and water lost.

Osmoregulation in plants is related to their habitat:

• Hydrophytes (water plants) live in water and have no problem getting water. They lose extra water through hydathodes and have few or no stomata on the lower surface.
• Mesophytes (plants of moderate conditions) control water loss by opening and closing stomata and by having a waxy cuticle.
• Xerophytes (desert plants) save water by having thick cuticle, sunken stomata, reduced or spiny leaves and deep or widespread roots.
• Halophytes (plants of salty soil) take in salt water and remove extra salt through salt glands or store it in vacuoles.

Osmoregulation in animals differs by environment. Freshwater animals take in water by osmosis and pass out a large volume of dilute urine. Marine bony fish lose water by osmosis, so they drink sea water and remove extra salt through the gills. Desert animals such as the camel save water by passing very concentrated urine and sweating little.

Excretion is the removal of metabolic waste products from the body. The main excretory organs of humans are the kidneys, which remove urea, extra water and salts. The lungs remove carbon dioxide and water vapour, and the skin removes water, salts and a little urea in sweat. Urea is made in the liver from the breakdown of excess amino acids.

The urinary system consists of two kidneys, two ureters, one urinary bladder and one urethra. Each kidney is bean shaped and lies on either side of the backbone below the ribs. In section it shows an outer cortex, an inner medulla and a central pelvis that leads into the ureter. Urine passes down the ureter to the bladder, where it is stored until it leaves through the urethra.

The nephron is the structural and functional unit of the kidney. Each kidney has about one million nephrons. Its parts are:

• Bowman's capsule: a cup shaped structure that surrounds a knot of capillaries called the glomerulus.
• Proximal convoluted tubule: a coiled tube that follows the capsule.
• Loop of Henle: a U shaped part that dips into the medulla.
• Distal convoluted tubule: a second coiled part in the cortex.
• Collecting duct: receives urine from many nephrons and carries it to the pelvis.

Urine is formed in three steps:

• Ultrafiltration: blood enters the glomerulus at high pressure. Water, glucose, amino acids, salts and urea pass into Bowman's capsule. Blood cells and large proteins stay in the blood.
• Selective reabsorption: the useful substances, all the glucose and amino acids and most of the water and salts, are taken back into the blood, mainly in the proximal tubule.
• Tubular secretion: some substances such as extra hydrogen ions and certain drugs pass from the blood into the tubule for removal.

Urine is about 95 percent water. It also contains urea, uric acid, creatinine and salts. The amount of water in urine is controlled by antidiuretic hormone (ADH) from the pituitary gland. When the body is short of water, more ADH is released, the collecting ducts reabsorb more water, and a small volume of concentrated urine is passed. When there is excess water, less ADH is released and dilute urine is passed in large amount.

Common kidney problems include kidney stones and kidney failure. Kidney stones form when salts such as calcium oxalate crystallise in the kidney, and they cause severe pain. Small stones pass out with urine, while larger ones are broken by ultrasound waves (lithotripsy) or removed by surgery. Kidney failure is treated by dialysis, in which an artificial kidney machine removes urea from the blood, or by a kidney transplant from a matching donor.

The skin is the largest organ of the body and helps in homeostasis. It has an outer epidermis and an inner dermis with sweat glands, sebaceous glands, hair follicles, blood vessels and nerve endings. It protects the body, senses touch, pain and temperature, makes vitamin D in sunlight, and controls body temperature.

Thermoregulation is the control of body temperature at about 37 °C, and it is directed by the hypothalamus in the brain:

• In hot conditions: sweat glands produce more sweat, whose evaporation cools the body. Arterioles near the skin widen (vasodilation), so more heat is lost from the skin.
• In cold conditions: sweating stops, arterioles in the skin narrow (vasoconstriction) to reduce heat loss, hair stands up to trap air, and shivering produces heat in the muscles.

Study tip: Practise labelling the nephron and a section of the kidney, since diagrams carry marks. Learn the three steps of urine formation in order, and write temperature control answers as a chain: stimulus, hypothalamus, response, result.

Definitions and flashcards

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Short questions with model answers

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Common mistakes

• Confusing excretion with egestion. Excretion removes metabolic wastes, while egestion removes undigested food.
• Saying that glucose is found in normal urine. It is fully reabsorbed, so its presence suggests diabetes.
• Mixing up vasodilation and vasoconstriction. Dilation increases heat loss, and constriction reduces it.
• Writing that blood cells pass through the glomerulus filter. Only small molecules are filtered.
• Forgetting that ADH increases water reabsorption, not water loss.

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