(a) Describe the process of osmo regulation in (I) a named unicellular organism (ii) man (b) Describe how the mammalian skin can react to regulate the body ...

(a) Osmoregulation is the control of the water and solute (salt) balance of an organism so that its internal osmotic pressure is kept steady.

(i) In a named unicellular organism (Amoeba / Paramecium)

Amoeba lives in fresh water. The surrounding water is more dilute (hypotonic) than the cell contents, so water continuously enters the cell by osmosis across the cell membrane. To avoid bursting, the excess water collects in a contractile vacuole. The vacuole gradually fills, moves to the surface and discharges the water to the outside (osmoregulation by active removal of water). This is repeated continually, using energy (ATP).

(ii) In man

Osmoregulation is carried out mainly by the kidneys under the control of antidiuretic hormone (ADH) from the pituitary gland.

• When the blood is too concentrated (little water), the pituitary releases more ADH, which makes the kidney tubules (nephrons) reabsorb more water back into the blood, so a small volume of concentrated urine is produced and water is conserved.
• When the blood is too dilute (excess water), less ADH is released, less water is reabsorbed, and a large volume of dilute urine is produced.
• Water is also lost through sweating and in expired air, and gained by drinking and from food; the kidney adjusts urine output to keep the balance.

(b) How mammalian skin regulates body temperature in a hot room

In a hot environment the skin acts to lose heat and cool the body:

• Vasodilation: the blood vessels (arterioles) supplying the skin surface widen, so more warm blood flows near the surface and heat is lost by radiation and convection, giving the skin a flushed appearance.
• Sweating: the sweat glands become active and pour out sweat onto the skin surface; as the sweat evaporates it takes latent heat from the body, cooling it.
• Hair lies flat: the erector (hair) muscles relax so the hairs lie flat, trapping less air, allowing heat to escape.
• Reduced metabolic heat: the rate of metabolism (heat production) is lowered.