Dear all,

Sorry for taking so long to post the answer, had some problem to log in yesterday.
:-) Here's the answer:

Describe how the following help to maintain a constant internal environment.
(a) the skin, [7]
(b) the kidneys. [3]
(N2003 2B 8a, bOr)

Examiner Report:
(a) Candidates omitted to say that its specific role is that of temperature regulation.
Misconception: 1: capillaries are capable of constriction.
2: heat is lost from the skin surface only when the body
temperature rises above normal.

• Arterioles are capable of constriction (vasoconstriction)
• Heat is lost from the skin surface all the time and more heat lose when the body temperature rises above normal.

(a)
Skin regulates the temperature of the body.
When body temperature rises above normal, temperature receptors in skin detect change and send nerve impulse to brain.
Hypothalamus of brain is stimulated and sends nerve impulses to relevant body parts.
Arterioles in the skin undergo vasodilation to increase blood flow to the skin.
Skin receives more heat and this is lost to the environment by radiation, convection and conduction.
Sweat gland more active, increase sweat production, a large amount of heat is lost as latent heat of vaporization.
Hair erector muscles in skin relax. OR
Metabolic rate decrease, decreases heat production.
Blood temperature decreases to normal
When body temperature decreases below normal, temperature receptors in skin detect change and send nerve impulse to brain.
Hypothalamus of brain is stimulated and sends nerve impulses to relevant body parts.
Arterioles in the skin undergo vasoconstriction to decrease blood flow to the skin.
Decrease blood flow to the skin and reduces the loss of heat by radiation, convection and conduction.
Sweat gland becomes less active and produces less sweat, reducing heat loss.
The hair erector muscles contract and make the hair erect and in order to trap the air around the skin as air is a poor conductor of heat.
Under normal circumstances when there is no excessive heat loss or heat gain, the skin continue to maintain the temperature by constantly but gradually losing heat to the environment as heat is constantly generated by the body.

(b)
Kidneys regulate the water potential in the blood. 1
When water potentials of blood increase, hypothalamus is stimulated to produce less ADH ½
Less water will be reabsorbed and more water excreted in urine ½
When water potentials of blood decrease, hypothalamus is stimulated to produce more ADH ½
More water will be reabsorbed and less water excreted in urine ½