Wednesday, 8 April 2009

iGCSE Biology - Homeostasis and Excretion


  • Homeostasis is the process by which the conditions in our body are kept constant.
  • Homeostasis is important because many bodily functions need precise conditions to occur successfully.
  • Urine contains:



               Sodium chloride



  • Each kidney is supplied blood through a renal artery.
  • The filtered blood leaves the kidneys through a renal vein.
  • The urine leaves the kidneys through the ureters and is then stored in the bladder.
  • The bladder has a tube leading to the outside called the urethra.
  • The urine is kept in the bladder due to two sphincter muscles on the wall of the urethra.
  • Structure of a kidney:

                     Innermost core: pelvis

               Middle layer: medulla

               Outer layer: cortex

  • Most of a nephron is contained in the cortex, but the loop of Henlé extends into the medulla.
  • Structure of a nephron:

At the start of a nephron is the Bowman’s capsule, which surrounds a ball of capillaries called the glomerulus.

High pressure in the glomerulus forces blood into the Bowman’s capsule.

A basement membrane in between the capillaries and the Bowman’s capsule filters the blood. Thus, only glomerular filtrate enters the Bowman’s capsule.

There are two coils in the nephron, separated by the loop of Henlé. It is here that all the “good” parts of the filtrate are reabsorbed into the bloodstream.

After the second coiled tubule the nephron joins a collecting duct which is connected to the pelvis.

·        The amount of water in the urine is controlled by anti-diuretic hormone (ADH).

·        If the body is low on water, the kidneys need to produce less water in the urine, and vice versa.

·        If there is low water in the body, the blood concentration increases, and this is sensed by the hypothalamus.

·        The hypothalamus then causes the pituitary gland to produce ADH.

·        ADH causes the tubules in the kidneys to become more permeable to water, so it can be reabsorbed into the blood.

·        If the blood is dilute, no ADH is secreted, and thus the body loses more water through its urine.

·        The body temperature of humans is controlled in the hypothalamus, the “thermoregulatory centre”.

·        The skin can be used to control heat loss from the body.

·        If the body is too hot:

Sweat glands produce larger amounts of sweat, and when this evaporates it cools the body down.

Hair erector muscles relax, causing hairs to lie flat against the skin. This stops a layer of trapped air forming (insulation).

Arterioles leading to capillary loops dilate, increasing blood flow to the skin’s surface. This allows more heat to be lost and is called vasodilation.

·        If the body is too cold:

Sweat glands stop producing sweat, so that its evaporation cannot cool the body down.

Hair erector muscles contract, causing hairs to become erect. This forms a layer of trapped air which acts as insulation.

Arterioles leading to capillary loops constrict, decreasing blood flow to the skin’s surface. This stops heat loss and is called vasoconstriction.

·        If the body is cold, it can also speed up its metabolism and cause shivering.