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Summary - CCEA Unit A2 1 - Physiology and Ecosystems - Homeostasis
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Summary notes covering the entire CCEA A21 Biology specification in regard to the Homeostasis section.
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HOMEOSTATIS AND THE KIDNEYULTRAFILTRATION
-Ultrafiltration is a non-selective process
- Blood enters the glomerulus through the afferent arteriole at high hydrostatic pressure caused by
the afferent arteriole being wider than the efferent arteriole and being so close to the heart.
- All small molecules such as glucose, water, ions, and amino acids from the blood plasma are forced
out of the capillary and into the cavity of the bowman’s capsule.
- Red blood cells and large proteins do not leave the capillary due to them being too large to pass
through the basement membrane – which acts as the effective filter.
- Ultrafiltration is a process aided by pores in the walls of the capillary and gaps between the
podocytes in the walls of the bowman’s capsule.
- The liquid in the cavity of the bowman’s capsule is now called the filtrate.
SELECTIVE REABSOPRPTION
- At the proximal convoluted tubule amino acids and glucose are selectively reabsorbed by
active transport and facilitated diffusion.
- At the end of the proximal convoluted tubule all amino acids and glucose will have been
selectively reabsorbed.
- Small molecules are reabsorbed by pinocytosis
- The active reabsorption of the substances lowers the solute potential of the blood, causing
most of the water to re-enter the blood capillaries by osmosis.
- Urea is not selectively reabsorbed at the PCT, however the loss of water from the filtrate
causes the concentration of urea to increase throughout the nephron.
ADH/OSMOREGULATON
(qu.) Explain the link between a more negative blood solute and osmoregulation in the kidney.
ANS – Osmoreceptors in the hypothalamus detect the low solute potential. ADH is released from the
pituitary gland. This increases the permeability of the collecting duct and distal convoluted tubule by
opening its aquaporins. More water leaves the filtrate to be reabsorbed by the blood in the medulla.
This results in a low volume of concentrated urine.
(qu.) Explain the link between a more positive solute potential and osmoregulation in the kidney.
ANS – Less ADH is released from the pituitary gland. Therefore, permeability decreases in the
collecting duct and distal convoluted tubule due to more aquaporins closing. Less water leaves the
filtrate to be reabsorbed by the blood in the medulla. This results in a large volume of dilute urine.
ADAPTIONS OF DESERT ANIMALS
Mammals which live in particularly dry areas have a longer loop of Henle and a thicker medulla.
The longer the loop of Henle the more ions can be pumped from the filtrate to create a very low
water potential in the medulla (salt bath). More water will leave the filtrate and enter the medulla,
therefore a reduced water loss by only producing a small volume of concentrated urine.
PARTS OF THE KIDNEY
Cortex – Filtration carried out by nephrons, dense capillary network which receives blood from the
renal artery.
Medulla – Divided into pyramids, nephron extends across the cortex into the medulla
Pelvis – Urine passes into pelvis before passing into ureter
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