6.4.3 Control of blood water potential Flashcards
(11 cards)
Osmoregulation
Osmoregulation is control of the water potential of the blood.
Osmoreceptors
Osmoreceptors detect changes in blood water potential. Mammalian osmoreceptors are found in the hypothalamus. When a mammal is dehydrated the water potential of its blood falls, causing the cell volume of the osmoreceptor to decrease as water moves from the osmoreceptor into the blood by osmosis down the water potential gradient. This triggers the hypothalamus to stimulate the pituitary gland, leading to increased secretion of antidiuretic
hormone (ADH) from the posterior pituitary gland.
ADH
ADH increases the permeability of the cells in the distal convoluted tubule and collecting duct by stimulating the inclusion of aquaporins in their plasma membranes, so more water is reabsorbed from the nephron through the interstitial fluid and into the renal blood capillaries by osmosis, down the water potential gradient.
Urea
(Urea is produced in the liver by deamination of amino acids.) Urea is removed from the blood at the Bowman’s capsule during ultrafiltration, as it is small enough to pass through the basement membrane. It is then not reabsorbed from the nephron, becoming more concentrated as water is reabsorbed by osmosis from the proximal convoluted tubule, descending limb of the loop of Henle, distal convoluted tubule and collecting duct.
Many aquatic organisms don’t need to produce urea. Deamination in these aquatic organisms produces ammonia, which is lost from the body by simple diffusion.
The nephron is composed of…
Glomerulus, Bowman’s capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct.
Bowman’s capsule
Site of ultrafiltration.
Bowman’s capsule is in the cortex of the kidney.
Blood arrives at the glomerulus in the afferent arteriole and leaves in the efferent arteriole. The efferent arteriole has a narrower lumen than the afferent arteriole, which creates a high hydrostatic pressure in the glomerulus, forcing fluid (glomerular filtrate) out of the blood through small fenestrations (gaps / pores) in the capillary endothelium, through the basement membrane which surrounds the glomerular capillaries, between the podocytes (which support the glomerular capillaries) and into the Bowman’s capsule.
The basement membrane acts as the filter. Blood cells, platelets and large plasma proteins such as albumin are too large to pass through, whilst water and it’s smaller solutes (glucose, ions, urea) do.
Proximal convoluted tubule
Site of selective reabsorption.
PCT is in the cortex of the kidney.
A ‘brush border’ composed of microvilli provide a large surface area for reabsorption. The plasma membranes of the microvilli are embedded with many channel proteins for facilitated diffusion and many carrier proteins for active transport, including the sodium-potassium pump. There are also many symport carrier proteins for co-transport (the cotransport reabsorbs glucose and amino acids in the same way as in the ileum).
PCT cells have many mitochondria to produce ATP for active transport, and abundant rough endoplasmic reticulum for the many ribosomes required to produce all the membrane bound transport proteins.
Glucose is reabsorbed by facilitated diffusion and cotransport (with sodium ions, which are active transported), whilst water is reabsorbed by osmosis down a water potential gradient.
Loop of Henle
Site of water reabsorption.
Loop of Henle extends into the kidney medulla.
In the ascending limb sodium ions are actively transported out of the nephron into the tissue fluid of the medulla. This lowers the water potential of the medulla tissue fluid, maintaining a water potential gradient between the descending limb and the tissue fluid (for the reabsorption of water).
In the descending limb water moves out of the nephron into the interstitial tissue fluid by osmosis down the water potential gradient.
The ascending limb is impermeable to water.
The loop of Henle is an example of a countercurrent multiplier, because the longer the loop of Henle, the lower the water potential in medulla tissue fluid:
A longer loop of Henle means a higher sodium ion concentration in the medulla, because more sodium ions are moved out into the medulla and the sodium ion gradient is maintained for a longer section of the nephron, so a water potential gradient is maintained for a longer section of the nephron, so more water is reabsorbed from the loop of Henle and collecting duct by osmosis.
Distal convoluted tubule
Site of water reabsorption.
DCT is in the cortex of the kidney.
The distal convoluted tubule is sensitive to ADH, responding in the same way as the collecting duct.
(The DCT is also involved in regulating blood pH).
Collecting duct
Site of water reabsorption.
The collecting duct extends into the kidney medulla, which has a low water potential because of the ascending limb of the loop of Henle.
Collecting duct cells are sensitive to ADH.
ADH increases the permeability of the collecting duct by stimulating the inclusion of aquaporins in teh plasma membrane of collecting duct cells so more water can be reabsorbed by osmosis down the water potential gradient.
Adaptations of animals living in arid conditions.
Some desert mammals have thicker medulla, longer loops of Henle and secrete large amounts of antidiuretic hormone (ADH), meaning that more water from their filtrate is reabsorbed by osmosis into the blood and less is lost in their urine.
Some desert animals also obtain water from aerobic respiration and condensation reactions.
