Role of the nephron in osmoregulation Flashcards Preview

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What is one of the main important role of the kidney

-one of the important functions of the kidney is to maintain the water potential of plasma and hence tissue fluid (osmoregulation)


What are the stages the nephron carries out in its role of osmoregulation?

-the formation of glomerular filtrate by ultrafiltration
-reabsorption of glucose and water by the proximal convoluted tubule
-maintenance of a gradient of sodium ions in the medulla by the loop of Henle
-reabsorption of water by the distal convoluted tubule and collecting ducts


What is the formation of glomerular filtrate by ultrafiltration?

-blood enters the kidney through the renal artery, which branches frequently to give around one million tiny arterioles, each of which enters a renal (bowman's) capsule of a nephron
-this arterial is called the afferent arteriole and it divides to give a complex of capillaries known as the glomerulus
-the glomerular capillaries later merge to form the efferent arterial, which then sub-divides again into capillaries, which wind their way around the various tubules of the nephron before combining to form the renal vain
-the walls of the glomerular are made up of epithelial cells with pores between them
-as the diameter of the affront arteriole is greater than that of the efferent arteriole, there is a build up of hydrostatic pressure within the glomerulus
-as a result, water, glucose and mineral ions are squeezed out of a capillary to form the glomerular citrate
-blood cells and proteins cannot pass across into the renal capsule as they are too large


what is the movement of filtrate out of the glomerulus resisted by?

-capillary epithelial cells
-connective tissue and epithelial cells of the blood capillary
-epithelial cells of the renal capsule
-the hydrostatic pressure of the fluid in the renal capsule space
-the low water potential of the blood in the glomerulus


Which modifications reduce the barrier to the flow of filtrate?

-the inner layer of the renal capsule is made up of highly specialised calls called podocytes, these cells, have spaces between them, this allows citrate to pass beneath them and through gaps between their branches, citrate passes between these cells rather than through them
-the endothelium of the glomerular capillaries has spaces up to l00nm wide between its cells, again, fluid can therefore pass between, rather than through, these cells, as a result the hydrostatic pressure of the blood in the glomerulus is sufficient to overcome their resistance and so filtrate passes from the blood into the renal capsule, the filtrate which contains urea does not contain cells or plasma proteins which are too large to pass across the connective tissue
-many of the substances in the 125cm3 of filtrate passing out of blood each minute are extremely useful to the body and reabsorbed


What is the reabsorption of glucose and water by the proximal convoluted tubule?

-in the proximal convoluted tubule nearly 85% of the citrate is reabsorbed back into the blood
-ultrafiltration operates on the basis of size of molecule - small ones are removed
-some, such as urea, are wastes, but most are useful and so are reabsorbed


How are the proximal convoluted tubules adapted to reabsorb substances into the blood by having epithelial cells?

-microvilli to provide a large surface area to reabsorb substances from the citrate
-infolding at their bases to give a large surface area to transfer reabsorbed substances into blood capillaries
-a high density of mitochondria to provide ATP for active transport


What is the process of re-absorbance of substances into the blood through proximal convoluted tumbles?

-sodium ions are actively transported out of the cells lining the proximal convoluted tubule into blood capillaries which carry them away, the sodium ion concentration of these cells is therefore lowered
-sodium ions now diffuse down a concentration gradient from the lumen of the proximal convoluted tubule into the epithelial lining cells but only through special carrier proteins by facilitated diffusion
-these carrier proteins are of specific types, each of which carries another molecule (glucose or amino acids or chloride ions, etc.) along with the sodium ions which is known as co-transport
-the molecules which have been co-transported into the cells of the proximal convoluted tubule then diffuse into the blood, as a result all the glucose and most other valuable molecules are reabsorbed as well as water


What haooens to the water then enters the nephrons?

-about 180dm3 of water enters the nephrons each day
-of this volume, only about 1dm3 leaves the body as urine
-85% of the reabsorption of water occurs in the proximal convoluted tubule
-the remainder is reabsorbed from the collecting duct as a result of the functioning of the loop of Henle


What is maintenance of a gradient of sodium ions by the loop of Henley?

-the loop of Henle is a hairpin-shaped tubule that extends into the medulla of the kidney
-it is responsible for water being reabsorbed from the collecting duct, thereby concentrating the urine so that it has a lower water potential than blood
-the concentration of the urine produced is directly related to the length of the loop of Henle


What are the regions of the loop of Henley?

-the resending limb, which is narrow, with thin walls that are highly permeable to water
-the ascending limb, which is wider, with thick walls that are impermeable to water


How does the loop of Henley act as a counter-current multiplier?

1-sodium Ions are actively transported out of the ascending limb of the loop of Henle using ATP provided by the many mitochondria in the cells of its wall
2-this creates a low water potential (high ion concentration) in he region of the medulla between the two limbs (called the interstitial region) in normal circumstances water would pass out of the ascending limb by osmosis, however the thick walls are almost impermeable to water and so very little, if any escapes
3-the walls of the descending limb are however very permeable to water and so passes out of the citrate by osmosis into the interstitial space, this water enters the blood capillaries in this region by osmosis and is carried away
4-the filtrate progressively loses water in this way as it moves done the descending limb lowering its water potential
-it reaches its lowest water potential at the tip of the hairpin
5-at the base of the ascending limb, sodium ions diffuse out of the filtrate and as it moves up the ascending limb these ions are also actively pumped out and therefore the citrate develops a progressively higher water potential
6-in the interstitial space between the ascending limb and the collecting duct there is a gradient of water potential with the highest water potential (lowest concentration of ions) in the cortex and an increasingly lower water potential (higher concentration of ions) the further into the medulla one goes
7-the collecting duct is permeable to water and so, as the citrate moves down it, water passes out of it by osmosis
-this water passes by osmosis into the blood vessels that occupy this space and is carried away
8-as water passes out of the filtrate its water potential is also lowered in the interstitial space and so water continues to move out by osmosis down the whole length of the collecting duct
-the counter-current multiplier ensures that there is always a water potential gradient drawing water out of the tubule


What is the antidiuretic hormone?

-the water that passes out of the collecting duct by osmosis does so through channel proteins that are specific to water (aquaporins)
-antidiuretic hormone can alter the number of these channels and so control water loss
-by the time the filtrate, now called urine, leaves the collecting duct on its way to the bladder, it has lost most of its water and so it has a lower water potential is more concentrated than the blood


What is the distal convoluted tubule?

-the cells that make up the walls of the distal convoluted tubule have microvilli and many mitochondria that allow them to reabsorb material rapidly from the filtrate, by active transport
0the main role of the distal tubule is to make final adjustments to he water and salts that are reabsorbed and to control the pH of the blood by selecting which ions to reabsorb
-to achieve this, the permeability of its walls becomes altered under the influence of various hormones


What is the counter-current multiplier?

-you may remember that when two liquids flow in opposite directions past one another, the exchange of substances or heat between them is greater than if they flowed in the same direction next to each other
-in the case of the loop of Henle, the counter-current flow means that the filtrate in the collecting duct with a lower water potential
-this meets interstitial fluid that has an even lower water potential
-this means that although the water potential gradient between the collecting duct and interstitial fluid is small, it exists for the whole length of the collecting duct
-there is therefore a steady flow of water into the interstitial fluid, so that around 80% of the water enters the interstitial fluid and hence the bold
-if the two flows were in the same direction (parallel less of the water would enter the blood