16.6 Role of nephron in osmoregulation Flashcards
What is the function of the kidney
To maintain water potential of plasma and hence tissue fluid
(osmoregulation)
What are the stages of the nephron carrying out its role of osmoregulation
- The formation of glomerular filtrate by ultrafiltration
- Reabsorption of glucose and water by proximal convoluted tubule
- Maintenance of a gradient of sodium ions in the medulla by the loop of Henle
- Reabsorption of water by distal convoluted tubule and collecting ducts
Describe the structure of Glomerulus capillaries
-Blood enters kidney through renal artery which branches into millions of arterioles
- Each of these enter the renal Bowman’s capsule of a nephron
- This arteriole is called afferent arteriole and it divides into complex of capillaries called Glomerulus
- The glomerular capillaries later merge to form the efferent arteriole which then sub divides again into capillaries
- These wind around tubules of nephron before combining to form renal vein
How is glomerular filtrate made by ultrafiltration
Walls of glomerular capillaries are made of endothelial cells with pores between them
Diameter of afferent arteriole is greater than that of efferent arteriole
This makes a build up of hydrostatic pressure within the glomerulus.
This pressure squeezes out water, glucose and mineral ions to form the glomerular filtrate.
Blood cells and large proteins are too big to pass out of renal capsule
What is the movement of the glomerular filtrate out of the glomerulus resisted by
- Capillary endothelial cells
- Connective tissue and endothelial cells of blood capillary
- Epithelial cells of renal capsule
- Hydrostatic pressure of the fluid in the renal capsule space
- Low water potential of the blood in the glomerulus
Although there is lots of resistance as shown, what modifications reduce this barrier to the flow of filtrate
- Inner layer of renal capsule is made of specialised cells called podocytes
- These have spaces between them to allow filtrate to pass beneath them and through the gaps between their branches
- Filtrate passes between these cells rather than through them
- The endothelium of the glomerular capillaries have spaces up to 100nm wide between its cells so fluid can pass between rather than through these cells.
As a result the hydrostatic pressure of the blood in the glomerulus is sufficient to overcome the resistance and so filtrate passes from the blood to bowman’s capsule.
The filtrate which contains urea doesn’t contain plasma proteins or blood cells which are too large to pass through tissue.
What is the proximal convoluted tubules role
- Most of filtrate is reabsorbed into the blood.
- Most molecules are useful and are reabsorbed however eg urea are wastes
What are the adaptations of the epithelial cells in convoluted tubule
- Microvilli provide large surface area to reabsorb substances from the filtrate
- Infoldings at their bases give a large surface area to transfer reabsorbed substances into blood capillaries
- A high density of mitochondria provide ATP for active transport
What is the process of reabsorption in the proximal convoluted tubule
- Sodium ions are actively transported out of the cells lining the tubule into blood capillaries.
- These carry them away creating diffusion gradient
- Sodium ions now diffuse down from lumen of tubule to epithelial cells lining it but only through carrier proteins by facilitated diffusion.
- These carrier proteins are specific types each of which carry sodium ions and another molecule eg chloride ions. So this is co transport
- The molecules that have been co transported into the cells of the proximal convoluted tubule then diffuse into the blood. As a result all the glucose and other valuable molecules are reabsorbed as well as water.
What is the loop of Henle
Hairpin shaped tubule that extends into 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 the blood.
What are the two regions of the loop of Henle
Descending limb: Which is narrow, with thin walls that are highly permeable to water
Ascending limb: Which is wider, with thick walls that are impermeable to water
The loop of Henle acts as a countercurrent multiplier.
Explain the process
- Sodium ions actively transported out of ascending limb of the loop of Henle using ATP provided by mitochondria in cells in its walls.
- This creates low water potential in the region of the medulla between the two limbs. Ascending limb is impermeable so water has to move out of the filtrate in descending limb by osmosis.
- So filtrate loses water as you get further down descending limb which lowers its water potential at the bottom.
- At base of ascending limb sodium ions diffuse out of filtrate and as you move up ascending limb, these ions are actively pumped out. So filtrate gets higher water potential at the top of ascending limb.
- In the interstitial space between ascending limb and collecting duct there is a gradient of water potential: Highest potential in the cortex, and it decreases as you go further down into medulla.
- The collecting duct is permeable to water so filtrate moves down it and water leaves it by osmosis and into the blood vessels that occupy this space.
- As water passes out of the filtrate its water potential is lowered however the water potential is also lowered in the interstitial space so water continues to move by osmosis down the whole length of collecting duct. Counter current multiplier ensures there is always a concentration gradient.
What are aquaporins
Channel proteins for water.
These line the collecting duct so water can move out of them by osmosis.
What does the distal convoluted tubule do
- Cells that make up the walls of it have microvilli and many mitochondria that allow them to reabsorb material rapidly from the filtrate by active transport.
- Main role of it is to make final adjustments to the water and salts that are reabsorbed and to control PH of blood by selecting which ions to reabsorb.
What is the counter current multiplier
When two liquids flow in opposite directions past each other, the exchange of substances is greater than if they flowed in the same direction.
In the loop of Henle, this means the filtrate in the collecting duct with a lower water potential meets the interstitial fluid that has an even lower water potential.
This means there is a gradient so water moves from collecting duct to interstitial fluid.
This gradient is maintained for the whole length of the duct so there is a steady flow of water into the interstitial fluid.
