Control of blood water potential Flashcards
Define osmoregulation
Controlling the water potential of the blood
Structure of kidney
- Capsule
- Cortex
- Medulla
- Renal pelvis: cavity collects urine into ureter
- Ureter: tube that carries urine to bladder through urethra
- Renal artery: supplies kidney with oxygenated blood from heart
- Renal vein: returns deoxygenated blood from kidney to heart
Structure of nephron
- Bowman’s capsule: cup-shaped, surrounds glomerulus (branched knot of capillaries), inner layer of podocytes
- Proximal convoluted tubule: series of loops surrounded by capillaries, walls made of epithelial cells with microvilli
- Loop of Henle: loop extends from cortex into medulla, made up of descending limb (narrower) + ascending limb (wider)
- Distal convoluted tubule: similar to PCT but fewer capillaries
- Collecting duct: DCT from several nephrons empty into collecting duct, which leads into pelvis of kidney
Describe the blood vessels associated with nephron
Wider afferent arteriole- Branch of renal artery that delivers blood to glomerulus for ultrafiltration
Narrower efferent arteriole- Vessel leaving the glomerulus that branch to form a capillary network that surrounds the tubules
How is glomerular filtrate formed - Ultrafiltration
- Walls of glomerular capillaries are made of epithelial cells with pores in between them (permeable)
- As the diameter of the afferent arteriole is greater than the efferent, theres a build up of hydrostatic pressure within the glomerulus
- So, anything smaller than 69,000RMM such as: glucose, water, amino acids, urea, mineral ions, are squeezed out of the capillary to form the glomerular filtrate
How are cells of bowman’s capsule adapted for ultrafiltration
- Bowman’s capsule is made up of podocytes. The spaces between these cells (larger SA) + their branches allow filtrate through
- Endothelium of glomerular capillaries has spaces between cells (slit pores) - permeable
- Filtrate can pass through basement membrane
Process of selective reabsorption
Occurs in PCT
- Sodium ions are actively transported out of the PCT cells into the capillary
- Sodium ions diffuse down the conc gradient from the PCT lumen into the epithelial cells of the PCT through carrier proteins
- These carrier proteins co-transport glucose, amino acids, chloride ions etc along with the sodium ions
- Glucose can than diffuse from PCT epithelial cells into bloodstream
How are cells in PCT adapted for selective reabsorption?
- Microvilli : large SA for co-transporter proteins
- Many mitochondria: ATP for active transport of glucose into intercellular spaces
- Folded basal membrane: Large SA
What is the function of the loop of Henle + the 2 regions?
Descending limb: narrow with thin walls, highly permeable to water
Ascending limb: wider with thick walls, impermeable to water
Function: to maintain a sodium ion gradient
Describe how the loop of Henle maintains a sodium ion gradient
- Mitochondria in the walls of the cells provide energy to actively transport sodium ions out of ascending limb into the interstitial space
- This creates a lower water potential in the interstitial space of medulla
- The permeable walls of the descending limb lose water by osmosis to the interstitial space, which then enters the capillaries
- The filtrate progressively loses water in the descending limb, lowering the water potential
- At the base of the ascending limb, sodium ions diffuse out of the filtrate. As it moves up the limb, sodium ions are actively pumped out, developing a higher water potential
- In the interstitial space between the collecting duct + the ascending limb, there is a gradient of water potential with the highest water potential in the cortex + the lowest in the medulla
- The collecting duct is permeable to water + so the filtrate moves out into the blood vessels by osmosis
- The water potential is lowered as water moves out the filtrate. The counter current multiplier moves blood away, ensuring a water potential gradient out of the tubule
Describe how water is reabsorbed by DCT + collecting duct
- Water passing out the collecting duct does so through water channels called aquaporins
- ADH can alter the number of these channels + so can control water loss
- The epithelial cells of the DCT have mitochondria + microvilli that allow further absorption of substances by active transport
- Water + salts are selectively reabsorbed to control pH. To achieve this the walls can be influenced by various hormones
Describe the countercurrent multiplier
Filtrate in collecting duct is always beside an area of interstitial fluid that has a lower water potential
This maintains a water potential gradient for the maximum reabsorption of water
What causes a rise in solute conc + lowering of water potential?
- Too little water being consumed
- Too much sweating
- Excess ion consumption
Where is ADH produced
ADH is produced in the hypothalamus + then it moves into the posterior pituitary + is released into the capillaries + into the blood
How does body respond to a fall in water potential
- Osmoreceptors in blood detect fall in water potential + lose water by osmosis causing them to shrink
- This triggers the production of ADH
- ADH passes to the posterior pituitary gland where it is secreted into the bloodstream
- ADH passes to the kidney, increasing the permeability of the DCT + collecting duct
- Collecting ducts cell membrane has protein receptors that bind to ADH, leading to activation of phosphorylase
- Activated phosphorylase causes vesicles containing aquaporins to fuse with the cell membrane which increase the permeability of water
- ADH increases the permeability to urea, which passes out, lowering the water potential of the fluid around the duct
- The combined effect is that more water leaves the collecting duct filtrate by osmosis down the water potential gradient, entering the blood
- As the reabsorbed water came from the blood in the first place, it will only prevent any further reduction in water potential
- Osmoreceptors send impulses to thirst region of brain
- Osmoreceptors detect a rise in water potential of blood + send fewer impulses to pituitary
- Pituitary reduces ADH release so permeability of collecting duct returns to normal
