The control of blood water potential Flashcards
What is osmoregulation
The homeostatic control of the water potential of the blood.
What are the basic structural and functional units of the kidneys called
nephrons
What is the kidney made up of
- Fibrous capsule: an outer membrane that protects the kidney.
- Cortex: a lighter coloured outer region made up of renal (Bowman’s) capsules, convoluted tubules and blood vessels.
- Medulla: a darker coloured inner region made up of loops of Henle, collecting ducts and blood vessels.
- Renal pelvis: a funnel-shaped cavity that collects urine into the ureter.
- Ureter: A tube that carries urine to the bladder.
- Renal artery: supplies the kidney with blood from the heart via the aorta.
- Renal vein: returns blood to the heart via the vena cava.
What is a Nephron
- The nephron is the functional unit of the kidney.
- It is a narrow tube up to 14mm long, closed at one end, with two twisted regions separated by a long hairpin loop.
List and describe the things that each nephron is made up of
- renal (Bowman’s) capsule: the closed end at the start of the nephron. It is cup-shaped and surrounds a mass of blood capillaries known as glomerulus. The inner layer of the renal capsule is made up of specialised cells called podocytes.
- Proximal convoluted tubule: A series of loops surrounded by blood capillaries. Its walls are made of epithelial cells which have microvilli.
- Loop of Henle: a long, hairpin loop that extends from the cortex into the medulla of the kidney and back again. It is surrounded by blood capillaries.
- Distal convoluted tubule: a series of loops surrounded by blood capillaries. Its walls are made of epithelial cells, but it is surrounded by fewer capillaries than the proximal tubule.
- Collecting duct: a tube into which a number of distal convoluted tubules from a number of nephrons empty. It is lined by epithelial cells and becomes increasingly wide as it empties into the pelvis of the kidney.
List and describe the blood vessels associated with each nephron
- Afferent arteriole: A tiny vessel that ultimately arises from the renal artery and supplies the nephron with blood. The afferent arteriole enters the renal capsule of the nephron where it forms the glomerulus.
- Glomerulus: A many-branched knot of capillaries from which fluid is forced out of the blood. The glomerular capillaries recombine to form the efferent arteriole.
- Efferent arteriole: A tiny vessel that leaves the renal capsule. It has a smaller diameter than the afferent arteriole and so causes an increase in blood pressure within the glomerulus. The efferent arteriole carries blood away from the renal capsule and later branches to form the blood capillaries.
- Blood capillaries: A concentrated network of capillaries that surrounds the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule and from where they reabsorb mineral salts, glucose and water. These capillaries merge together into venules that in turn merge together to form the renal vein.
List the series of stages by which the nephron carries out its role in osmoregulation
1)The formation of glomerular filtrate by ultrafiltration.
2) Reabsorption of glucose and water by the proximal convoluted tubule.
3) Maintenance of a gradient of sodium ions in the medulla by the loop of Henle.
4) Reabsorption of water by the distal convoluted tubule and collecting ducts.
What are the walls of the glomerular capillaries made up of
Endothelial cells with pores between them
Describe the formation of glomerular filtrate by ultrafiltration
- As the diameter of the afferent arteriole is greater than that of the efferent arteriole, there is a build up of pressure within the glomerulus.
- As a result, water, glucose and mineral ions are squeezed out of the capillary to form the glomerular filtrate.
- Blood cells and large proteins cannot pass into the renal capsule as they are too large.
- The movement of the filtrate out of the glomerulus is resisted by the:
1) Capillary endothelial cells.
2) Connective tissue and endothelial cells of the blood capillary.
3) Epithelial cells of the renal capsule.
4) The hydrostatic pressure of the fluid in the renal capsule space.
5) The low water potential of the blood in the glomerulus. - There are some modifications which resist this barrier to the flow of filtrate and thus said in its formation:
1) The inner layer of the renal capsule is made up of highly specialised cells called podocytes. These cells have spaces between them which allows filtrate to pass beneath them and through gaps between their branches. Filtrate passes between these cells rather than through them.
2) The endothelium of the glomerular capillaries has spaces up to 100 nm 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 the resistance and so filtrate passes from the blood into the renal capsule.
- This filtrate, which contains urea, does not contain cells or plasma proteins which are too large to pass across the connective tissue.
Describe the reabsorption of glucose and water by the promixal convoluted tubule
- In the proximal convoluted tubule nearly 85% of the filtrate is reabsorbed back into the blood as there are useful molecules in the filtrate.
- How this happens:
- sodium ions are actively transported out of the cells lining in the proximal convoluted tubule into blood capillaries which carry them away.
- The sodium ion concentration in 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, amino acids, chloride ions etc) along with the sodium ions by 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.
How much water enters the nephrons and is reabsorbed per day on average
- About 180 dm^3 of water enters the nephrons each day.
- Of this, only about 1 dm^3 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.
Describe what the loop of Henle is
- 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 the blood.
- Th concentration of the urine produced is directly related to the length of the loop of Henle.
Describe the two regions that the loop of henle is made up of
1) The descending limb which is narrow, with thin walls that are highly permeable to water.
2) The ascending limb which is wider, with thick walls that are impermeable to water.
Describe how the loop of Henle acts 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 the region of the medulla between the two limbs (called the interstitial region).
3) 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 escapes.
4) The walls of the descending limb, however, are very permeable to water and so it passes out of the filtrate, by osmosis, into the interstitial space. This water enters the blood capillaries in this region by osmosis and is carried away.
5) The filtrate progressively loses water in this way as it moves down the descending limb, lowering its water potential. It reaches its lowest water potential in the tip of the hairpin.
6) 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 filtrate develops a progressively higher water potential.
7) 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 ion conc) in the cortex and an increasingly lower water potential (higher ion conc) the further into the medulla one goes.
8) The collecting duct is permeable to water and so, as the filtrate moves down it, water passes out of it by osmosis. The water passes by osmosis into the blood vessels that occupy this space, and is carried away.
9) As water passes out of the filtrate its water potential is lowered. However, the 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.
Through what does the water that passes out of the collecting duct by osmosis pass through and what can alter these
- the water that passes out of the collecting duct by osmosis does so through channel proteins that are specific to water (aquaporins).
- Antidiuretic hormone (ADH) can alter the number of these channels and so control water loss.