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What are the functions of the kidney?

1. Excretion: Removal of nitrogenous waste.
2. Osmoregulation: Control of blood/body water potential.
3. Regulation of blood pH.


What is the structure of the kidney?

- Outside of kidney surrounded by fibrous layer called the capsule.
- The outermost layer is the cortex and is where the renal corpuscules, PCTs (Proximal Convoluted Tubules) and DCTs (Distal Convoluted Tubules) are located.
- The second layer is the medulla and is where the Loop of Henle and collecting ducts are located.
- The innermost layer is the pelvis, which is where the renal artery and vein connects. It leads to the ureter.


What is the structure of a nephron?

1. A nephron starts with the Bowman's capsule, which contains the glomerulus and is where the glomerular filtrate is formed.
2. Filtrate then enters the PCT, where selective reabsorption occurs and most solutes are reabsorbed.
3. Filtrate then flows into the descending limb of of the loop of Henle, where water is removed by osmosis into surroundings.
4. Filtrate flows into ascending limb which is split into the thin and thick sections. The thin section is permeable to water but the thick is not, and is where active transport of NaCl happens.
5. Filtrate then flows into DCT where more adjustments are made to the salt concentrations.
6. Filtrate flows into the collecting ducts which reabsorbs more water (as needed).
7. Collecting ducts join at pelvis which leads into ureter.


What is the structure of blood vessels in the kidneys?

1. Blood enters kidneys through renal artery.
2. Renal artery branches off into smaller arterioles (afferent arterioles).
3. Afferent arterioles branch further to form a network of fine capillaries called the glomerulus, which sits inside Bowman's capsule.
4. Capillaries recombine to form efferent arterioles which leads away from Bowman's capsule.
5. Blood then joins network of capillaries called the peritubular capillaries which wrap around the nephron and reabsorb substances.
6. Capillaries join together to form venules, which combine to form renal vein; taking blood away from kidneys.


How does filtrate content change throughout nephron?

- 85% of the filtrate is reabsorbed along the PCT (isotonic)
- Water potential decreased along descending limb as salts diffuse in and water diffuses out.
- Water potential increases again along ascending limbs as water is prevented from leaving and NaCl is removed (hypotonic).
- Water potential decreases agains as water leaves the collecting ducts by osmosis.
- Concentrated (hypertonic) urine is formed


How is high pressure generated in the glomerulus?

The afferent arteriole is much wider than the efferent arteriole, which creates a bottleneck effect in the glomerulus. As blood enters at a higher rate than can be removed, pressure increases beyond pressure inside Bowman's capsule, which creates pressure gradient for ultrafiltration.


What are the filtration barriers during ultrafiltration?

- Endothelium of capillaries consist of gaps which fluid is able to pass through. Offers no real barrier as blood cells can easily squeeze past.
- Basement membrane (made from fine collagen fibres) holding together endothelium is only real filtration barrier. Only molecules small enough are able to fit through gaps. Red blood cells and large plasma proteins cannot fit through and remain in capillaries.
- Podocytes make up epithelium of Bowman's capsule. They have finger-like projections called major processes. These ensure there are gaps between the cells which allow glomerular filtrate to freely enter Bowman's capsule.


What is the content of glomerular filtrate?

- Water.
- Glucose.
- Amino acids.
- Vitamins.
- Hormones.
- Urea.
- Inorganic ions (salts).
- Small proteins.


How much of each substance is reabsorbed into the blood from PCT?

Glucose - All
Amino acids - All
Water - Most
Urea - Some
Inorganic ions - some
Small proteins - All
Vitamins - All
Hormones - Some


How are epithelial cells lining the PCT adapted for selective reabsorption?

- Microvilli increases the surface area which results in larger exchange surface and greater rate of diffusion.
- Plasma membrane contains lots of co-transporter proteins used to reabsorb glucose and salts.
- Contains lots of mitochondria to produce energy as ATP for use in active transport.
- One layer thick to minimise diffusion distance.
- Nuclei contain instructions to manufacture transport membranes.
- Basal membranes on capillary side of cells also folded to increase surface area.
- Tight junction between cells to force filtrate through epithelial cells to control absorption rather than leaking into capillaries directly.


How is water reabsorbed in the PCT?

Due to ultrafiltration, there is a very low water potential in the capillaries next to epithelial cells, much lower than the glomerular filtrate (especially due to Na+ and glucose being removed). Water potential gradient set up across the epithelial cells between capillaries and PCT which results in water moving out of PCT, across epithelial cells and into blood by osmosis.


How is glucose and Na+ ions reabsorbed?

1. Sodium-potassium pump in basal membrane pumps Na+ ions out of epithelial cells by active transport, which maintains low Na+ concentration in cell.
2. Na+ ions diffuse into epithelial cells through co-transporter proteins, down concentration gradient.
3. Movement of Na+ ions provides energy for glucose to be pumped into epithelial cells, even against concentration gradient (indirect active transport).
4. Concentration of glucose increases in cell so it diffuses out and into tissue fluid. It then diffuses into blood and is carried away.
5. This process may be accelerated by active transport.


How are amino acids reabsorbed?

Amino acids also enter the cells through co-transporter proteins with Na+ ions and then diffuse into blood down concentration gradient. This process can also be accelerated through active transport.


How are larger molecules like small proteins reabsorbed?



What is the headpin countercurrent multiplier effect?

The arrangement of tubules in a tight hairpin shape. This means that fluid is always flowing in the opposite direction to each other and there is always a concentration gradient between the fluid in the two limbs of the loop of Henle. This makes it much easier for salts to be transferred from one limb to the other, and thus lowering the water potential of the medulla.


What is the process of water reabsorption?

1. Water moves, by osmosis, out of the descending limb and into surrounding tissue fluid. This concentrates filtrate.
2. The fluid reaching the bottom of the loop of Henle is very concentrated due to loss of water. Medulla is also very concentrated due to accumulation of Na+ and Cl- ions by diffusion and active transport.
3. Fluid enters ascending limb. As it moves up, Na+ and Cl- ions are pumped out and into medulla by active transport.
4. Active transport of Na+ and Cl- out of filtrate results in accumulation of these ions in surrounding medulla. This increases water potential in filtrate and decreases water potential in medulla, but water is prevented from leaving by osmosis as walls of ascending limb are impermeable.
5. Fluid at top of ascending limb very dilute.
6. Fluid empties into collecting duct.
7. Reabsorption of water in the collecting duct is controlled by ADH which makes the walls more/less permeable.


How does the loop of Henle aid in the reabsorption of water?

Reabsorption of water is aided by the low water potential in the medulla created by the countercurrent multiplier effect in the loop of Henle (Active transport of ions out of ascending limb), which sets up a water potential gradient between collecting duct and medulla, causing water to leave by osmosis. This helps produce very concentrated (hypertonic) urine.


Overall, how is the PCT adapted to selective reabsorption?

1. Larger surface area created by microvilli on membrane of epithelial cells, which maximise rate of reabsorption.
2. Large surface area created by long PCT maximises reabsorption.
3. Constant concentration gradient produced by flowing blood and flowing filtrate.
4. Concentration gradient created by active transport of Na+ ions into the blood.
5. Diffusion distance decreased by having only 2 layers of cells between blood and filtrate.
6. Diffusion distance decreased by blood vessels being next to the PCT.


What is osmoregulation?

Control and maintenance of constant water levels in the body.


How is water gained?

- Food.
- Drinks.
- Metabolism + respiration.


How is water lost?

- Urine.
- Sweat.
- Expiration.
- Faeces.


What happens when there is too little water in the body?

1. Low water potential of blood causes osmoreceptor cells in hypothalamus to shrink.
2. This stimulates the neurosecretory cells and an action potential is created.
3. Action potential causes more ADH to be released into the bloodstream from the posterior pituitary gland.
4. ADH travels to the collecting ducts via blood.
5. ADH binds to complementary receptors on the plasma membrane of cells lining collecting ducts.
6. Walls of collecting ducts are made more permeable.
7. More water is reabsorbed from collecting ducts.
8. Blood water potential increases to normal while small volume of concentrated urine is produced.


What happens when there is too much water in the body?

1. High water potential of blood causes osmoreceptor cells in the hypothalamus to swell.
2. This inhibits the neurosecretory cells.
3. Less ADH is released into the bloodstream from posterior pituitary gland due to lack of action potentials.
4. Less ADH travels to the collecting duct via blood.
5. Reduced level of ADH in the blood causes the walls of the collecting duct to become less permeable.
6. Less water is reabsorbed from the collecting ducts.
7. Blood water potential decreases to normal while large volume of dilute urine is produced.


What hormone is responsible for controlling reabsorption of water from the kidneys?

Anti-Diuretic Hormone (ADH).


How does ADH make the walls of collecting duct more/less permeable?

- When ADH binds to receptors on cells lining the collecting ducts, a series of enzyme-catalysed reaction occur.
- This causes vesicles containing aquaporins (water channels) to be moved to membrane lining collecting ducts.
- These vesicles fuse with membrane so that aquaporins are present in walls of collecting ducts which allows water to be absorbed at quicker rate, so more is reabsorbed.
- As ADH is broken down and concentration drops, collecting duct membranes fold inwards, removing aquaporins from walls and reducing amount of water reabsorbed.
- If less ADH is released from pituitary gland, the net concentration of ADH drops in blood, so less aquaporins are present on walls, making walls of collecting ducts less permeable.


What are the most common causes of kidney failure?

- Diabetes mellitus.
- Hypertension.
- Infection.


What is dialysis?

Process of using partially permeable membrane and dialysis fluid to filter blood.


What are the principles behind dialysis?

- Partially permeable membrane allows solutes such as glucose, urea and salts to pass through but not large red blood cells and plasma proteins.
- Dialysis fluid contains right concentrations of different solutes in blood and no urea.
- Urea diffuses out of blood and into dialysis fluid, down concentration gradient.
- Concentration of other solutes in blood also adjusted by diffusion according to right concentrations in dialysis fluid.


How is urine used in pregnancy tests?

- During pregnancy, human Chorionic Gonadotrophin (hCG) is excreted from urine.
- A dipstick containing monoclonal antibodies specific to hCG is dipped in urine.
- hCG acts as the antigen and attaches to antibodies to form antigen-antibody complex. Antibodies also have coloured markers attached to other end.
- As urine is drawn up stick by capillary action, it brings hCG-antibody complexes with it.
- As the complexes move up, they encounter line of immobilised antibodies also specific to hCG.
- Immobilised antibodies attach to hCG on hCG-antibody complexes, holding complexes in position so that coloured markers make coloured line.
- Above test line is a control line with immobilised antibodies specific to monoclonal antibodies themselves. These hold any antibodies not attached to test line.
- Coloured markers form a line indicating the test is working.


How is urine used to test for anabolic steroids?

- Gas chromatography is used.
- Urine vaporised and dissolved in gaseous solvent.
- Mixture passed though tube lined with absorption agent.
- Different molecules come out of gas at different times and is absorbed by agent at different places.
- This allows for creation of chromatograms.
- Urine chromatograms compared against drug chromatogram to test for any drugs.