B6 THE KIDNEY

Where do the kidneys receive their oxygenated blood from?

the renal artery

How is blood delivered to the kidneys?

under high pressure

What’re kidneys main roles in homeostasis?

• removing urea and nitrogenous waste

• osmoregulation

What is the role of the cortex?

• outer layer

• where ultrafiltration occurs

• has a dense network of capillaries

• receives blood from the renal artery

What is the role of the medulla?

• inner layer

• nephrons extend to this layer from the cortex

• forms renal pyramids

What is the role of the pelvis?

• renal pyramids project into the pelvis

• urine passes into the pelvis before passing down the ureter

What’s a nephron?

• each kidney contains millions

• each tube is surrounded by a capillary network

• substances can be exchanged between the capillaries and the nephrons

What occurs in ultrafiltration?

1. plasma in the capillary carries urea, water, glucose, sodium ions, large proteins and blood cells

2. high hydrostatic pressure in the glomerulus forces small ions through the gaps in the endothelium cells (through basement membrane and between the podocytes)

3. blood cells and large protein molecules cannot cross

What causes the high hydrostatic pressure in the arteriole?

• the afferent arteriole is wider than the efferent arteriole

• there is a high blood pressure in the capillaries of the glomerulus

• pressure forces fluid containing water and small soluble molecules out of the blood

What acts as a filter during ultrafiltration?

• the capillary endothelium and the wall of the bowmans capsule are formed from a single layer of cells with gaps between them

• separating them is a basement membrane which acts as a molecular sieve

What useful substances also leave the blood during ultrafiltration?

• glucose

• amino acids

What is selective reabsorption?

where certain useful substances are reabsorbed back into the blood

How and where are the glucose and amino acids transported back into the blood?

• at the proximal convoluted tubule

• by facilitated diffusion and active transport

How are glucose and amino acids transported back into the blood?

1. Na+ ions are actively transported from epithelial cells to capillary using energy

2. this creates a sodium ion concentration gradient

3. facilitated diffusion of Na+ ions into the epithelial cell

4. glucose/amino acids are co-transported with Na+ ions

5. glucose/ amino acids diffuse down the concentration gradient and into the capillary via facilitated diffusion

How are epithelial cells well adapted for reabsorption?

• microvilli provide a large surface area for absorption

• many mitochondria provide ATP for active transport against their concentration gradient

• many carrier proteins for active transport of Na+ ions out of the epithelial cell

Where is the loop of henle?

runs from the cortex, down into the medulla and then back into the cortex

What’s the function of the loop of henle?

creates an area of high solute concentration in the interstitial fluid (tissue fluid) of the medulla so that it has a more negative water potential compared to the filtrate in the nephron, causing more water to be reabsorbed from the nephron

how does the loop of henle achieve the area of high solute concentration?

• cells in the descending limb are permeable to water

• cells in the ascending limb are impermeable to water but are full of mitochondria for active trransport

How does the loop of henle maintain the gradient of sodium ions?

• as the filtrate travels up the ascending limb of the loop, sodium ions are actively transported out of the nephron and into the tissue fluid of the medulla

• this lowers the water potential in the interstitial fluid

• there is a steep water potential gradient between the filtrate and the interstitial fluid

• water cant move out of the ascending limb down the water potential gradient as the walls are impermeable

• due to the descending limb being permeable to water, water can move out descending limb by osmosis

What occurs if there is a fall in the water potential of the blood?

1. osmoreceptors in the hypothalamus detect the fall in water potential

2. ADH is secreted into the capillaries by the posterior pituitary gland

3. ADH travels to kidneys in the blood and increases the permeability of the cells of the distal convulated tubule and collecting duct to water

How does ADH work?

it causes vesicles bound with aquaporin proteins to fuse with the cell surface membranes of cells of the distal convoluted tubule and collecting duct

What is the main role of the distal convoluted tubule?

makes final adjustments to the water and salts that’re reabsorbed, by altering the permeability of its walls due to hormones

What happens in the interstitial space?

there is a water potential gradient with the highest water potential in the cortex and the lowest in the medulla

What happens in the collecting duct?

its permeable to water so water continues to move out by osmosis due to the countercurrent multiplier ensuring that there is always a water potential gradient along the entire collecting duct

What happens to remaining fluid?

passes down the collecting duct, drains into the ureter and then into the bladder where it is exreted as urine

What should the urine contain?

urea, excess salts and excess water

What’s the mark scheme answer for how is urea removed from the blood?

1. hydrostatic pressure

2. causes ultrafiltration in the bowmans capsule

3. through basement membrane

4. enabled by small size of the urea molecule

What’s the mark scheme answer of describe how ultrafiltration produces glomerular filtrate?

1. hydrostatic pressure

2. small molecules

3. pass through the basement membrane

4. protein too large to go through

5. presence of podocytes in basement membrane

What is the mark scheme answer for how the cells of the proximal tubule are adapted for reabsorption of glucose?

1. many mitochondria provide ATP for active transport

2. many carrier protein sfor facilitated diffusion

3. microvilli provide large surface area

Whats the mark scheme answer for how does the lack of insulin affect reabsorption of glucose in the kidneys of a person with diabetes?

1. high concentration of glucose in blood

2. high concentration in tubule

3. reabsorbed by facilitated diffusion

4. this requires proteins

5. these proteins are saturated

6. not all glucose is reabsorbed

What is the mark scheme answer for why glucose is found in the urine of a person with untreated diabetes?

1. high concentration of glucose in blood

2. not all the glucose is reabsorbed at the proximal convoluted tubule

3. co-transport proteins are saturated

What is the mark scheme answer for maintaining a concentration gradient of sodium ions in the medulla?

1. sodium ions are actively transported out of the ascending limb into interstitial fluid using ATP

2. lowers the water potential between the filtrate and the interstitial fluid leading to a steep water potential gradient so water moves out of the permeable descending limb by osmosis

3. as the ascending limb is impermeable to water then little or no water leaves that part of the loop

4. water from the descending limb then enters capillaries returning to the blood

What’s the mark scheme answe for explain the role of the loop of henle in the absorption of water from the filtrate?

1. in thee ascending limb sodium ions are actively removed

2. ascending limb is impermeable to water

3. in descending limb sodium ions diffuse in

4. descending limb is permeable to water

5. low water potential in the medulla

6. the longer the loop the deeper into the medulla

7. water leaves the collecting duct

8. by osmosis

What is the mark scheme answer for how does ADH cause movement of water from the lumen to the collecting duct?

1. ADH causes vesicles containing aquaporin to be inserted into membrane

2. water enters cell through aquaporin

3. by osmosis

4. to capillary

5. via intersitial fluid