mechanisms in excretion

Question 1

step 1: ultrafiltration in the renal (Bowman’s) capsule

Answer 1

• in the glomerulus, water and relatively small molecules of the blood plasma, such as ions and mineral salts, urea, glucose and amino acids plass out of the capillaries into the lumen of the capsule. this filtrate is known as glomerular filtrate
• red blood cells, white blood cells, platelets and proteins are retained in the glomerulus as they are too large to be passed out
• process is described as ultrafiltration, powered by the high pressure of blood

Question 2

step 2: selective reabsorption in the proximal convoluted tubule

Answer 2

• proximal convoluted tubule is the longest section of nephron.
• a large part of the glomerular filtrate (most of the mineral salts, all of the glucose and amino acids) is reabsorbed back into the capillary network via diffusion and active transport (selective reabsorption)
• most of the water is absorbed by osmosis
  water is also subsequently absorbed at the loop of Henle

Question 3

step 3: water conservation in the loop of Henle

Answer 3

ascending limb

• Na+ and Cl- are pumped out of the filtrate into the interstitial fluid through active transport
• water molecules remain in the filtrate since walls of ascending limb are impermeable to water

descending limb
- due to high Na+ and Cl- concentration in interstitial fluid, water molecules are passed out from filtrate via osmosis (interstitial fluid has lower water potential)

Question 4

step 4: blood pH and ion concentration regulation in the distal convoluted tubule

Answer 4

• cells of the tubule walls adjust the composition of the blood (note control of pH by controlled secretion of H+ and reabsorption of HCO3- so that pH remains in range of 7.35 - 7.45)
• in distal convoluted tubule, concentration of useful ions is regulated
• concentration of K+ is adjusted by secretion of excess in plasma into the filtrate
• concentration of Na+ and Cl- in the body is regulated by varying the quantity of sodium and chloride ions reabsorbed from filtrate

Question 5

step 5: water reabsorption in the collecting duct

Answer 5

• when we have drunk a lot of water, hypothalamus detects this and stops the posterior pituitary gland from secreting ADH
• when we have taken in little water, sweated excessively or eaten salty food, hypothalamus detects this and directs the posterior pituitary gland to secrete ADH