Chapter 44: Osmoregulation and Excretion

Question 1

Key steps of excretory system

Answer 1

1. Filtration
2. Reabsortion
3. Secretion
4. Excretion

Question 2

Renal structures

Answer 2

1. Renal cortex- houses the glomerulus and convoluted tubules
2. Renal medulla- houses the medullary collecting ducts, loops of Henle, and vasa recta
3. Renal artery- blood towards the kidney and supplies the adrenal gland
4. Renal vein- carries blood away from the kidney to the inferior vena cava
5. Ureter
6. Renal pelvis- area at the center of the kidney where urine collects as it is funneled to the ureter

Question 3

Nephron types

Answer 3

Cortical nephrons- comprise the majoroity of nephrons; only reach a short way into the medulla

Juxtamedullary nephrons- extend deep into the medulla; essential for the production of hyperosmotic urine

Question 4

Nephron structures

Renal flow path

Answer 4

Blood

1. Branch of renal artery
2. Afferent arteriole- supplies the glomerulus
3. Glomerulus- ball of capillaries
4. Efferent arteriole- Capillaries that converge after leaving the glomerulus
5. Peritubular capillaries- supply the proximal and distal convoluted tubules
6. Vasa recta- branch off the efferent arteriole that supply the medulla and the loop of Henle
7. Branch of renal vein

Filtrate

1. Bowman’s capsule- cup-shaped swelling that surrounds the glomerulus
2. Proximal convoluted tubulue- collects filtrate from Bowman’s capsule
3. Loop of Henle- major site for the recovery of water
4. Distal convoluted tubule- receives filtrate from the loop of Henle
5. Collecting duct

Question 5

Proximal Tubule

Answer 5

Recaptures ions, water, glucose, amino acids, K⁺ ions, and other essential nutrients

Molecules are transported actively and passively from the filtrate into the interstitial fluid and then capillaries

• Na⁺ ions are transported actively
• This drives the passive transport of Cl⁻

Maintains blood pH by secreting H⁺ and reabsorbing about 90% of HCO₃⁻

Question 6

Descending limb of the loop of Henle

Answer 6

Reabsorption of water continues through channels formed by aquaporins

However, almost no channels for salts cause filtrate to become increasingly concentrated

Movement is driven by the high osmolarity of the interstitial fluid in the medulla 1,200mOsm/L

Question 7

Ascending limb of the loop of Henle

Answer 7

Transport epithelia lack aquaporins

Has two specialized regions:

• Thin segment- NaCl that has become highly concentrated passively diffuses back out of the filtrate
• Thick segment- Transport epithelia actively transport NaCl causing the filtrate to become more dilute

Question 8

Distal Tubule

Answer 8

Regulates K⁺ & NaCl concentrations of body fluids

Controlled secretion of H⁺ and reabsortion of HCO₃⁻ contributes to pH regulation

Question 9

Collecting duct

Answer 9

Carries filtrate through the medulla to the renal pelvis

Hormonal control of permeability and aquaporin insertion determines the extent to which the urine is concentrated

• Conserving water- transport epithelia remain impermeable to salt and water is further removed from filtrate
• Dilute urine- collecting duct actively absorbs salts and does not have aquaporins inserted to prevent water reabsortion

Question 10

Osmolarity gradients

Answer 10

Proximal tubule- filtrate volume decreases as water and salt are reabsorbed; osmolarity remains the same

Descending loop- solutes become more concentrated due to water leaving the tubule by osmosis

Ascending loop- maintains a high osmolarity in the interstitial fluid of the renal medulla; dilutes the filtrate

The countercurrent multiplier system involving the loop of Henle maintains a high salt concentration in the kidney

• Allows the vasa recta to supply the kidney with nutrients, without interfering with the osmolarity gradient

Question 11

Antidiuretic hormone

Answer 11

Osmoreceptor cells in the hypothalamus monitor blood osmolarity and regulate release of ADH from the posterior pituitary

Binding of ADH to receptor molecules leads to a temporary increase in the number of aquaporin proteins in the membrane of collecting duct cells to recapture more water

Normal blood osmolarity is 275−295 mOsm/L

As osmolarity falls a negative feedback mechanism reduces the osmoreceptor cell activity in the hypothalmus and ADH secretion is reduced

Question 12

The renin-angiotensin-aldosterone system

Answer 12

The RAAS responds to a drop in blood volume and pressure by increasing water and Na⁺ reabsortion

The juxtaglomerular apparatus (JGA) senses a drop in blood pressure in the afferent arteriole and releases the enzyme renin

Renin initiates a sequence of steps that cleave angiotensinogen to yield angiontensin II

Angiotensin II acts as a hormone and triggers vasoconstriction− increasing blood pressure and decreasing renal blood flow

Angiotensin II also stimulates the release of the mineralcorticoid aldosterone by the adrenal glands which caused the distal tubes and collecting ducts to reabsorb more Na⁺ and water

Question 13

Atrial natriuretic peptide (ANP)

Answer 13

Atrial natriuretic peptide is released by myocardial cells in the walls of the atria in response to an increase in blood pressure and volume

ANP opposes the RAAS and inhibits the release of renin from the JGA, inhibits NaCl reabsortion in the collecting ducts, and reduces aldosterone release from the adrenal glands