Topic 4 Excretory System: Nephrons

Question 1

• Composed of a renal corpuscle and renal tubule

- Function to reabsorb nutrients, salts, and water

Answer 1

Nephrons

Question 2

• Contains glomerulus, which acts as a sieve, and Bowman’s capsule, which encloses the glomerulus. Bowman’s capsule contains 2 arterioles: an afferent arteriole that leads into the glomerulus, and an efferent arteriole that leads out of the glomerulus
• Hydrostatic pressure forces plasma through the fenestrations (small pores) of the glomerular endothelium and into Bowman’s capsule. These fenestrations screen out blood cells and large proteins from entering Bowman’s capsule. The fluid that does get in is called the filtrate/primary urine. Podocytes are cells in Bowman’s capsule that filter blood by holding back large molecules (proteins) and allowing smaller molecules (sugars, water, salts) through. After the efferent arteriole passes out of the glomerulus, it just webs around the entire nephron structure as the peritubular capillaries (which surround the proximal convoluted tubule and distal convoluted tubule and reabsorb materials) and vasa recta (which surrounds the Loop of Henle in the kidney’s medulla and maintains the concentration gradient) before dumping back into the renal branch of the renal vein
• When substances are reabsorbed from the tubules (or secreted into them), it takes place in the interstitium. It is this network of capillaries that lets us reabsorb into/secrete from the blood.
• The renal corpuscle leads to…

Answer 2

1. Renal Corpuscle

Question 3

1. Proximal convoluted tubule (PCT)
2. Loop of Henle
3. Distal convoluted tubule (DCT
4. Collecting duct

Answer 3

Parts of Renal Tubules

Question 4

• Where active reabsorption of almost all glucose, amino acids, and some NaCl, as well as passive reabsorption of K+ and HCO3- begins. Water follows these ions out so the cortex is not salty. Most reabsorption takes place here.
  a. Drugs, toxins, NH3 also get secreted into the filtrate; H+ ions are secreted in as well via antiport with Na+
  b. The nest result of the PCT is to reduce the amount of filtrate, but the concentration stays roughly the same
  c. PCT cells have a lot of mitochondria due to all of the active reabsorption that takes place here

Answer 4

1. Proximal convoluted tubule (PCT

Question 5

• makes up a majority of the nephron
  a. Descending loop: only permeable to water (but this water is picked up by the vasa recta, so the medulla stays salty) via lots of aquaporins. The solute concentration in the tube increases as a result
  b. Ascending loop: makes the renal medulla salty: first passively and then actively by pumping out NaCl. The ascending loop is also impermeable to water. Solute concentration in the tube decreases as a result

Answer 5

2. Loop of Henle

Question 6

• more reabsorption of glucose, ions and water occurs here so the cortex isn’t salty. The filtrate (NaCl and HCO3-) gets actively pulled out and reabsorbed into the body, and K+/H+ are actively secreted into the tubule. Some water passively gets pulled out, but overall, the filtrate concentration is lowered
  a. Aldosterone, and to a lesser extent ADH, can act on the end of the distal tubule to increase its permeability to water, which is normally not permeable. Aldosterone increases the amount of Na+/K+ antiport — more K+ gets secreted into the tubule as more Na+ is resorbed from the tubule. Water follows the Na+ out and the concentration of the filtrate increases. We’ll get into this more in the endocrinology section! But for now, let’s follow the distal convoluted tubule into the collecting duct.

Answer 6

3. Distal convoluted tubule (DCT)

Question 7

• collects the remaining filtrate. What happens here (concentrated or dilute urine) is highly dependent on what hormones are acting on it.
  a. We can have resorption of NaCl at the upper part of the medulla, and the collecting duct is largely impermeable to water unless ADH acts on it. The body uses ADH to control how much water we retain.
  b. Urea is also resorbed here which maintains the medulla’s osmolarity (although sometimes it can re- enter the tubule at the Loop of Henle — a phenomenon known as urea recycling)

Answer 7

4. Collecting duct

Question 8

1. Urea first descends to the medulla (salty part) where antidiuretic hormones (ADH/vasopressin) can make more water leave from urine by increasing permeability of the collecting duct (via increased aquaporins) → urine is even more concentrated. Note that one collecting duct is shared by many nephrons.
2. Aldosterone can also act on the collecting duct by increasing Na+ reabsorption, resulting in water passively following Na+
3. By the time urine emerges, it usually has varying amounts of: H2O, urea, NaCl, K+, and creatinine
   - Note: Alcohol blocks the creation of vasopressin and leads to more urine output since less H2O is resorbed by the body!

Answer 8

Path of Urea through collecting duct