Lecture 4 – Tubular Reabsorption/Secretion

1) What’s the difference between paracellular and transcellular transport?  What are the two membranes a substance would have to cross during transcellular transport?

Paracellular transport = substances pass between epithelial cells.
Transcellular transport = substances move through the epithelial cells, crossing the luminal membrane and then the basolateral membrane.

2) How do the peritubular capillaries facilitate reabsorption of substances/fluid from the renal tubule?

Peritubular capillaries have low hydrostatic pressure and high osmotic pressure, which facilitates reabsorption of substances from the renal tubule into the blood.

3) What is transport maximum and what is it dependent upon?

Transport maximum (Tm) = the maximum amount of a substance that can be reabsorbed in a given time. It depends on the number of transport proteins available for that substance

4) What is renal threshold? What happens to a substance if it is in the tubular fluid, but the plasma is already at renal threshold for that substance?

Renal threshold = the maximal plasma concentration a substance can reach before it is no longer reabsorbed.
If plasma is at renal threshold, any of that substance in the tubular fluid will not be reabsorbed and will be excreted in urine.

5) Which substances will be completely reabsorbed from the tubular fluid (if the blood isn’t at renal threshold)?

All nutrients such as glucose and proteins are completely (100%) reabsorbed from the tubular fluid in the PCT (if below renal threshold)

6) To reabsorb glucose what ion is necessary and why?

Na+ (sodium) is necessary because Na+ moving down its gradient provides the energy for secondary active transport of glucose through a Na+/glucose symporter.

7) Are peptides reabsorbed in their peptide form? If not, what must happen to the peptide before it can be reabsorbed into the blood?

No, peptides are not reabsorbed in their peptide form. They are digested by lysosomes into amino acids, which then move by facilitated diffusion into the blood.

8) Which part of the renal tubule reabsorbs most of the filtered sodium? And in what parts of the renal tubule is sodium reabsorption regulated?

Most Na+ is reabsorbed in the PCT (65%).
Regulated Na+ reabsorption occurs in the DCT, collecting tubules (CT), and collecting ducts (CD).

9) What two transport proteins are needed to reabsorb sodium and which one uses up 80% of the energy used by the kidneys?

The two transport proteins are the Na+ channels and the Na+/K+ pumps.
The Na+/K+ pumps use 80% of the kidney’s active transport energy.

10) What two hormones regulate the amount of sodium excreted in our urine?

The two hormones that regulate Na+ excretion are Aldosterone and Atrial Natriuretic Peptide (ANP).

11) Which cells of the DCT and CT/CD are stimulated by aldosterone and what do they do once stimulated?

Principal cells in the DCT and CT/CD are stimulated by Aldosterone.
They synthesize more Na+ channels and Na+/K+ pumps, leading to Na+ reabsorption and K+ secretion.

12) How is potassium affected by aldosterone?

Aldosterone causes K+ secretion from principal cells into the tubular fluid

13) Which two portions of the renal tubule are affected by ANP?

Atrial Natriuretic Peptide (ANP) affects the PCT and CT, inhibiting Na+ reabsorption.

14) Obligatory water reabsorption occurs because water follows which ion?

Obligatory water reabsorption occurs because water follows Na+ by osmosis

15) In what region of the renal tubule is most water reabsorbed? Which regions of the renal tubule are impermeable to water?

Most water is reabsorbed in the PCT (65%).
The ascending limb of the nephron loop and the DCT are impermeable to water

16) What hormone controls facultative water reabsorption? If more of this hormone were secreted, what would happen to the urine (becomes more dilute or more concentrated?)

Antidiuretic Hormone (ADH) controls facultative water reabsorption.
If more ADH is secreted, urine becomes more concentrated (darker yellow

17) Which cells of the CT and CD are affected by ADH and what do they do when stimulated?

Principal cells in the CT and CD are affected by ADH.
When stimulated, they increase aquaporins, allowing more water reabsorption

18) What is a diuretic and in what two ways do they work?

A diuretic is a substance that increases urine production.
They work by inhibiting ADH release or inhibiting Na+ reabsorption, which reduces water reabsorption.

19) In which region of the renal tubule is most potassium reabsorbed and what is K⁺ reabsorption dependent on?

Most K+ is reabsorbed in the PCT (60–80%), and it is dependent on Na+ movement (and the resulting water movement)

20) Which two cells within the CT and CD affect potassium levels and what do they each do?

In the CT and CD, Type A intercalated cells continuously reabsorb K+, while Principal cells secrete K+ (regulated by Aldosterone).

21) How does PTH affect the reabsorption of calcium and phosphate? How does this affect blood calcium levels?

PTH inhibits phosphate (PO43-) reabsorption in the PCT and stimulates Ca2+ reabsorption in the DCT.
This causes blood calcium levels to increase.

22) In which region of the renal tubule is most bicarbonate reabsorbed? Describe the process of bicarbonate reabsorption.

Most bicarbonate (HCO3-) is reabsorbed in the PCT (80–90%).
Process: HCO3- binds H+ → forms carbonic acid (H2CO3) → breaks into CO2 + H2O → CO2 diffuses into cell → reforms HCO3- + H+ → HCO3- enters blood, H+ returns to tubular fluid (with carbonic anhydrase).

23) Which regions of the renal tubule are important in pH balance?

pH balance is regulated in the DCT, CT, and CD.

24) How do Intercalated A cells affect blood pH? What about intercalated B cells?

Type A intercalated cells secrete H+ into the tubular fluid and reabsorb HCO3- into the blood → raises blood pH.
Type B intercalated cells secrete HCO3- into tubular fluid and reabsorb H+ into blood → lowers blood pH.

25) Urea is not only secreted as a waste product but then also ‘reabsorbed’. How much filtered urea is excreted with the urine?

50% of filtered urea is excreted in urine (after partial reabsorption and recycling)

26) Besides nitrogenous wastes, what other types of substances are secreted out of the blood and into the tubular fluid by the kidneys?

The kidneys also secrete drugs (e.g., antibiotics, morphine, chemotherapy drugs, marijuana chemicals) and bioactive substances like urobilin, hormone metabolites, and hormones (e.g., epinephrine, prostaglandins, hCG).

27) Renal plasma clearance is the comparison of how much of a particular substance was filtered from the glomerulus versus how much of that substance ended up in the urine. If the renal plasma clearance of a substance is 0, what does that mean?

If renal plasma clearance = 0, that means the substance is completely reabsorbed (e.g., glucose).

28) The interstitial concentration gradient of the medulla is important to what function of the kidneys? What are the three things that establish and maintain the concentration gradient?

The interstitial concentration gradient is important for facultative water reabsorption and ADH function.
It is established by:

1. Countercurrent multiplier (nephron loop)

2. Countercurrent exchange system (vasa recta)

3. Urea recycling

29) From what part of the renal tubule is urea actively moved from the tubular fluid and into the interstitial fluid around the tubules?  The urea then diffuses back into the tubular fluid in what part of the renal tubule?

Urea moves from the collecting tubule (CT) into the interstitial fluid, then diffuses back into the thin ascending limb of the nephron loop.

30) Why does urea play such an important role in the interstitial concentration gradient?

Urea makes up half of the solutes that create the interstitial concentration gradient, helping maintain osmotic pull for water reabsorption.

31) As the tubular fluid flows through the descending limb of the nephron loop, what is diffusing out of the renal tubule (and into the interstitial space)? What is actively pumped out of the tubular fluid as it flows through the ascending limb?

In the descending limb, water diffuses out into the interstitial space.
In the ascending limb, salts are actively pumped out, but water remain

32) As blood is traveling through the vasa recta near the ascending limb of the nephron loop, what is moving into the blood? What is moving out of the blood?

Near the ascending limb, salts move into the blood, and water moves out of the blood.

33) As blood is traveling through the vasa recta near the descending limb of the nephron loop, what is moving into the blood? What is moving out of the blood?

Near the descending limb, water moves into the blood, and salts move out into the interstitial fluid.