Lecture 46 - Urinary System: Tubular Reabsorption and Secretion

Question 1

What is diabetes mellitus

Answer 1

It is a disorder of glucose metabolism associated with elevated plasma glucose levels

Question 2

How do we detect that someone has diabetes mellitus (especially T1DM)

Answer 2

By urinary symptoms. In diabetes mellitus, there is:

• Glycosuria (glucose in urine)
• Polyuria (increased urine production)

Question 3

What is the renal tubule’s (and collecting duct’s) 3 general functions?

Answer 3

1. Remove nutrients and proteins from tubular fluid
2. Reabsorb filtered fluid (water and ions)
3. Concentrate waste products in tubular fluid

Question 4

What is the proximal convoluted tubule (PCT) responsible for? Where is it found?

Answer 4

It’s found within the renal cortex and is closely associated with peritubular capillaries

It’s responsible for reabsorption and secretion within a nephron. The PCT is the only part of the nephron capable of absorbing organic substances such as nutrients and filtered proteins

Question 5

What type of epithelium lines the PCT? What is the function of this type of epithelium and explain its brush border

Answer 5

Leaky simple cuboidal epithelium with microvilli

In a leaky epithelium, the tight junctions allow high bulk flow of water and ions between the cells (paracellular flow). The apical surface of PCT epithelial cells has a “brush border” (microvilli), which massively increases its surface area, inc. absorption and secretion

Question 6

Where are co-transporters for filtered nutrients (ex. glucose) found? Where are transporters found?

Answer 6

Co-transporters for Na+ and glucose or Na+ and amino acids are found on the apical surface of the PCT epithelium

Transporters for glucose or amino acids are found on the basolateral surface

Question 7

What is the function of co-transporters in the PCT?

Answer 7

Co-transport with sodium allows for 99% of filtered nutrients to be actively removed from the tubular fluid through transcelllular transport

Question 8

Describe the path of glucose in the PCT and its associated co-transporters/transporters

Answer 8

Co-transporters will rapidly remove glucose against its concentration gradient from the tubular fluid in the lumen of the PCT cell due to favourable gradient for Na+ into the ICF. Glucose in the ICF diffuses into the ISF using passive (facilitated) transport. Glucose in the ISF is returned to peritubular capillaries through diffusion

Question 9

Where does reabsorption of water occur?

Answer 9

It occurs in the PCT through osmosis via the paracellular route and recaptures 2/3 of total filtered fluid per day

Question 10

What occurs when glucose (and other small molecules) are pumped out of the lumen into PCT cells (and then on into ISF)

Answer 10

The osmolarity of tubular fluid will dec. and the osmolarity of ICF (intracellular fluid) and ISF will inc.

This osmotic gradient draws water into ISF (through the leaky tight junctions of the epithelium)

Question 11

T/F? In diabetes mellitus, extra filtered glucose leads to insufficient urine production

Answer 11

False - in diabetes mellitus, extra filtered glucose leads to excess urine production

Question 12

Describe what happens if plasma (and filtrate) concentrations of glucose are high

Answer 12

Glucose transport by PCT will saturate, since transporter and co-transporter proteins have limited number of binding sites available (ie. they are saturable)

If glucose is not reabsorbed by the PCT, tubular fluid osmolarity will remain high and water will also remain

Question 13

Name 3 things that happens if filtered glucose is not all pumped out of the lumen into PCT cells

Answer 13

1. The osmolarity of tubular fluid will remain high
2. The osmotic gradient from lumen to ISF will be reduced
3. Less water will be reabsorbed by the PCT epithelium and more will remain within the lumen (turning into urine)

Question 14

How do PCT cells handle albumin and small-medium sized plasma proteins?

Answer 14

They can reabsorb albumin and other small-medium sized plasma proteins by receptor-mediated endocytosis

From there they are either returned to the plasma (albumin) or broken down and excreted by tubular cells (most other proteins)

Question 15

What is the secondary function of the proximal and distal convoluted tubules

Answer 15

They secrete:

• Nitrogenous wastes (ex. urea, uric acid, ammonium)
• Metabolic acids
• Toxins/drugs

Question 16

How do PCT and DCT cells regulate pH?

Answer 16

Epithelial cells in the convoluted tubules express carbonic anhydrase, which allows them to convert CO2 to H2CO3 (ie. H+ and HCO3-)

H+ can be excreted into the lumen via counter-transporters, neutralized, and excreted. This allows for HCO3- reabsorption (as CO2)

Question 17

What is produced when amino acid catabolism occurs within PCT and DCT?

Answer 17

It makes ammonium ions (NH4+) and ammonia (NH3). NH4+ is excreted by counter-transport and NH3 diffuses through the plasma membrane

Question 18

How does the PCT (and to some extent the DCT) secrete toxins and drugs?

Answer 18

Through general organic cation and anion transporters. Kidney tubular cells express:

• Transporter and pump proteins that can transport a wide variety of organic cations (OC+)
• Transper and pump proteins that can transport a wide variety of organic anions (OC-)