Physiology 2 - Reabsorption & Secretion

Question 1

What are the 2 main methods of reabsorption and where does it mainly take place?

Answer 1

• Carrier mediated transport
  
  • Glc/AA/Organic Acids/Sulphate and phosphate ions
    
    • Carrier proteins enable larger molecules such as glucose to cross the membrane
• Active Transport
  
  • Na+

Mainly occurs in the Proximal Tubule

Question 2

Explain the concept of a Renal Plasma Threshold?

Answer 2

Carrier Mediated Transport systems have a Maximum transport capacity (Tm) before they become saturated

• This is called the Renal plasma theshold
  
  • E.g. Glc transports have a Threshold of 10mmol/l. So if plasma [Glc] is higher than this (not physiological) the excess is excreted in urine
    
    • If plasma [glc} = 15 mmoles/l, 15 will be filtered and 10 reabsorbed and 5 excreted

Question 3

Is Tm above or below the physiological concentration?

Answer 3

It depends on the substance

• AA and Glc Tm is above physiological levels so excretion should not occur
• Phosphate and Sulphate ions have Tm below physiological [plasma] to ensure excretion

Question 4

Explain how soidum ions are reabsorped?

Answer 4

Active Transport

• Na/KATPase pumps Na from the tubular cells into the interstitial fluid
  
  • Generates a concentration gradient which draws more Na out the tubule into the cells

Question 5

How much Na ions are reabsorbed daily?

Answer 5

180 L/day (filtrate) x 142 mmole/l (plasma [conc] of Na+) = 25560 mmoles/day

• 99.5% is reabsorbed

Question 6

Why will Na+ not be absorbed by all cells?

Answer 6

Not all cells have the same permeability

• Brush border of the proximal tubule cells has a higher permeability to Na+ ions than most other membranes in the body
  
  • Partly because of the enormous surface area offered by the microvilli and the large number of Na+ ion channels
  • Facilitate the passive diffusion of Na+

Question 7

What is a side effect of Na+ being reabsorped?

Answer 7

Creates an electrical gradient down which Cl- and other anions can be reabsorbed

Question 8

What is a consequence of N and Anion reabsorption?

Answer 8

Movement of so many ions creates an osmotic force that causes water to be reabsorped

Question 9

What happens after the water is reabsorped?

Answer 9

Loss of water causes the remaining substances in the tubule to be concentrated

• Creating outward conc. gradients allowing them to be reabsorped (E.g. K+/Ca2+/Urea/Glc)

Question 10

Summarize the resorption of sodium and its effects?

Answer 10

1. Na reabsorped by active transport usin Na/KATPase
2. Na movement generates an electrical gradient that allows anions to be reabsorped
3. Na/Anion movement generates an osmotic force that reabsorps H2O
4. aH20 movement concentrates the remaining substances, creating conc. gradients that allow thier resorption

Question 11

What effects rate of absorption of the remaining solutes after H2O is reabsorped?

Answer 11

1. Amount of H2O removed and therefore Conc gradient
2. Permeability of membrane to the particular solute

Question 12

How are active transport of sodium and carrier mediated transport of substances like Glc/AA linked?

Answer 12

Na enters the tubular cells from the tubule through symporters with substances such as glucose (SGLT)

• Hence Na+ deficiency in the tubule will affect glucose etc resorption

Question 13

What is Tubular secretion?

Answer 13

Transport of substances from the peritubular capillaries into the tubule lumen

• Opposite of reabsorption

Question 14

What part of the nephron lacks secretion?

Answer 14

The Loop of henle (only reabsorption occurs there)

Question 15

What substances use secretion most and why?

Answer 15

• Protein bound substances which need to be actively secreted as they’re too big for filtration
• Harmful substances that have to be prevented from reabsorping

Question 16

By what mechanism are substances secreted?

Answer 16

Carrier-mediated secretory mechanisms (Like reabsorptive ones they have a Tm)

Question 17

Drugs are also secreted by the nephron, kinda odd we have mechanisms to remove substances we should never have in our system, why is this?

Answer 17

Our Secretory Carrier Mechanisms are just very non-specific so for instance the organic acid mechanism is also capable of secreting aspirin

Question 18

How is potassium handled by our kidneys?

Answer 18

• Filtered at the glomerulus
• Reabsorped by the Proximal tubule
• Secreted in the Distal Tubule

Question 19

What controls how much K we secrete?

Answer 19

• Renal tubule K+ concentration (more K+= More K+ secreted)
• Aldosterone (High ECF [K+] stimulates aldosterone-secreting cells).

Question 20

How is H+ secretion different?

Answer 20

ITs actively secreted as part of the acid/base balance from tubule cells rather than the peritubular capillaries

Question 21

Most drugs and pollulants are non-polar (i.e. lipid soluble) meaning the membrane is highly permeable and they’re easily reabsorped. How does our body enable us to excrete them?

Answer 21

The liver metabolizes such compounds down to polar compounds that can’t be reabsorped and so are secreted then excreted

Question 22

What is an anion?

Answer 22

Anion = -vely charged ions