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 for things like Glc/AA/Organic Acids/Sulphate and phosphate

Active Transport for Na

Mainly 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 your plasma Glc is higher than this (not physiological) the excess is excreted in urine

Question 3

Is Tm above or below the physiological concentration?

Answer 3

It depends on the substance
AA & Glc Tm is above physiological levels so excretion should not occur

But 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.
This generates a concentration gradient which draws more Na out the tubule into the cells

Question 5

What is a side effect of Na+ being reabsorped?

Answer 5

It creates an electrical gradient down which Cl- and other anions can be reabsorped

Question 6

What is a consequence of Na& Anion reabsorption

Answer 6

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

Question 7

What happens after the water is reabsorped?

Answer 7

Loss of water causes the remaining substances in the tubule to be concentrated. Creatin outward conc. gradients allowing them to be reabsorped
(E.g. K+/Ca2+/Urea/Glc)

Question 8

Summarize the resorption of sodium and its effects?

Answer 8

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) H20 movement concentrates the remaining substances, creating conc. gradients that allow thier resorption

Question 9

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

Answer 9

1) Amount of H2O removed and therefore Conc gradient

2) Permeability of membrane to the particular solute

Question 10

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

Answer 10

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 11

What is Tubular secretion?

Answer 11

Transport of substances from the peritubular capillaries into the tubule lumen
Opposite of reabsorption

Question 12

What part of the nephron lacks secretion?

Answer 12

The Loop of henle (only reabsorption occurs there)

Question 13

What substances use secretion most and why?

Answer 13

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 14

By what mechanism are substances secreted?

Answer 14

Carrier-mediated secretory mechanisms

Like reabsorptive ones they have a Tm

Question 15

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 15

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

Question 16

How is potassium handled by our kidneys?

Answer 16

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

Question 17

What controls how much K we secrete?

Answer 17

Renal tubule K+ concentration (more K+= More K+ secreted)

Aldosterone (High ECF [K+] stimulates aldosterone-secreting cells).

Question 18

How is H+ secretion different?

Answer 18

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

Question 19

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 19

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

Question 20

What is reabsorbed?

Answer 20

99% H2O, 100% glucose, 99.5% Na+, 50% urea filtered

Question 21

How does the kidney regulate the reabsorption of sodium/potassium

Answer 21

• Tm is set at a level where normal plasma concentration = saturation
• Any extra is excreted in urine
• Phosphate levels are also regulated via the PTH so: high PTH decreases reabsorption

Question 22

How much sodium is filtered per day?

Answer 22

Most filtered: 180 l/day x 142 mmoles/l = 25560 mmoles/day

Question 23

How is sodium is reabsorbed?

Answer 23

99.5% is reabsorbed

65-75% is reabsorbed in the proximal tubule

Question 24

Where are the active sodium pumps located?

Answer 24

1-Active sodium pumps are located in the basolateral membrane (in contact with the intersistial fluid)
Lots of mitochondria found at the basolateral membrane
This transports lots of sodium into the epithelial cells
2-This creates a diffusion gradient for the sodium to then be transported passively into the the tubule cells
3-From the tubule cells the sodium will then repeat stage 1

Question 25

How come sodium can move passively in the basolateral membrane?

Answer 25

Usually sodium is not permeable to membranes so cannot move passively.
The brush border of the proximal tubule has a higher permeability due to large SA:
-Microvilli
-lots of sodium ion channels for passive diffusion

Question 26

what happens when potassium levels are abnormal?

Answer 26

Normal ECF concentration= 4mmmol/l

Anything over 5.5 mmol/l- hyperkalemia: decreased resting membrane of excitable cells leads to ventricular fibrillation

Anything under 5.5mmol/l- hypokalemia- increases resting membrane of excitable cells leads to hyperpolarisaion= arrhythmias and death.

Question 27

How is K+ controlled by aldosterone

Answer 27

K+ secretion is also controlled by aldosterone:

• An increase in K+ bathing the cells which produce aldosterone causes an increase in aldosterone production
• Aldosterone travels to the kidneys
• Stimulate K+ secretion in the renal tubule
• also stimulates the reabsorption of sodium at the distal tubule via a reflex pathway