Lecture 39: Plasma volume and sodium homeostasis

Question 1

What is the major ion in the extracellular fluid?

What is its major role/function?

Answer 1

Soduim

• is the major contributor to the osmolarity of the extracellular fluid
• change in the concentration of sodium in the ECF will change the osmolarity of the ECF and potentially cause cells to swell or shrink

Question 2

How do we change the osmolarity of the cell?

How do we change the volume of the ECF?

Answer 2

Osmolarity change: release of antidiuretic hormone changes the amount of water in the ECF which corrects the osmolarity of the ECF

Volume change: kidneys adjust their sodium excretion output which can alter the volume of ECF

Question 3

In order to monitor changes in sodium concentration, we monitor changes in plasma volume.
- changes in plasma volume are sensed at with three main sites?

Answer 3

Low pressuure baroreceptors: found in the vena cava, right atrium and pulmonary blood vessels

High blood pressure baroreceptors: found in the carotid sinus and aortic arch

Intra-renal baroreceptors: afferent arteriole and macula densa

Question 4

What are the mechanisms for regulating Na+ and plasma fluid volume?

Answer 4

• Atrial natriuretic peptide (ANP/ANH)
• Renal sympathetic nerves
• Anitdiuretic hormone
• Renin-Angiotensin
• Aldosterone

Question 5

What is aldosterone?

How does it regulate Na+ and plasma fluid volume?

Answer 5

Aldosterone is a steroid hormone which is secreted from the adrenal gland. Secretion is stimulated by angiotensin II and increased extracellular K+
- Aldosterone acts on the distal tubule and collection duct = increases reabsorption and secretion of K+

Question 6

How does ADH regulate Na+ and plasma fluid volume ?

Answer 6

= Alters fluid balance to help correct volume
ADH increases the permeability of the collecting duct to water, so more water is either retained or excreted —-> leads to a change in TBW, which helps restore ECF osmolarity to normal

Question 7

How does ANP regulate Na+ and plasma fluid volume

Answer 7

The atria secrete ANP in response to increased volume which:
- increases filtered load of Na+
- decreases tubular reabsorption of Na+
- decreases renin secretion
which results in = increase in sodium excretion, water is lost, volume and blood pressure is reduced

Question 8

How do the renal sympathetic nerves regulate Na+ and plasma fluid volume ?

Answer 8

The renal sympathetic nerves are activated in response to low volume
- decrease filtered load of Na+
- increase tubular reabsorption of Na+
- increase renin release
which results in = decreased Na+ excretion, Na+ is retained with water and volume increases

Question 9

What is renin-angiotensin ?

Answer 9

Renin is an enzyme secreted by the JGA which cleaves angiotensinogen into Angiotensin I
- This angiotensin I is then cleaved to active angiotensin II by the angiotensin converting enzyme ACE.

Question 10

What can active angiotensin do? What are its effects?

Answer 10

• potent vasoconstricter of blood vessels
• It increases sodium reabsorption in the proximal tubule
• It increases the release of aldosterone from the adrenal glands

= increase volume by decreasing Na+ and water secretion to preserve/restore volume

• stimulates reabsorption of Na+ and decreases filtered load of Na+
• increases aldosterone secretion with also stimulate Na+ reabsorption