Lecture 18: regulation of ECF volume Flashcards
What ions are the main osmotically active solutes of the ECF?
Na+ - as it makes up 90 % of the ECF, the body content of Na+ is the primary determinant of ECF volume
What happens to the ECF volume is the ECF content changes?
- if the ECF content changes, then the ECF volume will also changes
- EG - if we increase the body Na+ content by ingesting NaCl, then water will be retained to restore normal blood osmolality so then the body fluid volume will increase
What is the effective cirulating volume?
- the ECV is the component of the ECF which is perfusing the tissues
- the control of the ECV is dependent on sodium balance
What are the 3 ways by which Na+ balance is regulated?
1.** renal nerves**
2.** RAAS system** - renin-angiotensin-aldosterone system
3. natriuretic peptides
What are the fast, intermediate and slow responses to a** change in arterial pressure**?
- a change in arterial pressure causes a fast baroreceptor reflex to restore the pressure
- it causes an intermediate response - through renal nerves and changes in peripheral resistance to restore pressure
- After hours or days, there are prolonged renal actions that involve the excretion of salt and water
What is the RAAS system?
- Renin
- Angiotensin
- Aldosterone system
responds to a lower bp to bring pressure back up
Where is renin stored and released from?
the granular cells of the juxtaglomerular apparatus
What are the 3 main mechanisms that stimulate renin release into the circulation?
- a decrease in volume of ECF causes increased sympathetic nerve activity
- as a result of a decrease in pressure & ECV, there is a decrease in the wall tension in the afferent arterioles which is sensed by the pressure sensitive granular cells
- macula densa mechanism
How is sympathetic activity related to an increase in renin release?
- when the effective circulating volume is reduced, the ECF volume is reduced which leads to a reduction in blood pressure
- this leads to the activation of the baroreceptor reflex
- the reflex leads to increased sympathetic activity
- NA binds to the beta 1 receptors on the granular cells which stimulates the release of renin
What enzyme does renin cleave & what product is formed as a result of this?
angiotensinogen
* angiotensin I is formed as a result of this
How does angiotensin I get converted to angiotensin II?
through the angiotensin converting enzyme (ACE)
What are the main functions of angiotensin II?
- it causes systemic vasoconstriction which increases total peripheral resistance and leads to an increased blood pressure
- it stimulates the adrenal cortex to produce aldosterone
- it increases PCT Na+ reabsorption by stimulating the Na+-H+ antiporter
acts on the hypothalamus to increase thirst
increases secretion of ADH by the posterior pituitary gland
Describe the function/mechanism of aldosterone
- aldosterone freely crosses cell membranes and binds to **mineral corticoid receptors **
- In the nucleus, the receptor acts as a transcription factor that promotes gene expression and synthesis of Na+-K+ATPase pump and** K+ channels and also ENac channels **
What other factors relating to cincentration of substances in the plasma determine how much aldosterone is secreted?
- a small increase in plasma K+ concentration increases aldosterone release
- a decrease in plasma Na+ concentration increases aldosterone release
How do natriuretic peptides regulate Na+ balance?
- Np’s act against the RAAS system
- they inhibit renin secretion by granular cells
- they** inhibit aldosterone secretion**
- they **inhibit NaCl reabsorption **by the collecting duct
- therefore they work to increase Na+ excretion by increasing the GFR and reducing Na+ reabsorption
