Regulation of ECF and NaCl

Question 1

Effective circulating volume (ECV)

Answer 1

Portion of ECF volume that is in the arterial system under particular pressure and is effectively perfusing the tissues

The ECV is not a measurable and distinct body fluid compartment

About 0.7 L of vascular volume forms the ECV

Question 2

Congestive Heart Failure

Answer 2

Patients with CHF have low effective circulating volume due to decrease cardiac output

Na+ and fluid retention resulting in edema in which venous and capillary hydrostatic pressure increase

Patients continue to retain Na+, increasing the ECF volume without correcting the effective circulating volume

Question 3

Ways a decreased ECV is counteracted

Answer 3

1) Activation of RAAS
2) Stim of the sympathetic nervous system via the baroreceptor reflex
3) Increased ADH secretion
4) Increased renal fluid retention via altered starlings forces in the peritubular capillaries

Question 4

Osmoreceptors

Answer 4

Neurons that initiate the reflexes controlling ADH secretion in response to changes in osmolality

They detect changes in the plasma osmolality

Located in supraoptic and pararventricular areas of the hypothalamus

Functions: Regulate ADH release and regulate thirst

Question 5

ADH Secretion

Answer 5

Controlled by osmoreceptors and baroreceptors (osmoreceptors are more sensitive)

When blood pressure or volume are reduced, ADH released to increase fluid reabsorption

Question 6

Sensed volume on osmolality and ADH release

Answer 6

Decreases of sensed volume “sensitize” the system, so small changes of osmolality induce larger amounts of ADH release

If the body senses volume expansion, there is a higher threshold for release of ADH and less vigorous response to progressive hyperosmolarity

Question 7

Baroreceptors

Answer 7

Arterial baroreceptors sense pressures in the aorta and carotid arteries and send afferent info to the brainstem vasomotor center, which then regulates CV and renal processes via autonomic efferents

Cardiopulm baroreceptors sense pressure in the cardiac atria and pulm arteries, thereby being responsive to the filling of the vascular tree

Cardiopulm baroreceptors influence the hypothalamus which regs ADH

Question 8

ADH responds to changes in plasma osmolality

Answer 8

1) Changes in Na intake or output that result in changes in total body Na content can cause a change in plasma osmolality
2) Body alters water intake and renal water reabsorption to maintain an isotonic ECF
3) osmolality is maintained at the expense of ECF volume

Decrease in total body Na content–> eventual ECF contraction

Gain in total body Na–> eventual ECF expansion

Question 9

Secretion of Renin

Answer 9

Three major mechanisms for reg of renin release:

1) Perfusion pressure
- Low perfusion in afferent arterioles stims renin secretion while high perfusion inhibits renin secretion

2) Sympathetic nerve activity
- Activation of the sympathetic nerve fibers in the afferent arterioles increases renin secretion

3) NaCl delivery to macula densa
- When NaCl is decreased, renin secretion is stimulated and vice versa

Question 10

ANP

Answer 10

ANP is produced in the atrial cardiac muscle cells and induces natriuresis

Released from heart muscle cells when they are mechanically stretched by expansion of the circulating plasma volume

Directly inhibits Na+ reabsorption in the distal part of the nephron, thus increasing Na+ excretion and H2O excretion in the urine

Increase Na+ excretion in the urine by inhibiting RAAS
Inhibits release of ADH and renin

Question 11

Na+ excretion with volume expansion

Answer 11

1) Decreased activity of the renal sympathetic nerves
2) Increased release of ANP and BNP from the heart and urodilatin by the kidneys
3) Inhibition of AVP secretion from the posterior pituitary and decreased AVP action on the collecting duct
4) Decreased renin secretion and thus decreased production of angiotensin II
5) decreased aldosterone secretion, which is a consequence of reduced angiotensin II levels, and elevated natriuretic peptide levels

Question 12

Na+ excretion with volume contraction

Answer 12

1) Increased renal sympathetic nerve activity
2) Increased secretion of renin, which results in elevated angiotensin II levels and thus increased secretion of aldosterone by the adrenal cortex
3) Stim of AVP secretion by the posterior pituitary

Question 13

Hyponatremia

Answer 13

Serum sodium of <135 mEq/L and serum osmolality of <280 mEq/L
Describes state when sodium concentration in the plasma (outside the cell) is lower than normal

Cells swell due to water influx