Osmolarity and Volume

role of kidneys in Na+ homeostasis

regulate ECF osmolarity and volume

• a change in ECF [Na+] will change ECF osmolarity - kidneys regulate ECF osmolarity by adjusting [Na+] of ECF

• a change in ECF Na+ amount will change ECF volume (*intravascular volume) - kidneys can regulate ECF volume by adjusting ECF Na+ amount (via how much Na+ is excrete in urine)

relationship between [Na+] and osmolarity

[Na+] inc → osm inc

[Na+] dec → osm dec

fluids flow from low to high osm

low out = swelling = bad

high out = shrink = bad

hypernatremia

high ECF [Na+]

fluid leaves cells = shrink

• rupture of cerebral vesseles/hemmorage

• muscle weakness

• behavioral changes/ataxia

• coma → death

hyponatremia

low ECF [Na+]

fluid into cells = swelling

• muscle weakness

• seizures

• cerebral/pulmonary edema

which statement about hyponatremia is most accurate?

a. ECF [Na+] will be higher than normal

b. osmolarity of extracellular fluid is likely to decrease

c. osmoreceptors in hypothalamus will shrink

d. ADH release will be increased

e. the animal will feel thirsty

b. osmolarity of extracellular fluid is likely to decrease

defense against osmolarity changes

kidneys can regulate ECF osmolarity by changing ECF Na+ concentration

[Na+] is the amount of Na+ in a unit volume of solvent (water)

1. osmoreceptors in hypothalamus shrink/swell as ECF osmolarity changes: elevated osmolarity = ADH release

2. stimulation of thirst mechanism: animal consumes fluid

hypervolemia

high ECF volume

• ascites

• pulmonary edema

hypovolemia

low ECF volume

• hypovolemic shock

• organ damage

struggle to get nutrients and O2 to tissues and to get waste away, heart is working v. v. hard

disorders of ECF Na+ amount → change in ECF volume

changes in ECF Na+ amount can cause volume changes - excessive salt consumption, excessive renal loss of Na+, etc.

ECF volume disorders can also be caused by other things - blood loss, vomiting, liver failure, etc

defense against volume changes

baroreceptors: stretch receptors in heart, aorta, and carotid sinus

• sympathetic flow → inc Na+ reabsorption, inc volume

• natriuretic peptide release → dec Na+ reabsorption, dec volume

juxtaglomerular apparatus: stretch receptors in afferent arterioles

• Renin-Angiotensin System → inc Na+ reabsorption, inc volume

three mechanisms regulate GFR and reabsorption to control renal Na+ retention

Na+ retention effect on ECF volume

• inc Na+ retention = inc ECF Na+ amt = inc ECF volume

• dec Na+ retention = dec ECF Na+ amt = dec ECF volume

sympathetic flow

increased sympathetic flow to kidneys increases ECF volume

baroreceptors sense low ECF volume → sympathetic flow to kidney inc → promotes renal reabsorption of Na+

3 mechanisms norepinephrine

• is a vasoconstrictor

• promotes renin release

• directly increases Na+ reabsorption from proximal tubule

norepinephrine is a vasoconstrictor

preferentially constricts efferent arteriole to increase GFR

alters starlings forces (at proximal tubule) at peritubular capillaries to increase Na+ reabsorption

how Starlings forces are affected

changes driven by efferent arteriole constriction

• capillary hydrostatic pressure (Pc) - decrease : less blood in arteriole

• tubular hydrostatic pressure (Pt) - increase

• capillary hydrostatic pressure (πc) - increase : protein in capillary stays, fluid leaves

• tubular oncotic pressure (πt) : not effected or slight, inc volume may reduce [protein]

net effect of increased sympathetic flow is to promote Na+ (and water) reabsorption from proximal tubule into peritubular capillaries

norepinephrine promotes renin release

triggers the renin-angiotensin system

renin release from kidneys inc ECF volume

low ECF volume triggers renin release from juxtaglomerular apparatus, RAS effectors inc renal absorption of Na+

norepinephrine directly increases Na+ reabsorption from proximal tubule

stimulates Na+/H+ antiporter

natriuretic peptides

trigger Na+ excretion in urine

decrease ECF volume

high ECF volume stretches cardiac muscle, releasing natriuretic peptides, promotes excretion of Na+

which statement about hypovolemia is accurate?

a. it is defined as high ECF volume

b. it is caused by low ECF Na+ concentration

c. the animal will respond increasing sympathetic flow to the kidneys

d. to correct it the Renin-Angiotensin system will be inhibited

c. the animal will respond increasing sympathetic flow to the kidneys

Low ECF volume id returned to normal by which of the following?

a. stimulation of Renin-Angiotensin-Aldosterone System (RAAS)

b. inc natriuretic peptide release

c. inc sympathetic flow to kidneys

a. stimulation of RAAS

AND

c. inc sympathetic flow to kidneys