Lectures 35 and 36: Fluid and Electrolyte Balance

Regulation of MAP

1. Cardiovascular —> Baroreceptor reflex —> Fast

• Cardiac output and TPR

2. Kidneys —> Renin, ANG II, Aldosterone —> Slow

• Regulated by Aldosterone (ANG II), ADH, ANP

Pathway for decreased BP/volume 

• Decrease in blood pressure/volume → detected by JG cells in kidneys → release renin → angiotensinogen produced from liver and renin produce ANG I → ACE (enzyme) converts ANG I to ANG II → affects CVCC, Arterioles, Adrenal cortex, Hypothalamus 

  • Adrenal cortex → release aldosterone → acts on P cells in distal tubules → increase Na+ reabsorption

  • Hypothalamus → increase thirst and increase ADH 

  • CVCC of increase heart rate and vasoconstriction arterioles result in increased blood pressure 

Adrenal Cortex

Released aldosterone —> Acts on P cells in distal tubules —> increase Na+ reabsorption —> Water follows —> increase blood volume —> Increase blood pressure

Hypothalmus on posterior pituitary

Increase ADH —> act on collecting duct cells —> increase aquaporins —> increase water reabsoprtion —> more water in collecting duct for increased blood volume —> increased blood pressure

Pathway for increased plasma osmolarity

Activates hypothalmus for increase in ADH want increase water reabsorption

Inhibits adrenal cortex because you don’t want aldosterone for increase Na+ reabsorption as it will increase osmolarity

Increased plasma osmolarity inhibits

adrenal cortex —> decrease aldosterone

Pathway for increased BP/volume

Increased BV —> Increase BP —> Increased atrial stretch —> release ANP —> dilate afferent arteriole and relax mesangial cells —> increase GFR —> excrete more volume —> lower BV —> lower BP

JG cells

sit on afferent arteriole

sense decreased blood pressure to release renin

Aldosterone

Increases Na+ reabsorption

Can easily diffuse across membrane due to cholesterol backbone —> can affect transcription

Increases channel activity

Absorb more Na+, more K+ secreted

ANP

Inhibits adrenal cortex, hypothalamus and JG cells

Stops production of renin

Decreases BV

ADH

acts on collecting ducts and increases aquaporins onto the cell

Pathway for decreased GFR

Detected by macula densa cells —> increase in renin —> increase BV —> increase BP

Dehydration

Increase in plasma osmolarity and decrease in blood volume

What is diuretic?

Increase in output of urine

Osmoregulation

Body fluid osmolarity is maintained at 290 mOsm/L (300)

Deviations in osmolarity produce

hormonal responses that can change water reabsorption by kidneys

What is responsible for maintaining constant body fluid osmolarity?

Water reabsorption

Water reabsorption happens at

late distal tubule and collecting duct

Range of urine osmolarity

very wide range (50 to 1200 mOsm)

_____ urine → urine osmolarity = blood osmolarity

_____ urine → urine osmolarity > blood osmolarity

_____urine → urine osmolarity < blood osmolarity

Isoosmotic urine

Hyperosmotic urine

Hypoosmotic urine

What is main determinant to osmolarity of urine?

ADH

SIADH

• Increase in ADH —> high water reabsorption (pulling water out from tubular structures) —> hyperosmotic urine and low plasma osmolarity

Central diabetes insipidus (brain injury)

• Decrease in ADH —> low water reabsorption (urinating lots of water) —> hypoosmotic urine and high plasma osmolarity

Loop of Henle Functions

Site of production of dilute urine

Create/maintain osmotic gradient in medulla

Mechanisms of Loop of Henle

Countercurrent multiplication (reference to loop of Henle)

Countercurrent exchange (reference to vasa recta)

Vasa recta

Peritubular capillaries that surround loop of Henle

Countercurrent multiplication

• Loop of Henle

• Active process (gradient present) that establishes medullary osmotic gradient

Countercurrent exchange

Vasa Recta

Passive process that helps maintain gradient

Freely permeable to small solutes and water

Blood flow through vasa recta is slow and solutes and water can move in and out, allowing for efficient countercurrent exchange

How to explain countercurrent?

Loop of Henle

• As descending limb, only permeable to water and imperpeable to solutes so only water can move out

• At bottom of hairpin: very concentrated

• As ascending limb, only permeable to solutes and impermeable to water so solutes can leave and go to vasa recta

• Urine leave loop of henle, very hyposmotic at 100 mOsM —> can go to distal and collecting duct where water regulation occurs

Vasa Recta

• Solutes to vasa recta —> becomes more hypoosmotic but because water can go back in —> goes back to normal osmolarity

Goal of countercurrent exchange

Create hyperosmotic hairpin at bottom to establish gradient for water out into plasma

Create hypoosmotic urine for water reabsorption