06: Regulation of ECF

Where is most of the filtrate reabsorbed

PCT

What is reabsorbed by the thin descending loop of Henle

Water

Wat is reabsorbed by the thick ascending loop of Henle

Salt

Where do aldosterone and vasopressin act

Principal cells of the collecting duct

Hormone that is key for water reabosprtion

ADH (vasopressin)

Purpose of urine concentration

Water conservation

What feature of the kidney determines how concentrated urine can get

How salty the medulla is— the body can only concentrate the urine up until it is the same as the medullary osmotic gradient

Why do animals have to get rid of their daily obligatory urine volume

They have to be able to eliminate nitrogenous waste (toxins)

Why can’t you drink sea water to survive

It takes more water to excrete the solutes ingested than the water it provides you

Components that make up the countercurrent exchange mechanism

Loop of Henle and peritubular capillaries (vasa recta)

Type of nephron that contributes to water conservation

Juxtamedullary nephrons

How does USG relate to urine concentration

The more concentrated the urine, the higher the USG

How does countercurrent exchange concentrate urine

• Pumps in the ascending loop of Henle and DCT pushes solutes into the interstitium

• The salty interstitium then pulls water from the PCT and descending loop of Henle

• The concentrated urine from the PCT and descending loop then flows through, and more solutes are pumped out into the interstitium, to then pull more water from the PCT and descending loop

What ultimately determines how concentrated the urine is coming out of the collecting ducts

Medullary osmotic gradient and ADH

What osmolyte (not Na⁺) plays a significant role in the medullary osmotic gradient in the later tubules of the nephron and collecting duct

Urea

Mechanisms for urea transport in the DCT and collecting ducts

• Passive transport

• Transporters activated by ADH

Important vascular features that maintain the medullary osmotic gradient

• Slow blood flow through the medulla

• Vasa recta facilitating the countercurrent exchange, minimizing wash out

Effect of ADH on the urine osmolarity

ADH increases urine osmolarity

Major factors that affect the kidney’s ability to concentrate/dilute urine

• Amount of ADH

• Response to ADH

• Impaired countercurrent exchange

Types of diabetes insipidus

• Central: no ADH produced

• Nephrogenic: no kidney response to ADH

ADH analog used to treat central diabetes insipidus

Desmopressin

Mechanism to trigger thirst

Increased ECF osmolarity → cells near the third ventricle shrink → ADH release → H₂O reabsorption → thirst stimulated

Stimulus other than thirst that causes ADH release

Low blood pressure

Stimulus for ADH release in response to low blood pressure

Aortic baroreceptors

Is ADH released more strongly in response to changes in osmolarity or changes in blood pressure

Changes in osmolarity

When there is high Na⁺ concentrations in the blood, is ADH or aldosterone more effective in maintaining homeostasis

ADH