Osmoregulation and Excretion

Page
9

Thirst

Drinking reduces

blood osmolarity

to set point.

Osmoreceptors in

hypothalamus trigger

release of ADH.

Increased

permeability

Pituitary

gland

ADH

Hypothalamus

Distal

tubule

H2O reab-

sorption helps

prevent further

osmolarity

increase.

STIMULUS:

Increase in blood

osmolarity

Collecting duct

Homeostasis:

Blood osmolarity

(300 mOsm/L)

(a)

Exocytosis

(b)

Aquaporin

water

channels

H2O

H2O

Storage

vesicle

Second messenger

signaling molecule

cAMP

INTERSTITIAL

FLUID

ADH

receptor

ADH

COLLECTING

DUCT

LUMEN

COLLECTING

DUCT CELL

Slide 56

The Renin-Angiotensin-Aldosterone System

The renin-angiotensin-aldosterone system RAAS is part of a complex feedback circuit that functions in homeostasis.

A drop in blood pressure near the glomerulus causes the juxtaglomerular apparatus = JGA to release the enzyme renin.

Renin triggers the formation of the peptide angiotensin II.

Slide 57

Angiotensin II

Raises blood pressure and decreases blood flow to the kidneys

Stimulates the release of the hormone aldosterone, which increases blood volume and pressure.

Slide 58

Regulation of blood volume and pressure by RAAS The Renin-Angiotensin-Aldosterone System

Renin

Distal

tubule

Juxtaglomerular

apparatus (JGA)

STIMULUS:

Low blood volume

or low blood pressure

Homeostasis:

Blood pressure,

volume

Liver

Angiotensinogen

Angiotensin I

ACE

Angiotensin II

Adrenal gland

Aldosterone

Arteriole

constriction

Increased Na+

and H2O reab-

sorption in

distal tubules

Slide 59

Homeostatic Regulation of the Kidney

ADH and RAAS both increase water reabsorption, but only RAAS will respond to a decrease in blood volume.

Another hormone, atrial natriuretic peptide ANP, opposes the RAAS.

ANP is released in response to an increase in blood volume and pressure and inhibits the release of renin.

Slide 60

Summary Review

Animal

Freshwater

fish

Bony

marine

fish

Terrestrial

vertebrate

H2O and

salt out

Salt in

(by mouth)

Drinks water

Salt out - active

transport by gills

Drinks water

Salt in

H2O out

Salt out

Salt in

H2O in

active transport

by gills

Does not drink water

Inflow/Outflow

Urine

Large volume