24-1

e. Kidney Function

(1) Glomerulus: filtration

(2) PCT: tubular reabsorption

(3) Loop of Henle

(a) descending loop: filtrate concentrates

(b) ascending loop: filtrate dilutes

Constant recycling of salt creates

“standing salt gradient” in kidney medulla

24-2

PCT DCT

LOOP

CD

GLOM

24-3

300

NaCl H2O

24-4

300

1000

700

400

100300

600

900

1200

1400

H2O

H2O

H2O

H2O

NaCl

NaCl

NaCl

24-5

300

1000

700

400

100300

600

900

1200

1400

300

600

900

1200

1400

NaCl

NaCl

NaCl

24-6

(4) Distal convoluted tubule

both active absorption and secretion

(a) control ion concentrations in filtrate

e.g., Na+, Cl-, K+, HCO3-, H+

regulates blood pH and ion composition

(b) removes wastes from blood by secretion

At end of DCT, filtrate is back to 300 mosm/L

24-7

300

1000

700

400

100

300

300

600

900

1200

1400

NaCl H2O

24-8

(5) Collecting Duct

Filtrate in CD passes down through standing salt gradient in medulla ECF

Water will leave filtrate by osmosis

Water picked up by blood and returned to general circulation

Salts and wastes remain behind to form a concentrated urine

24-9

(6) Summary

All kidneys:

elimination of wastes

conservation of needed salts and nutrients

Looped kidneys (birds and mammals only):

self generating osmotic gradient gives ability to concentrate urine

massive savings of water in animals with high waste production and water

loss

24-10

f. Control of kidney function

Control water reabsorption in collecting duct

Neurohypophysis/neural lobe

arginine vasopressin (AVP) in mammals

arginine vasotocin (AVT) in all others

9 amino acid peptide

Control: endocrine reflex arc

24-11

Decreased BloodPressure

Increased BloodOsmolarity

AorticBaroreceptors

CNSChemoreceptors

AVP RELEASE

+

+ +

+

24-12

AVP action

increase permeability of cells to water

via cAMP, induces production of proteins

“water channels”

works in kidney, bladder, skin

kidney

increases permeability of cells of collecting duct to water

24-13

300 mosm 300 mosm

300 mosm

NO AVPlow water

permeability

Large amountsof dilute urine

DIURESIS

HIGH AVPhigh water

permeability

H2O to circulation

1200 mosm

Small amounts ofconcentrated urine

ANTIDIURESIS

AVP = AntiDiuretic Hormone (ADH)

ECF

300

800

1200

24-14

2. Bladder

homeotherms

storage organ for hyperosmotic urine

poikilotherms

epithelium is thin, contains ion pumps

Na+, Cl-, pumped to blood, H2O follows

H2O permeability controlled by AVT

AVT increases, water uptake increases

24-15

3. Integument (skin)

barrier to environment

most vertebrates: impermeable to salts, H2O

waxy coating, dead cells, scales, mucus

amphibians: no barrier to H2O

lose H2O rapidly

can take up H2O along with ions

AVT: increases skin permeability

24-16

4. Salt Glands

marine elasmobranchs: rectal gland

marine reptiles and birds: facial

24-17

All drink sea water to gain water

no freshwater access

high salt load

no looped kidney: dilute urine

Excrete salt load from salt glands

concentrated saline solution excreted

active transport of Na+/Cl- to outside

2-3 X osmolarity of blood plasma

24-18

5. Gut

primary location for water and salt uptake

ion pumping into animal

water entry by osmosis

24-19

6. Gills

pump ions in or out

water follows by osmosis

24-20

C. Osmoregulatory Environments

1. Sea water (1000 mosm/L)

Problem: if blood pOs does not equal water pOs

then water and solutes will diffuse across gill

2 strategies:

a. conform

let blood osmolarity = environment

hagfish: plasma = 1000 mosm salt

sharks: plasma = 500 mosm salt, 500 mosm urea

kidney must still regulate blood composition

24-21

b. Regulate at 300-350 mosm/L

gill in salt water:

osmotic H2O loss from blood

passive salt gain from environment

response of marine fish:

(1) drink sea water

gain H2O and salt in gut

(2) excrete Na+, Cl- by pumping out at gills

(3) divalent ions, wastes excreted in urine

24-22

Marine fish kidney

adapted to minimize water loss in urine

very low glomerular filtration, down to 0

Overall strategy:

gain water and salt by drinking

excrete salt gained

conserve water at all locations

24-23

2. Fresh water (<100 mosm/L)

All animals regulate at 300-350 mosm/L

Problem

diluting: lose salt, gain H2O at gills

Solution

don’t drink

gain salt through diet

conserve salt

pump in at gills

reabsorb from bladder

24-24

Freshwater fish excrete excess water at kidney

adapted to maximize water loss in urine

very high glomerular filtration, no loop

“copious amounts of dilute urine”

24-25

Additional osmoregulatory problem

disposal of nitrogenous wastes

protein catabolism makes ammonia

increases osmotic pressure of blood

toxic

24-26

All fish

ammonia highly soluble in water

diffuses out of blood at gills

Fish are “ammonotelic”

excrete ammonia as nitrogenous waste

24-27

3. Terrestrial Environments

a. Take up as much water as possible

(1) Drink

(2) Eat

H2O trapped in food as humidity

H2O generated by biochemical breakdown of complex nutrients

“metabolic water”

60 ml H2O/100 g dry barley

(3) Absorb: amphibian skin, bladder

24-28

b. Reduce water loss

(1) Avoid hot environments (nocturnal)

(2) Impermeable skin

(3) Kidney

Produce a concentrated urine: loop

Kidney water conservation ability:

U/P ratio = urine osmolarity

plasma osmolarity

reptiles: no loop, U/P up to 1

birds: small loop, U/P up to 6

mammals: great loop, U/P up to 25

24-29

(4) Respiratory system

nasal labyrinth

inhale: air warmed and humidified

exhale: H2O condensed by cooling

Respiration still primary H2O loss in xeric environments

24-30

(5) Nitrogenous wastes

terrestrial: retain and detoxify ammonia

Mesic environments:

ammonia converted to urea in liver

less toxic

concentrated in urine

“ureotelic”

24-31

Xeric environments

urea still requires too much water

ammonia is converted in liver to uric acid

precipitates as insoluble salt after kidney

“uricotelic”

Huge water savings

Ammonia: 500 mls H2O to excrete 1 g N

Urea: 50 mls H2O to excrete 1 g N

Uric Acid: 10 mls H2O to excrete 1 g N

24-32

Ammonia

fish

aquatic amphibian larvae

crocodilians

Urea

mesic reptiles and amphibians

mammals

Uric acid

birds

xeric reptiles and amphibians

24-33

d. Store H2O: amphibians

store H2O in bladder and lymph

tolerate dehydration

draw water out of lymph

draw water out of urine from bladder

allow blood osmolarity to rise to 600