Chapter 32

Osmoregulation and Excretion

afferent arteriolealdosteroneammoniaangiotensin IIanhydrobiosisantidiuretic hormone (ADH)Bowman’s capsule collecting ductcortical nephronscountercurrent multiplier systemdistal tubuleefferent arterioleexcretionfiltratefiltrationglomerulusjuxtaglomerular apparatus (JGA)juxtamedullary nephronsloop of HenleMalpighian tubulemetanephridiumnephron

osmoconformerosmolarityosmoregulationosmoregulatorperitubular capillariesprotonephridiumproximal tubulerenal arteryrenal cortexrenal medullarenal pelvisrenal veinsecretionselective reabsorptiontransport epitheliumultrafiltrationureaureterurethrauric acidurinary bladder

Overview/Background• Osmolarity vs Tonicity• both compare solute concentrations separated by a

membrane• osmolarity includes total solutes; both permeable and

impermeable• tonicity just includes permeable solutes• ex: a solution can be both hyperosmotic and isotonic

• Osmoregulation• Regulate solute concentrations• balance the gain and loss of water• controlled movement of solutes

• Excretion• Gets rid of metabolic wastes

THE INTERNAL ENVIRONMENT

WATER GAIN (milliliters)

WATER LOSS (milliliters)

by ingesting solids by ingesting liquids

by metabolism

in urine in feces

by evaporation

6.0 0.0

54.0 60.0

850 1,400

350 2,600

13.5 2.6

43.9 60.0

1,500 200 900

2,600

KANGAROO RAT HUMAN

Homeostasis - maintaining internal conditions,

i.e. water concentration, temperature, pH, glucose level.

Proteins Nucleic acids

Amino acids Nitrogenous bases

–NH2 Amino groups

Most aquatic animals, including most bony fishes

Mammals, most amphibians, sharks, some bony fishes

Many reptiles (including

birds), insects, land snails

Ammonia Urea Uric acid

NH3NH2

NH2

O C

C

CN

CO

N

H H

C O

NC

HN

O

H

Nitrogenous Wastes

• Where do they come from?• products of proteins and

nucleic acids• What’s wrong with them?• toxic on their own

Nitrogenous waste type correlated with evolutionary history and habitat

Mammals: Functional Unit of the Excretory System• Nephrons and associated blood vessels• Mammals: paired kidneys• water balance and salt regulation

• Each kidney supplied by:• renal artery

• Drained by:• renal vein

KIDNEY (one of a pair)

URETER (one of a pair)

URINARY BLADDER

URETHRA

adrenal gland

abdominal aorta

inferior vena cava

Components:

1) Kidneys filter blood in structures called nephrons. 2) Ureter 3) Bladder 4) Urethra

Urinary SystemFunctions:

• Balancing intake & output of water & salts.

• Separation of harmful & useful material. • Produce hormones to regulate blood pressure (adrenaline & aldosterone)

Waste material: urea which is a metabolic by-product of amino acids (proteins) metabolism.

Human Kidney Structure

ureter

renal pelvis

renal vein

renal artery

cortex

medulla

glomerulus (coded red)

proximal tubule (orange)

start of distal tubule (brown)

loop of Henle (yellow)

collecting duct

Nephron

Kidney Layers:

- cortex - medulla - pelvis

Nephrons - Basic functional units of kidney.

Bowman’s capsule

glomerular capillaries

COLLECTING DUCT

Nephrons Are Filtering Units

Each nephron consists of following:

- Bowman’s capsule

- Proximal tubule - Loop of Henle - Distal tubule - Collecting duct

- Capillaries: * Glomerular capillaries

* peritubular capillaries

DISTAL TUBULE

Artery Vein

Filtration of the Blood

• Blood pressure forces fluid from the blood in the glomerulus --> Bowman’s capsule• Nonselective filtration (=“ultrafiltration”)• The filtrate mirrors the concentration of various solutes in

the blood plasma• From Bowman’s capsule --> three regions of the nephron• proximal tubule -->• loop of Henle -->• distal tubule

• Then several nephrons -->• into a collecting duct

Urine Formation

Four Steps in Urine Formation:

1) Filtration (of solutes out of blood)

2) Reabsorption (of solutes into blood)

3) Secretion (of solutes out of blood)

- at glomerulus: small molecules & H2O move out of

blood into kidney tubules.

- at proximal tubules & loop of Henle: solutes & H2O

move back into blood.

- H+ and K+ are actively transported into nephron’s

lumen. 4) Excretion

Nephrons - Filtering Units

Bowman’s capsule

+ glomerular capillaries

= glomerulus

blood vessel entering

blood vessel leaving

COLLECTING DUCT

2) Reabsorption

1) Filtration

3) Secretion

Urine Formation

• Blood enters nephron thru glomerulus —› Bowman’s capsule.

• Blood cells, large solutes & proteins stay in blood. (size matters)

• Filtrate moves from Bowman’s capsule into proximal tubule.

Bowman’s capsule glomerular

capillaries

collecting duct

1) Filtration:

Urine Formation

Na+

Na+

Na+

Na+

Cl-

H2O

Interstitial fluid

KID

NEY

TU

BU

LE

Na+

Na+

•is the process of solutes getting back into blood capillaries.

BLO

OD

CA

PILL

ARY

Blo

od c

apill

ary

2) Reabsorption:

Urine Formation

2) Reabsorption:

• Na+, amino acids & glucose (filtrate) make their way back into blood by active transport.

• water follows these materials into capillaries by osmosis (99 %).

• most of reabsorption occurs in proximal tubule.

Bowman’s capsule glomerular

capillaries

collecting duct

Na+

glucose

H2O

Urine FormationBowman’s capsule

3) Secretion (by blood capillaries):

• occurs in the tubule wall, but in opposite direction of reabsorption.

• capillaries move H+ , K+ , urea and drugs into nephron’s lumen.

K+

H+ Drugs

• If hypothalamus detects a drop in extracellular fluid,

• ADH binds to receptors on distal tubule & collecting duct

• More water is reabsorbed from tubule into blood capillaries —› urine is more concentrated.

• Alcohol & caffeine = inhibitors of ADH.

H2O

H2O

H2O

H2O

ADH

Hormonal Control On Urine Formation (Antidiuretic Hormone)

• it calls for secretion of antidiuretic hormone (ADH).

•if decrease in Na+ level then adrenal gland releases:

•aldosterone

•distal tubules & collecting ducts reabsorb more Na+

Na+

Na+

Na+

Na+

Hormonal Control On Urine Formation (Aldosterone)

aldosterone

From Blood Filtrate to Urine: A Closer Look

s

sg

From Blood Filtrate to Urine: A Closer Look

• Role o/t descending limb of the Loop of Henle:• Reabsorption of

water• the longer the loop,

the more water is reabsorbed

• Countercurrent system:• maintains high salt

concentration in kidney interior

From Blood Filtrate to Urine: A Closer Look

• Role o/t ascending limb of the loop of Henle:• Salt diffuses out of the

permeable tubule into the interstitial fluid

• not permeable to water• The distal tubule• regulates K+ and NaCl

From Blood Filtrate to Urine: A Closer Look

• Role o/t collecting duct• Carries the filtrate through

the medulla to the renal pelvis

• permeable to water• not permeable to salt• permeable to urea

• helps maintain countercurrent gradient

REGULATION OF KIDNEY

FUNCTION

• Osmolarity of the urine is regulated by • nervous and hormonal control

• water and salt reabsorption in the kidneys

• Antidiuretic hormone (ADH)• Increases water reabsorption

• in the distal tubules and• collecting ducts of the

kidney