Osmoregulation and Excretion

The urinary system

Osmoregulation

All animals face the same central problem of osmoregulation. Over time, the rates of water uptake and

loss must balance. Animal cells—which lack cell walls—swell

and burst if there is a continuous net uptake of water, or shrivel and die if there is a substantial net loss of water.

35.1

Osmolarity

Osmolarity is defined as Moles of solute (salt) per liter of solution

(water) A value of 300 mosm/L

35.1

Excretion

The removal of nitrogenous waste (from the break down of amino acids primarily)

Urea is the waste product in humans

35.1

Proteins Nucleic acids

Amino acids Nitrogenous bases

–NH2

Amino groups

Most aquaticanimals, includingmost bony fishes

Mammals, mostamphibians, sharks,some bony fishes

Many reptiles(includingbirds), insects,land snails

Ammonia Urea Uric acid

NH3 NH2

NH2O C

C

CN

CO N

H H

C ONC

HN

OH

Types of Waste Products

Among the most important wastes are the nitrogenous breakdown products of proteins and nucleic acids

35.2

Ammonia

Animals that excrete nitrogenous wastes as ammonia Need access to lots of water Release it across the whole body surface

or through the gills

35.2

Urea

The liver of mammals and most adult amphibians Converts ammonia to less toxic urea

Urea is carried to the kidneys, concentrated And excreted with a minimal loss of water

35.2

Uric Acid Insects, land snails, and many reptiles,

including birds Excrete uric acid as their major nitrogenous

waste Uric acid is largely insoluble in water

And can be secreted as a paste with little water loss

35.2

The Excretory System

The Steps in Urine Production Filtration, pressure-filtering of body fluids

producing a filtrate Reabsorption, reclaiming valuable solutes from

the filtrate Secretion, addition of toxins and other solutes

from the body fluids to the filtrate Excretion, the filtrate leaves the system

35.3

Most excretory systems produce urine by refining a filtrate derived from body fluids

Filtration. The excretory tubule collects a filtrate from the blood.Water and solutes are forced by blood pressure across the selectively permeable membranes of a cluster of capillaries and into the excretory tubule.

Reabsorption. The transport epithelium reclaims valuable substances from the filtrate and returns them to the body fluids.

Secretion. Other substances, such as toxins and excess ions, are extracted from body fluids and added to the contents of the excretory tubule.

Excretion. The filtrate leaves the system and the body.

Capillary

Excretorytubule

Filtra

teU

rine

1

2

3

4

35.3

Structure and Function

The mammalian excretory system centers on paired kidneys Which are also the principal site of water

balance and salt regulation

35.4

Anatomy

Each kidney Is supplied with

blood by a renal artery and drained by a renal vein

Posterior vena cava

Renal artery and vein

Aorta

Ureter

Urinary bladder

Urethra

(a) Excretory organs and major associated blood vessels

Kidney

35.4

(b) Kidney structure

UreterSection of kidney from a rat

Renalmedulla

Renalcortex

Renalpelvis

The mammalian kidney has two distinct regions

An outer renal cortex and an inner renal medulla

35.4

The nephron is the functional unit of the vertebrate kidney It consists of a single long tubule and a ball

of capillaries called the glomerulusJuxta-medullarynephron

Corticalnephron

Collectingduct

To renalpelvis

Renalcortex

Renalmedulla

20 µm

Afferentarteriolefrom renalartery

Glomerulus

Bowman’s capsule

Proximal tubule

Peritubularcapillaries

SEM

Efferentarteriole fromglomerulus

Branch ofrenal vein

DescendinglimbAscendinglimb

Loopof

Henle

Distal tubule

Collectingduct

(c) Nephron

Vasarecta(d) Filtrate and

blood flow35.5

Filtration of the Blood

Filtration occurs as blood pressure Forces fluid from the blood in the

glomerulus into the lumen of Bowman’s capsule

35.5

Filtration of the Blood

Filtration of small molecules is nonselective And the filtrate in Bowman’s capsule is a

mixture that mirrors the concentration of various solutes in the blood plasma

35.5

Pathway of the Filtrate From Bowman’s

capsule, the filtrate passes through three regions of the nephron The proximal tubule, the

loop of Henle, and the distal tubule

Fluid from several nephrons flows into a collecting duct

35.5

Blood Vessels Associated with the Nephrons

Each nephron is supplied with blood by an afferent arteriole a branch of the renal artery that subdivides into the capillaries

The capillaries converge as they leave the glomerulus forming an efferent arteriole

The vessels subdivide again forming the peritubular capillaries, which surround the proximal and distal tubules

35.5

Proximal tubule

Filtrate

H2OSalts (NaCl and others)HCO3

–

H+

UreaGlucose; amino acidsSome drugs

KeyActive transport

Passive transport

CORTEX

OUTERMEDULLA

INNERMEDULLA

Descending limbof loop ofHenle

Thick segmentof ascendinglimb

Thin segmentof ascendinglimb

Collectingduct

NaCl

NaCl

NaCl

Distal tubuleNaCl Nutrients

Urea

H2O

NaClH2O

H2OHCO3 K+

H+ NH3

HCO3

K+ H+

H2O

1 4

32

3 5

From Blood Filtrate to Urine: A Closer Look

35.6

Secretion and reabsorption in the proximal tubule Substantially alter the volume and composition

of filtrate Reabsorption of water continues

As the filtrate moves into the descending limb of the loop of Henle

35.6

From Blood Filtrate to Urine: A Closer Look

As filtrate travels through the ascending limb of the loop of Henle Salt diffuses out of the permeable tubule into the

interstitial fluid The distal tubule

Plays a key role in regulating the K+ and NaCl concentration of body fluids

The collecting duct Carries the filtrate through the medulla to the renal

pelvis and reabsorbs NaCl

35.6

From Blood Filtrate to Urine: A Closer Look

Water Conservation

The mammalian kidney Can produce urine much more

concentrated than body fluids, thus conserving water

35.7

Solute Gradients and Water Conservation

In a mammalian kidney, the cooperative action and precise arrangement of the loops of Henle and the collecting ducts Are largely responsible for the osmotic

gradient that concentrates the urine

35.7

Two solutes, NaCl and urea, contribute to the osmolarity of the interstitial fluid which causes the reabsorption of water in the kidney and concentrates the urine

H2O

H2O

H2O

H2O

H2O

H2O

H2O

NaCl

NaCl

NaCl

NaCl

NaCl

NaCl

NaCl

300

300 100

400

600

900

1200

700

400

200

100

ActivetransportPassivetransport

OUTERMEDULLA

INNERMEDULLA

CORTEX

H2O

Urea

H2OUrea

H2O

Urea

H2O

H2O

H2O

H2O

1200

1200

900

600

400

300

600

400

300

Osmolarity of interstitial

fluid(mosm/L)

300

35.7

The countercurrent multiplier system involving the loop of Henle Maintains a high salt concentration in the

interior of the kidney, which enables the kidney to form concentrated urine

35.7

From Blood Filtrate to Urine: A Closer Look

The collecting duct, permeable to water but not salt Conducts the filtrate through the kidney’s

osmolarity gradient, and more water exits the filtrate by osmosis

35.7

From Blood Filtrate to Urine: A Closer Look

Urea diffuses out of the collecting duct As it traverses the inner medulla

Urea and NaCl Form the osmotic gradient that enables the

kidney to produce urine that is hyperosmotic to the blood

35.7

From Blood Filtrate to Urine: A Closer Look