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Chapter 44. Osmoregulation and Excretion. Overview: A Balancing Act. Physiological systems of animals operate in a fluid environment Relative concentrations of water and solutes must be maintained within fairly narrow limits.

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chapter 44

Chapter 44

Osmoregulation and Excretion

overview a balancing act
Overview: A Balancing Act
  • Physiological systems of animals operate in a fluid environment
  • Relative concentrations of water and solutes must be maintained within fairly narrow limits
slide3
Freshwater animals show adaptations that reduce water uptake and conserve solutes
  • Desert and marine animals face desiccating environments that can quickly deplete body water
slide5
Osmoregulation regulates solute concentrations and balances the gain and loss of water
  • Excretion gets rid of metabolic wastes
concept 44 1 osmoregulation balances the uptake and loss of water and solutes
Concept 44.1: Osmoregulation balances the uptake and loss of water and solutes
  • Osmoregulation is based largely on controlled movement of solutes between internal fluids and the external environment
osmosis
Osmosis
  • Cells require a balance between osmotic gain and loss of water
  • Various mechanisms of osmoregulation in different environments balance water uptake and loss
osmotic challenges
Osmotic Challenges
  • Osmoconformers, consisting only of some marine animals, are isoosmotic with their surroundings and do not regulate their osmolarity
  • Osmoregulators expend energy to control water uptake and loss in a hyperosmotic or hypoosmotic environment
slide9
Most animals are stenohaline; they cannot tolerate substantial changes in external osmolarity
  • Euryhaline animals can survive large fluctuations in external osmolarity
marine animals
Marine Animals
  • Most marine invertebrates are osmoconformers
  • Most marine vertebrates and some invertebrates are osmoregulators
slide12
Marine bony fishes are hypoosmotic to sea water
  • They lose water by osmosis and gain salt by diffusion and from food
  • They balance water loss by drinking seawater
le 44 3a

LE 44-3a

Gain of water and

salt ions from food

and by drinking

seawater

Osmotic water loss

through gills and other parts

of body surface

Excretion of salt ions

and small amounts

of water in scanty

urine from kidneys

Excretion of

salt ions

from gills

Osmoregulation in a saltwater fish

freshwater animals
Freshwater Animals
  • Freshwater animals constantly take in water from their hypoosmotic environment
  • They lose salts by diffusion and maintain water balance by excreting large amounts of dilute urine
  • Salts lost by diffusion are replaced by foods and uptake across the gills
le 44 3b

LE 44-3b

Osmotic water gain

through gills and other parts

of body surface

Uptake of

water and some

ions in food

Uptake of

salt ions

by gills

Excretion of

large amounts of

water in dilute

urine from kidneys

Osmoregulation in a freshwater fish

animals that live in temporary waters
Animals That Live in Temporary Waters
  • Some aquatic invertebrates in temporary ponds lose almost all their body water and survive in a dormant state
  • This adaptation is called anhydrobiosis
le 44 4

LE 44-4

100 µm

100 µm

Dehydrated

tardigrade

Hydrated tardigrade

land animals
Land Animals
  • Land animals manage water budgets by drinking and eating moist foods and using metabolic water
le 44 5

LE 44-5

Water

balance in a

kangaroo rat

(2 mL/day)

Water

balance in

a human

(2,500 mL/day)

Ingested

in food

(750 mL)

Ingested

in food (0.2 mL)

Ingested

in liquid

(1,500 mL)

Water

gain

Derived from

metabolism (1.8 mL)

Derived from

metabolism (250 mL)

Feces (100 mL)

Feces (0.09 mL)

Urine

(1,500 mL)

Urine

(0.45 mL)

Water

loss

Evaporation (900 mL)

Evaporation (1.46 mL)

le 44 6

LE 44-6

4

3

Water lost per day

(L/100 kg body mass)

2

1

0

Control group

(Unclipped fur)

Experimental group

(Clipped fur)

transport epithelia
Transport Epithelia
  • Transport epithelia are specialized cells that regulate solute movement
  • They are essential components of osmotic regulation and metabolic waste disposal
  • They are arranged in complex tubular networks
  • An example is in salt glands of marine birds, which remove excess sodium chloride from the blood
le 44 7a

LE 44-7a

Nasal salt gland

Nostril

with salt

secretions

le 44 7b

LE 44-7b

Lumen of

secretory tubule

Vein

Capillary

Artery

Secretory

tubule

NaCl

Transport

epithelium

Direction

of salt

movement

Blood

flow

Secretory cell

of transport

epithelium

Central

duct

concept 44 2 an animal s nitrogenous wastes reflect its phylogeny and habitat
Concept 44.2: An animal’s nitrogenous wastes reflect its phylogeny and habitat
  • The type and quantity of an animal’s waste products may greatly affect its water balance
  • Among the most important wastes are nitrogenous breakdown products of proteins and nucleic acids
le 44 8

LE 44-8

Nucleic acids

Proteins

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

forms of nitrogenous wastes
Forms of Nitrogenous Wastes
  • Different animals excrete nitrogenous wastes in different forms: ammonia, urea, or uric acid
ammonia
Ammonia
  • Animals that excrete nitrogenous wastes as ammonia need lots of water
  • They release ammonia across the whole body surface or through gills
slide29
Urea
  • The liver of mammals and most adult amphibians converts ammonia to less toxic urea
  • The circulatory system carries urea to the kidneys, where it is excreted
uric acid
Uric Acid
  • Insects, land snails, and many reptiles, including birds, mainly excrete uric acid
  • Uric acid is largely insoluble in water and can be secreted as a paste with little water loss
the influence of evolution and environment on nitrogenous wastes
The Influence of Evolution and Environment on Nitrogenous Wastes
  • The kinds of nitrogenous wastes excreted depend on an animal’s evolutionary history and habitat
  • The amount of nitrogenous waste is coupled to the animal’s energy budget
concept 44 3 diverse excretory systems are variations on a tubular theme
Concept 44.3: Diverse excretory systems are variations on a tubular theme
  • Excretory systems regulate solute movement between internal fluids and the external environment
excretory processes
Excretory Processes
  • Most excretory systems produce urine by refining a filtrate derived from body fluids
  • Key functions of most excretory systems:
    • Filtration: pressure-filtering of body fluids
    • Reabsorption: reclaiming valuable solutes
    • Secretion: adding toxins and other solutes from the body fluids to the filtrate
    • Excretion: removing the filtrate from the system
le 44 9

LE 44-9

Capillary

Filtration

Excretory

tubule

Filtrate

Reabsorption

Secretion

Urine

Excretion

survey of excretory systems
Survey of Excretory Systems
  • Systems that perform basic excretory functions vary widely among animal groups
  • They usually involve a complex network of tubules
protonephridia flame bulb systems
Protonephridia: Flame-Bulb Systems
  • A protonephridium is a network of dead-end tubules lacking internal openings
  • The smallest branches of the network are capped by a cellular unit called a flame bulb
  • These tubules excrete a dilute fluid and function in osmoregulation
le 44 10

LE 44-10

Nucleus

of cap cell

Cilia

Interstitial fluid

filters through

membrane where

cap cell and tubule

cell interdigitate

(interlock)

Tubule cell

Flame

bulb

Protonephridia

(tubules)

Tubule

Nephridiopore

in body wall

metanephridia
Metanephridia
  • Each segment of an earthworm has a pair of open-ended metanephridia
  • Metanephridia consist of tubules that collect coelomic fluid and produce dilute urine for excretion
le 44 11

LE 44-11

Coelom

Capillary

network

Bladder

Collecting

tubule

Nephridio-

pore

Nephrostome

Metanephridium

malpighian tubules
Malpighian Tubules
  • In insects and other terrestrial arthropods, Malpighian tubules remove nitrogenous wastes from hemolymph and function in osmoregulation
  • Insects produce a relatively dry waste matter, an important adaptation to terrestrial life
le 44 12

LE 44-12

Digestive tract

Rectum

Hindgut

Intestine

Midgut

(stomach)

Malpighian

tubules

Anus

Feces and urine

Salt, water, and

nitrogenous

wastes

Malpighian

tubule

Rectum

Reabsorption of H2O,

ions, and valuable

organic molecules

HEMOLYMPH

vertebrate kidneys
Vertebrate Kidneys
  • Kidneys, the excretory organs of vertebrates, function in both excretion and osmoregulation
concept 44 4 nephrons and associated blood vessels are the functional unit of the mammalian kidney
Concept 44.4: Nephrons and associated blood vessels are the functional unit of the mammalian kidney
  • The mammalian excretory system centers on paired kidneys, which are also the principal site of water balance and salt regulation
  • Each kidney is supplied with blood by a renal artery and drained by a renal vein
  • Urine exits each kidney through a duct called the ureter
  • Both ureters drain into a common urinary bladder

Animation: Nephron Introduction

le 44 13

LE 44-13

Posterior vena cava

Renal artery and vein

Kidney

Renal

medulla

Aorta

Renal

cortex

Ureter

Renal

pelvis

Urinary bladder

Urethra

Ureter

Excretory organs and

major associated blood

vessels

Section of kidney from a rat

Kidney structure

Juxta-

medullary

nephron

Cortical

nephron

Afferent

arteriole

from renal

artery

Glomerulus

Bowman’s capsule

Proximal tubule

Peritubular capillaries

Renal

cortex

Collecting

duct

SEM

20 µm

Efferent

arteriole from

glomerulus

Renal

medulla

Distal

tubule

To

renal

pelvis

Collecting

duct

Branch of

renal vein

Descending

limb

Loop

of

Henle

Nephron

Ascending

limb

Vasa

recta

Filtrate and blood flow

structure and function of the nephron and associated structures
Structure and Function of the Nephron and Associated Structures
  • The mammalian kidney has two distinct regions: an outer renal cortex and an inner renal medulla
slide46
The nephron, the functional unit of the vertebrate kidney, consists of a single long tubule and a ball of capillaries called the glomerulus
filtration of the blood
Filtration of the Blood
  • Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule
slide48
Filtration of small molecules is nonselective
  • The filtrate in Bowman’s capsule mirrors the concentration of solutes in blood plasma
pathway of the filtrate
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
blood vessels associated with the nephrons
Blood Vessels Associated with the Nephrons
  • Each nephron is supplied with blood by an afferent arteriole, a branch of the renal artery that divides into the capillaries
  • The capillaries converge as they leave the glomerulus, forming an efferent arteriole
  • The vessels divide again, forming the peritubular capillaries, which surround the proximal and distal tubules
from blood filtrate to urine a closer look
From Blood Filtrate to Urine: A Closer Look
  • Filtrate becomes urine as it flows through the mammalian nephron and collecting duct
  • Secretion and reabsorption in the proximal tubule greatly alter the filtrate’s volume and composition
  • Reabsorption of water continues as filtrate moves into the descending limb of the loop of Henle
slide52
In the ascending limb of the loop of Henle, salt diffuses from the permeable tubule into the interstitial fluid
  • The distal tubule regulates the K+ and NaCl concentrations of body fluids
  • The collecting duct carries filtrate through the medulla to the renal pelvis and reabsorbs NaCl
le 44 14

LE 44-14

Proximal tubule

Distal tubule

Nutrients

NaCl

H2O

HCO3–

K+

HCO3–

H2O

NaCl

H+

H+

NH3

K+

CORTEX

Descending limb

of loop of

Henle

Thick segment

of ascending

limb

Filtrate

H2O

Salts (NaCl and others)

HCO3–

H+

Urea

Glucose; amino acids

Some drugs

NaCl

H2O

OUTER

MEDULLA

NaCl

Thin segment

of ascending

limb

Collecting

duct

Key

Urea

NaCl

Active transport

Passive transport

H2O

INNER

MEDULLA

slide54

Animation: Bowman\'s Capsule and Proximal Tubule

Animation: Loop of Henle and Distal Tubule

Animation: Collecting Duct

concept 44 5 the mammalian kidney s ability to conserve water is a key terrestrial adaptation
Concept 44.5: The mammalian kidney’s ability to conserve water is a key terrestrial adaptation
  • The mammalian kidney conserves water by producing urine that is much more concentrated than body fluids
solute gradients and water conservation
Solute Gradients and Water Conservation
  • The cooperative action and precise arrangement of the loops of Henle and collecting ducts are largely responsible for the osmotic gradient that concentrates the urine
  • NaCl and urea contribute to the osmolarity of the interstitial fluid, which causes reabsorption of water in the kidney and concentrates the urine
le 44 15 3

LE 44-15_3

Osmolarity of

interstitial

fluid

(mosm/L)

300

300

100

300

100

300

300

H2O

NaCl

H2O

CORTEX

Active

transport

200

400

400

400

H2O

NaCl

H2O

Passive

transport

NaCl

H2O

H2O

OUTER

MEDULLA

H2O

NaCl

H2O

400

600

600

600

H2O

NaCl

H2O

Urea

H2O

NaCl

H2O

700

900

900

Urea

H2O

H2O

NaCl

INNER

MEDULLA

Urea

1200

1200

1200

slide58
The countercurrent multiplier system involving the loop of Henle maintains a high salt concentration in the kidney
  • This enables the kidney to form concentrated urine
slide59
The collecting duct conducts filtrate through the osmolarity gradient, and more water exits the filtrate by osmosis
  • 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
regulation of kidney function
Regulation of Kidney Function
  • The osmolarity of the urine is regulated by nervous and hormonal control of water and salt reabsorption in the kidneys
  • Antidiuretic hormone (ADH) increases water reabsorption in the distal tubules and collecting ducts of the kidney

Animation: Effect of ADH

le 44 16a

LE 44-16a

Osmoreceptors

in hypothalamus

Thirst

Hypothalamus

Drinking reduces

blood osmolarity

to set point

ADH

Increased

permeability

Pituitary

gland

Distal

tubule

H2O reab-

sorption helps

prevent further

osmolarity

increase

STIMULUS

The release of ADH is

triggered when osmo-

receptor cells in the

hypothalamus detect an

increase in the osmolarity

of the blood

Collecting duct

Homeostasis:

Blood osmolarity

slide62
The renin-angiotensin-aldosterone system (RAAS) is part of a complex feedback circuit that functions in homeostasis
le 44 16b

LE 44-16b

Homeostasis:

Blood pressure,

volume

Increased Na+

and H2O reab-

sorption in

distal tubules

STIMULUS:

The juxtaglomerular

apparatus (JGA) responds

to low blood volume or

blood pressure (such as

due to dehydration or

loss of blood)

Aldosterone

Arteriole

constriction

Adrenal gland

Angiotensin II

Distal

tubule

Angiotensinogen

JGA

Renin

production

Renin

slide65
The South American vampire bat, which feeds on blood, has a unique excretory system
  • Its kidneys offload much of the water absorbed from a meal by excreting dilute urine
concept 44 6 diverse adaptations of the vertebrate kidney have evolved in different environments
Concept 44.6: Diverse adaptations of the vertebrate kidney have evolved in different environments
  • The form and function of nephrons in various vertebrate classes are related to requirements for osmoregulation in the animal’s habitat
le 44 18a

LE 44-18a

Bannertail kangaroo rat

(Dipodomys spectabilis)

Beaver (Castor canadensis)

le 44 18b

LE 44-18b

Rainbow trout

(Oncorrhynchus mykiss)

Frog (Rana temporaria)

le 44 18c

LE 44-18c

Roadrunner

(Geococcyx californianus)

Desert iguana

(Dipsosaurus dorsalis)

le 44 18d

LE 44-18d

Northern bluefin tuna (Thunnus thynnus)

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