chapter 26 maintaining the internal environment. how the animal body maintains homeostasis...

CHAPTER 26

MAINTAINING THE INTERNAL ENVIRONMENT

HOW THE ANIMAL BODY MAINTAINS HOMEOSTASIS

• Homeostasis may be defined as the dynamic constancy of the internal environment.• Conditions fluctuate continuously within narrow

limits.


• To maintain internal constancy, the vertebrate body uses:• Sensors that measure each condition of the

internal environment.• An integrating center that contains the set

point, or proper value for a particular internal condition.

• Effectors, which are muscles or glands that can change the value of the condition back toward the set point.• The activity of the effectors is influenced by the

effects they produce in a negative feedback loop.


• Regulating body temperature• Humans, as well as other mammals and birds,

are endothermic.• This means that they can maintain relatively

constant body temperature.• Other vertebrates are ectothermic, meaning

their body temperatures depend more or less on the environmental temperature.• But they can modify their behavior to affect

body temperature.


• Regulating blood glucose• Excess glucose is stored in the liver as glycogen

under the influence of the hormone insulin, which is released from the pancreas.• When glucose levels are low in the blood, the

pancreas releases the hormone glucagon, which stimulates the liver to convert glycogen back to glucose.

CONTROL OF BLOOD GLUCOSE LEVELS

REGULATING THE BODY’S WATER CONTENT

• Animals use various mechanisms for osmoregulation, the regulation of the body’s osmotic composition.• This refers to how much water and salt the

body contains.• The proper operation of many vertebrate organ

systems requires that the osmotic concentration of the blood be kept within narrow bounds.


• In many animals and single-celled organisms, the removal of water and salts from the body is coupled with the removal of metabolic wastes through the excretory system.

CiliumFeedercanal

Contractilevacuole

Excretorypore

Endoplasmicreticulum

Anteriorcontractilevacuole

Posteriorcontractilevacuole


• For example, protists, like Paramecium, employ contractile vacuoles.


• Flatworms employ a system of excretory tubules called protonephridia to expel fluids and wastes from the body.

Flamecell

Excretorypores

Collectingtubule

Cilia


• Other invertebrates have a system of tubules that open both to the inside and to the outside of the body.• In annelids, these

tubules are called nephridia.

Nephridium

Bladder Capillarynetwork

Pore forurine excretion

Coelomic fluid Nephrostome


• The excretory organs in insects are called Malpighian tubules, which are extensions of the digestive tract.

K+

Malpighiantubules

Waste molecules

Water

Mid gut

Waterand K+

AnusRectum

HindgutIntestine

RectumPoison sac

Mid gut

Air sac

Malpighiantubules


• Kidneys are the excretory organs in vertebrates.• Kidneys create a tubular fluid by filtration.• The filtrate contains many valuable nutrients in

addition to waste products.• Selective reabsorption ensures that these

nutrients and water are reabsorbed into the blood, while wastes remain in the filtrate.

EVOLUTION OF THE VERTEBRATE KIDNEY

• The kidney is a complex organ made up of many repeating units called nephrons.• Blood pressure forces the fluid in the blood

through a capillary bed at the top of each nephron, called a glomerulus.• The glomerulus excludes blood cells, proteins,

and other large molecules from the filtrate.• The remainder of the nephron tube reabsorbs

anything else useful from the filtrate

BASIC ORGANIZATION OF THE VERTEBRATE NEPHRON

H2O

Glomerulus Neck

Distal arm

Proximal arm

H2O

NaCl

NaCl

H2O

Collectingduct

H2O

H2O

H2O

Divalentions

Amino acids

Glucose

Intermediatesegment


• Only birds and mammals can reabsorb water from the glomerular filtrate to produce a urine that is hypertonic to (more concentrated than) blood.


• Kidneys are thought to have evolved first among the freshwater fish.• The body fluids of a freshwater fish have a

greater osmotic concentration than the surrounding water. So,• Water tends to enter the body from the

environment.• Solutes tend to leave the body and enter the

environment.


• Freshwater fish address these problems by• Not drinking water.• Excreting a large

volume of dilute urine.• Reabsorbing ions

(mainly NaCl) from the nephron.• Actively transporting

NaCl across the gills from the surrounding water into the blood.

Active tubularreabsorptionof NaClNaCl

NaCl

Kidney tubule

Largeglomerulus

Food,fresh water(passes overgills)

Gills:Active absorption ofNaCl, water entersosmotically

Freshwater fish

Urine

Kidney: Excretionof dilute urine

Intestinalwastes


• Marine fish probably evolved from freshwater ancestors.• Their bodies are

hypotonic to the surrounding seawater. So,• Water tends to leave

their bodies through osmosis across the gills.

• They lose water in their urine.

• To compensate, marine fish drink lots of seawater• They excrete isotonic

urine.

Marine fish

Food,seawater

Stomach:Passive reabsorptionof NaCl and water

Glomerulusreduced orabsent Active tubular

secretionof MgSO4

MgSO4

MgSO4

Gills:Active secretion ofNaCl, water loss

Intestinal wastes:MgSO4 voidedwith feces

Kidney:Excretion of MgSO4,urea, little water


• Elasmobranchs solve the osmotic problem posed by their seawater environment by reabsorbing urea from the nephron tubules.• The blood is approximately isotonic to the

surrounding sea.

Glomerulus

UreaUrea

Kidney tubule

Kidney

Cartilaginous fish


• The amphibian kidney is like that of freshwater fish.• Amphibians produce a very dilute urine and

actively transport Na+ across their skin.

• The kidneys of terrestrial reptiles reabsorb much of the salt and water in the nephron tubules.• Their urine is still hypotonic but they can absorb

additional water in the cloaca


• Because mammals and birds can produce hypertonic urine, they can excrete their waste products in a small volume of water.• The kidneys of some

mammals are even more extremely efficient at conserving water.• The kidneys of the

kangaroo rat are so efficient it never has to drink water; it can obtain all the water it needs from its food and aerobic cell respiration.

Salt glands

Salt secretion


• Birds have relatively few or no nephrons with long loops.• At most, they can only

reabsorb enough water to produce a urine that is about twice the concentration of their blood• Marine birds solve the

problem of water loss by drinking sea water and excreting excess salt through salt glands near the eyes.

THE MAMMALIAN KIDNEY

• In mammals, each kidney receives blood from a renal artery, and it is from this blood that urine is produced.• Urine drains from each

kidney through a ureter.• The ureters carry urine to

a urinary bladder.• Urine passes out of the

body through the urethra.


• Within the kidney, the mouth of the ureter flares open to form a funnel-like renal pelvis.• The renal tissue is

divided into:• An outer renal

cortex• An inner renal

medulla Ureter

Renalvein

Renalartery

Renalmedulla

Renalcortex

Nephron


• The mammalian kidney is comprised of roughly 1 million nephrons, each of which is composed of three regions:• Filter • The filtration device at the top of each nephron is

called the Bowman’s capsule which receives filtrate from the glomerular capillaries.

• Tube• The Bowman’s capsule is connected to a long renal

tubule, which includes the Loop of Henle, that acts as a reabsorption device.

• Duct • The renal tubule empties into a collecting duct that

operates as a water conservation device.


• There are five steps involved in the formation of urine in the kidney:

1. Pressure filtration2. Reabsorption of

water3. Selective

reabsorption4. Tubular secretion5. Further

reabsorption of water

http://youtu.be/TzwPmz5V6Xg

H2O

H2O

Na+

Cl–H2O

H2O

H2O

Na+

Cl–

2 3

4

5

To

tal

so

lute

co

nc

en

tra

tio

n (

mO

sm

)

300

600

Glomerulus Bowman'scapsule

Proximaltubule

Distal tubule

Nitrogenouswastes

Collectingduct

Urea

Inner medulla1200

Outer medulla

Cortex

Loop of Henle

1

ELIMINATING NITROGENOUS WASTES

• Amino acids and nucleic acids are nitrogen-containing molecules.• When animals metabolize these

substances, they produce nitrogen-containing by-products, called nitrogenous wastes, that must be eliminated by the body.


• The first step in the metabolism of amino acids and nucleic acids is the removal of the amino (—NH2) group.• This group is then combined with H+ to form

ammonia (NH3).• This takes place in the liver.


• Ammonia is quite toxic and is safe only in very dilute concentrations.• Fish and tadpoles, ammonia can be directly

eliminated across the gills or excreted in dilute urine.• In sharks, adult amphibians, and mammals, the

nitrogenous waste is eliminated as urea, which is less toxic.• Reptiles, birds, and insects excrete nitrogenous

wastes in the form of uric acid, which can be excreted with very little water.

NITROGENOUS WASTES

3 4

1

2

NH3HN

NH

O

O

HN

NH

O

O C

NH2

NH2

Amino acids and nucleic acids

Catabolism

Convertedto uric acid

Uric acid

Mammals, some others Reptiles and birds

Ammoniaby-product

Eliminateddirectly

Ammonia

Most fish

Convertedto urea

Urea

chapter 26 maintaining the internal environment. how the animal body maintains homeostasis...

Documents

animal body

vertebrate body

bodys water content

body temperatures

internal environment

bodys water content

bodys water content

gut water