HomeostasisChapter 30

HomeostasisHomeostasis refers to maintaining internal

stability within an organism and returning to a particular stable state after a fluctuation.

HomeostasisChanges to the internal environment come from:

Metabolic activities require a supply of materials (oxygen, nutrients, salts, etc) that must be replenished.

Waste products are produced that must be expelled.

HomeostasisSystems within an organism function in an

integrated way to maintain a constant internal environment around a setpoint.Small deviations in pH, temperature, osmotic

pressure, glucose levels, & oxygen levels activate physiological mechanisms to return that variable to its setpoint.Negative feedback

Osmoregulation & Excretion

Osmoregulation regulates solute concentrations and balances the gain and loss of water.

Excretion gets rid of metabolic wastes.

OsmosisCells require a balance between osmotic gain

and loss of water.

Water uptake and loss are balanced by various mechanisms of osmoregulation in different environments.

OsmosisOsmosis is the movement of water across a

selectively permeable membrane. If two solutions that are separated by a membrane

differ in their osmolarity, water will cross the membrane to bring the osmolarity into balance (equal solute concentrations on both sides).

Osmotic ChallengesOsmoconformers, which are only 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.

Osmotic RegulationMost marine invertebrates are osmotic

conformers – their bodies have the same salt concentration as the seawater.The sea is highly stable, so most marine

invertebrates are not exposed to osmotic fluctuations.

These organisms are restricted to a narrow range of salinity – stenohaline.Marine spider crab

Osmotic RegulationConditions along the

coasts and in estuaries are often more variable than the open ocean. Animals must be able to

handle large, often abrupt changes in salinity.

Euryhaline animals can survive a wide range of salinity changes by using osmotic regulation.Hyperosmotic

regulator (body fluids saltier than water)

Shore crab.

Osmotic RegulationThe problem of dilution is solved by pumping out

the excess water as dilute urine.

The problem of salt loss is compensated for by salt secreting cells in the gills the actively remove ions from the water and move them into the blood.Requires energy.

Osmotic Regulation - Freshwater

Freshwater animals face an even more extreme osmotic difference than those that inhabit estuaries.

Osmotic Regulation - Freshwater

Freshwater fishes have skin covered with scales and mucous to keep excess water out.

Water that enters the body is pumped out by the kidney as very dilute urine.

Salt absorbing cells in the gills transport salt ions into the blood.

Osmotic Regulation - Freshwater

Invertebrates and amphibians also solve these problems in a similar way.

Amphibians actively absorb salt from the water through their skin.

Osmotic Regulation – Marine

Marine bony fishes are hypoosmotic regulators.Maintain salt

concentration at 1/3 that of seawater.

Marine fishes drink seawater to replace water lost by diffusion.

Excess salt is carried to the gills where salt-secreting cells transport it out to the sea.More ions voided in feces or urine.

Osmotic Regulation – Marine

Sharks and rays retain urea (a metabolic waste usually excreted in the urine) in their tissues and blood.

This makes osmolarity of the shark’s blood equal to that of seawater, so water balance is not a problem.

Osmotic Regulation – Terrestrial

Terrestrial animals lose water by evaporation from respiratory and body surfaces, excretion (urine), and elimination (feces).

Water is replaced by drinking water, water in food, and retaining metabolic water.

Osmotic Regulation – Terrestrial

The end-product of protein metabolism is ammonia, which is highly toxic.Fishes can excrete ammonia directly because

there is plenty of water to wash it away.

Osmotic Regulation – Terrestrial

Terrestrial animals must convert ammonia to uric acid.Semi-solid urine – little water loss. In birds & reptiles, the wastes of developing

embryos are stored as harmless solid crystals.

Osmotic Regulation – Terrestrial

Marine birds and turtles have a salt gland capable of excreting highly concentrated salt solution.

Excretory ProcessesMost excretory

systems produce urine by refining a filtrate derived from body fluids (blood, hemolymph, or coelomic fluid).

Excretory ProcessesKey functions of most excretory systems are:

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.

Invertebrate Excretory Structures

Contractile vacuoles are found in protozoans and freshwater sponges.An organ of water balance – expels excess water

gained by osmosis.

Invertebrate Excretory Structures

The most common type of invertebrate excretory organ is the nephridium.The simplest

arrangement is the protonephridium of acoelomates and some pseudocoelomates.

Fluid enters through flame cells, moves through the tubules, water and metabolites are recovered and wastes are excreted through pores that open along the body surface.Highly branched due to

lack of circulatory system.

Invertebrate Excretory Structures

The metanephridium is an open system found in annelids, molluscs, and some smaller phyla.Tubules are open at

both ends.Water enters through

the ciliated, funnel shaped nephrostome.

The metanephridium is surrounded by blood vessels that assist in reclaiming water and valuable solutes.

Invertebrate Excretory Structures

In arthropods, antennal glands are an advanced form of the nephridial organ.No open

nephrostomes, hydrostatic pressure of the blood forms an ultrafiltrate in the end sac.

In the tubule, selective resorption of some salts and active secretion of others occurs.

Invertebrate Excretory Structures

Insects and spiders have Malpighian tubules that are closed and lack an arterial supply.

Salts (especially potassium) are secreted into the tubules from the hemolymph (blood). Water & other solutes

(including uric acid) follow.Water & potassium are

reabsorbed.Uric acid is expelled in feces.

Vertebrate KidneysKidneys, the excretory organs of

vertebrates, function in both excretion and osmoregulation.

Vertebrate KidneysNephrons 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.

Vertebrate KidneysEach kidney is

supplied with blood by a renal artery and drained by a renal vein.

Vertebrate KidneysUrine exits each kidney through a duct

called the ureter.

Both ureters drain into a common urinary bladder.

Structure and Function of the Nephron and Associated

StructuresThe mammalian kidney has two distinct

regions:An outer renal cortex An inner renal medulla

(b) Kidney structure

UreterSection of kidney from a rat

Renalmedulla

Renalcortex

Renalpelvis

Structure and Function of the Nephron and Associated

StructuresThe 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 BloodFiltration occurs as

blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman ’s capsule.

Pathway of the FiltrateFrom Bowman’s

capsule, the filtrate passes through three regions of the nephron:Proximal tubuleLoop of Henle Distal tubule

Fluid from several nephrons flows into a collecting duct.

From Blood Filtrate to Urine: A Closer Look

Filtrate becomes urine as it flows through the mammalian nephron and collecting duct.The composition of the filtrate is modified

through tubular reabsorption and secretion.Changes in the total osmotic concentration of

urine through regulation of water excretion.

From Blood Filtrate to Urine: A Closer Look

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.

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.

Conserving WaterThe mammalian kidney’s ability to

conserve water is a key terrestrial adaptation.

The mammalian kidney can produce urine much more concentrated than body fluids, thus conserving water.

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.

Solute Gradients and Water Conservation

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.

Solute Gradients and Water Conservation

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

The osmolarity of the urine is regulated by nervous and hormonal control of water and salt reabsorption in the kidneys.

Regulation of Kidney Function

Antidiuretic hormone (ADH) increases water reabsorption in the distal tubules and collecting ducts of the kidney.

Temperature RegulationAnimals must keep their bodies within a range

of temperatures that allows for normal cell function.

Each enzyme has an optimum temperature.Too low and metabolism slows.Too high and metabolic reactions become

unbalanced. Enzymes may be destroyed.

Temperature RegulationPoikilothermic animals’ body

temperatures fluctuate with environmental temperatures.

Homeothermic animals’ body temperatures are constant.

Temperature RegulationAll animals produce heat from cellular

metabolism, but in most this heat is lost quickly.Ectotherms – lose metabolic heat quickly, so

body temperature is determined by the environment.Body temp may be regulated

environmentally.Endotherms – retain metabolic heat and can

maintain a constant internal body temperature.

Ectothermic Temperature Regulation

Many ectotherms regulate body temperature behaviorally.Basking to increase temperature.Shelter in shade or coolness of a burrow to

decrease temperature.

Ectothermic Temperature Regulation

Most ectotherms can also adjust their metabolic rates to the environmental temperature.Activity levels can remain unchanged over a

wider range of temperatures.

Endothermic Temperature Regulation

Constant temperature in endotherms is maintained by a delicate balance between heat production and heat loss.Heat is produced by the animal’s metabolism.Producing heat requires energy – supplied by

food.Endotherms must eat more in cold weather.

Endothermic Temperature Regulation

If an animal is too cool, it can generate heat by increasing muscular activity (exercise or shivering). Heat is retained through insulation.

If an animal is too warm it decreases heat production and increases heat loss.

Adaptations for Hot Environments

Small desert mammals are mostly fossorial (living underground) or nocturnal.Burrows are cool and moist.

Adaptations to derive water from metabolism and produce concentrated urine & dry feces.

Adaptations for Hot Environments

Larger desert mammals (camels, desert antelopes) have different adaptations.Glossy, pallid color

reflects sunlight.Fat tissue is

concentrated in a hump, rather than being evenly distributed in an insulating layer.

Sweating and panting are ways of dumping heat.

Adaptations for Cold Environments

In cold environments, mammals reduce heat loss by having a thick insulating layer of fat, fur, or both.

Heat production is increased.

Extremities are allowed to cool.Heat loss is

prevented through countercurrent heat exchange.

Adaptations for Cold Environments

Small mammals are not as well insulated.Many avoid direct exposure to the cold by

living in tunnels under the snow.Subnivean environment.This is where food is located.

Adaptive HypothermiaEndothermy is energetically expensive.

Ectotherms can survive weeks without eating.Endotherms must always have energy

supplies.

Adaptive HypothermiaSome very small

mammals & birds (bats or hummingbirds) maintain high body temperatures when active, but allow temperatures to drop when sleeping.Daily torpor

Adaptive HypothermiaHibernation is a way to

solve the problem of low temperatures and the scarcity of food.True hibernators store fat,

then enter hibernation gradually.Metabolism & body slows to

a fraction of normal.Body temperature

decreases.Shivering helps increase

temperatures when they are waking up.

Adaptive HypothermiaOther mammals, such as bears, badgers,

raccoons and opossums enter a state of prolonged sleep, but body temperature does not decrease.

Adaptive HypothermiaAdverse conditions can also occur during the

summer.Drought, high temperatures.

Some animals enter a state of dormancy called estivation.Breathing rates and metabolism decrease.African lungfish, desert tortoise, pigmy mouse,

ground squirrels.