Chapter 44

Controlling the Internal Environment

homeostasis: "the steady-state physiological condition of the body"

ability to buffer actual cells and internal organs from environmental fluctuations

must balance uptake and loss for net homeostasis

covers the following topics:

· · osmoregulation

· · thermoregulation

A. Osmoregulation (Water Balance and Waste Disposal)

water will flow across a semi-permeable membrane from hypoosmotic to hyperosmotic

animals may deal with osmotic differences in their environment by being:

a) osmoconformers--are isoosmotic with their environment

b) osmoregulators--constantly adapt to differences between internal and external salinity; either:

constantly discharge water if environment is hypoosmotic ie. fresh water

constantly take in water to balance osmotic loss if ENVIRONMENT IS hyperosmotic eg. salt water, terrestrial animals

specific osmoregulatory problems:

I. Marine animals

first area of evolution--most are

a. osmoconformers

overall isoosmotic with surroundings

certain salt levels may differ greatly, must still regulate composition

eg. hagfish (Class Agnatha), most marine invertebrates are isoconformers

b. osmoregulators

sharks (Class Chondrichthyes)--hyperosmotic to their environment

rectal glands--pump salts out at anus

retain large amounts of urea (usually excreted by other animals); also produce and retain TMAO to protect proteins from urea

· · water enters body via osmosis (skin, gills)

do not drink, urinate heavily to balance osmotic water uptake (more drinking would cause even more salt uptake)

bony fish (Class Osteichthyes)--evolved in fresh water first, secondarily returned to salt water--Fig. 44.11

have many characteristics of fresh water osmoreg.

constantly lose water to surroundings (are hypoosmotic)

compensate by drinking; use epithelium of gills to pump salts out of body through capillary walls

marine birds, marine reptiles (iguanas, sea turtles)--Fig. 44.9

have nasal salt glands, excrete excess salt from drinking salt water

II. Fresh water animals--opposite problem, excess water uptake

protozoans (Amoeba, Paramecium)--contractile vacuoles

fishes--excrete large amounts of dilute urine

net loss of salts; gills pump Na+ and Cl- into the blood

salmon must undergo transition from one env. to another

gills become modified for salt uptake; cease drinking

III. Terrestrial animals

threat of desiccation most serious one to life

limited successful land colonizers to two main groups:

arthropods, vertebrates

selective adaptations

1. impervious coverings or surfaces to prevent drying

2. copious drinking under nervous and hormonal control

3. behavioral adaptations: nocturnal lifestyles

4. water-conserving adapt. of kidneys

Transport epithelia--tissue used by almost all osmoregulators

specialized epithelia regulate transport of salts, water between external and internal env.

single sheet of cells facing exterior or specialized channel to ext., joined by impermeable junctions

solutes must pass through selectively permeable membranes

molecular composition of plasma membrane determines osmoreg. function (membrane proteins for active transport); may be specialized for salt (osmoregulation) only, or for nitrogenous wastes as well

eg. nasal salt glands of marine birds, rectal glands of sharks

Excretory Systems in Invertebrates

A. Protonephridium ( network of closed tubules with no internal openings)--Fig. 44.15

in Platyhelminthes, rotifers, mollusk larvae, lancelets

osmoregulation only as its function; only some parasitic types use for nitrogenous wastes

eg. in flatworms

no circulatory system or coelom

most metabolic wastes excreted via gastrovascular cavity and mouth

flame cells directly monitor interstitial fluid

branched system of tubules throughout body; each terminal extension capped with a flame cells

interstitial fluid enters here, bundle of cilia w/in flame cells "beat" fluid into duct

join network of tubules ducted to epidermis via nephridiopore

excreted fluid very dilute--must reabsorb salts before excreted from body via epithelium (exact mechanism not known)

B. Metanephridium--earthworms, other annelids--Fig. 44.16

internal openings that collect body fluids

each body segment has a pair of metanephridia in coelomic fluid

network of capillaries of the closed circulatory system envelops each nephridium

drains to outside through nephridiopore

ciliated funnel, the nephrostome, collects coelomic fluid from segment just anterior to that containing metanephridium

removes essential salts and returns them to blood via capillaries

urine stored in storage bladder

urine excreted to outside is hypoosmotic to body fluids

function in osmoregulation to offset water uptake by skin

C. Malpighian Tubules--insects, other terrestrial arthropods--Fig. 44.17

essential water conservation adaptation of insects

remove nitrogenous wastes from hemolymph; osmoregulation also

open into digestive system at junction of mid- and hindgut

diverge (dangle) in coelomic fluid outside digestive system

lined with transport epithelium, transfers salts, etc. from hemolymph into tubule

epithelium of rectum returns most of salts to blood, water follows by osmosis

nitrogenous wastes eliminated nearly dry along with feces

D. Vertebrate Kidney--Fig. 44.18

composed of nephrons = excretory tubules of vertebrates + associated blood vessels

each kidney contains app. 1 million nephrons; collected into compact organs (kidneys) rather than scattered throughout body, due to efficiency of closed circulatory system

blood cycles through kidneys to remove nitrogenous wastes, function in osmoregulation (adjust concentration of salts in blood)

eg. Mammalian excretory system

blood enters each kidney via renal artery, exit via renal vein

receive 20% of blood circulated with each heartbeat (1100-2000L/day); concentrated into 180 L of filtrate within kidneys; finally concentrated to app. 1.5L of urine excreted per day (99% reabsorbed by blood)

urine formed within kidney passed through ureters to urinary bladder, through urethra to outside

1. Structure of a Nephron

A. renal tubules B. blood vessels

1. Bowman's capsule (blind end of renal tubule) n Glomerulus (ball of capillaries)

2. proximal convoluted tubule n peritubular capillaries

3. loop of Henle n vasa recta

4. distal convoluted tubule n peritubular capillaries

5. collecting duct

6. renal pelvis

7. ureter

substances transferred between the capillaries (plasma) and the renal tubule (filtrate) via the interstitial fluid

nephrons radiate out from center of kidney

cortex (outer zone) contains: 1, 2, 4 above

80% of human nephrons are cortical nephrons (much-reduced loops of Henle)

20% are juxtamedullary nephrons (loops of Henle extend into medulla for varying distances)--only mammals and birds have

** essential to excretion of hyperosmotic urine, conservation of water)

2. Physiology of the Nephron

nephrons regulate composition of blood with three processes: (Fig. 44.14)

filtration

secretion

reabsorption

1. filtration--blood pressure forces fluid from capillaries of glomerulus across epithelium of Bowman's capsule; enters lumen of renal tubule

non-selective towards small molecules (all small enough will enter eg. salts, sugars, vitamins, urea)

2. secretion--filtrate traveling through tubule joined by substances secreted by capillaries via interstitial fluid and across tubule epithelium

effect is net increase in solutes within tubule

*active + passive transport (selective)--eg. controlled secretion of H⁺ for pH maintenance

3. reabsorption--return of essential small molecules to interstitial fluid and blood; selective transport

occurs in convoluted tubules, loop of Henle, collecting duct

reabsorb almost all water, sugars, vitamins, organics

can selectively reabsorb or excrete certain ions for balance

transport epithelium of various regions crucial to differential transport properties of each--Fig. 44.19

1. Bowman's capsule = in osmolarity to blood plasma

2. proximal convoluted tubule

NH₃ secreted into filtrate from peritubular capillaries

controlled secretion of H⁺ for pH balance

drugs and other toxins from liver

actively transport glucose and amino acids from filtrate to peritub. capillaries; K⁺ also reabsorbed

reabsorption of NaCl, H₂O (75% and 70% from Bowman's capsule via brush border with microvilli on epithelial cells)

3. descending limb of loop of Henle

water reabsorption continues

freely permeable to water, not to salts

interstitial fluids maintained hyperosmotic to allow H₂O to diffuse out

filtrate increases in NaCl conc. as descends

4. ascending limb of loop of Henle

reverses permeability; permeable to salts, not water

first passive NaCl reabsorption, then active (filtrate again becoming more dilute)

5. distal convoluted tubule

vary amounts of K⁺ secreted into filtrate, Na⁺ reabsorbed

regulates pH by secreting H⁺, reabsorbing HCO₃^-

6. collecting duct (moving back toward medulla, renal pelvis)

epithelium permeable to water, not salt

loses more water by osmosis as hypertonic medulla entered

**concentrates urea in filtrate

epithelium at base of duct permeable to urea, some passes out

**helps maintain high osmolarity of medulla, very important in concentrating urine, making hyperosmotic to other body fluids

Comparative Nephron Physiology in vertebrates

1. mammals with most hyperosmotic urine: desert mammals eg. kangaroo rat;

very long loops of Henle, steep interstitial gradients

beavers: very dilute urine, short loops

2. birds: intermediate, have juxtamedullary loops of Henle, shorter loops, urine less conc. than mammals

3. reptiles: cortical nephrons only, urine isoosmotic to hypoosmotic (epithelium of cloaca reabsorbs water from urine, feces); excrete uric acid (insoluble form)

4. fresh-water fish--must excrete excess water; nephrons have cilia to sweep excess water; urine dilute; conserve ions by reabsorption in nephrons

5. amphibians--similar to fish, reabsorb water while on land from urinary bladder

6. saltwater (bony) fish--nephrons of many spp. lack glomeruli and Bowman's capsules; urine conc. by secreting ions into renal tubules; secrete very little water in urine for salt elimination (Ca²⁺, Mg²⁺, SO₄^2-); Na⁺ and Cl^-, NH₄⁺ excreted by gills

Nitrogenous Wastes--Fig. 44.10

N wastes are toxic by-products of protein, nucleic acid catabolism

deamination of proteins removes -NH₂ group to convert to carbohydrates, or use for energy

result is ammonia: NH₃, very toxic

form of excretion used depends on animal's evolution and habitat:

1. ammonia--used by most aquatic animals

small molecule, very water-soluble

easily permeates membranes eg. body surfaces (inverts., fish gills)

2. urea--mammals, adult amphibians; also some marine fishes, sea turtles

100,000X less toxic than NH₃

terrestrial animals cannot excrete NH₃ (too toxic, requires too much urine to dilute)

conserves water

urea produced in liver (combination of NH₃, CO₂), transported to kidneys for elimination

sharks, mammals retain urea (in blood or kidneys) for osmoregulation

amphibians that move from water to land switch from NH₃ to urea (Xenopus--South African clawed toad; African lungfish also)

3. Uric acid--land snails, birds, most terrestrial reptiles, insects

1000X less soluble in water than urea

can be excreted as a precipitate after water removal

birds and reptiles excrete into cloaca, where water is reabsorbed

mode of reproduction determines whether urea or uric acid used

vertebrates that lay shelled eggs produce uric acid (it precipitates out of solution, stored until hatching)

mammals and soft-shelled verts. can use urea or ammonia, which diffuses into maternal blood or out of egg shell