21-1 copyright 2010 mcgraw-hill australia pty ltd powerpoint slides to accompany biology: an...

21-1Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and SaintSlides prepared by Karen Burke da Silva, Flinders University

Chapter 21: Homeostasis of water and solutes

Homeostasis• In animals, each cell has a membrane which

separates the intracellular fluid from the surrounding extracellular fluid

• Extracellular fluid provides a protective internal environment in which the cells live

• Homeostasis is the term used to describe constancy of the extracellular environment in which the cells are located

• Regulatory mechanisms can maintain consistency in some aspects of the extracellular environment


Homeostasis (cont.)• Animals can either conform or regulate in regards

to certain aspects of their environment– Examples

Osmoconform or Osmoregulate Thermoconform or Thermoregulate Ionoconform or Ionoregulate


Fig. 21.1: Osmoconformer and osmoregulator



Water and solutes• All organisms contain

– high levels of water– ions

Na+, K+, Cl–, Ca2+, SO42–, PO4

3–

– organic solutes glucose, amino acids, proteins

• These substances are essential for life– few animals can survive dehydration or freezing


Exchange with the environment• Water and solutes are exchanged continuously

between an organism and its environment– intake of food and fluids– respiration– elimination of wastes

• Aquatic animals also – gain or lose water by osmosis– gain or lose solutes by diffusion

• Terrestrial animals also– lose water from body surface by evaporation

Fig. 21.4: Exchange with the environment



Intracellular environment• Intracellular fluid must have the same osmotic

concentration as extracellular fluid to prevent movement of water from one to the other– iso-osmotic

• Solute composition differs from that of extracellular fluid– higher levels of K+

– lower levels of Na+ and Cl–


Tonicity• Cell volume is determined by solute content in

intracellular fluid• Solutes move along diffusion gradients

– movement of a solute across membranes depends on the membrane’s permeability to that solute

– if solutes move across the membrane, water moves with them

• Isotonic solutions maintain solute and water balance across a membrane– cells maintain cell volume by changing concentration of

amino acids


Ionic and osmotic balance• Osmotic concentration of extracellular fluid of

osmoconformers is identical to that of the external environment

• Osmoregulators regulate the osmotic concentration

• Ion concentration in ionoconformers is identical to that of the external environment

• Ionoregulators regulate the ion concentration


Living in sea water• Many marine invertebrates are osmoconformers

and ionoconformers– no regulation of osmotic or ionic concentrations– extracellular fluid varies with external environment

• Almost all marine vertebrates– osmoconform and ionoregulate

cartilaginous fish (sharks, rays, chimaeras), coelacanth

– osmoregulate and ionoregulate most fish, amphibians, reptiles, birds, mammals


Fig. 21.5: Marine bony fish


Living in salt lakes• Hypersaline water has a greater salt concentration

than sea water• Animals that live in salt lakes must osmoregulate

and ionoregulate– fish excrete excess ions through salt pumps (chloride

cells) on gills– brine shrimp (Artemia) excrete excess ions through salt

pumps on appendages

• Water lost by osmosis is increased by drinking salty water


Living in fresh water• Because fresh water has a low solute

concentration, animals that live in it – gain water by osmosis– lose solutes by diffusion

• Freshwater animals must osmoregulate and ionoregulate– water is excreted as dilute urine– ions are absorbed from the gut or actively taken up

across the skin and gills

Fig. 21.7: Freshwater bony fishes



Moving between sea and fresh water• Euryhaline animals move between salt and fresh

waters– may change water balance strategies between different

environments

• Even those species that osmoregulate and ionoregulate in both environments must adjust pattern from fresh to salt– ‘freshwater’ hormone prolactin lowers water permeability

of skin, gills and gut and increases ion retention and uptake

– ‘seawater' growth hormone and cortisol increase ion excretion from gills


Living on land• Terrestrial animals face water loss through

– evaporation from lungs and skin surface– excretion in urine and faeces

• Water is usually replaced by drinking– in arid areas, food and metabolic water are important

sources– some invertebrates can absorb water directly from air

through skin of mouth or anus

• Water loss is minimised by producing solid or concentrated wastes


Nitrogenous wastes• Nitrogen produced by metabolism of protein must

be excreted from the body• Toxic ammonia (NH3) is the first product of protein

metabolism– excreted by aquatic animals

• Terrestrial organisms convert NH3 to a less-toxic form– soluble urea (CON2H4) in invertebrates and mammals– insoluble uric acid (C5H4O3N4) in reptiles and birds– insoluble guanine (C5H5ON5) in spiders and scorpions


Nitrogenous wastes (cont.)• Advantages and disadvantages associated with

different nitrogenous wastes

• Ammonia (NH3) – toxic, requires water for excretion, no energy expenditure

• Urea (CON2H4) – less toxic, less water required for excretion, some energy

expenditure

• Uric acid (C5H4O3N4) and guanine (C5H5ON5)– non-toxic, very little water required for excretion,

substantial energy expenditure

Fig. 21.10: Nitrogenous waste


Question 1:

The advantage of excreting wastes as urea rather than ammonia is that:

a) Urea is less toxic than ammonia.

b) Urea requires less water for excretion than ammonia.

c) Urea does not affect the osmolar gradient.

d) Urea can be exchanged for Na+.

e) Both A and B are advantageous.



Excretion• Excretion regulates the internal environment by

– controlling body water– maintaining solute composition– excreting metabolic waste products and unwanted

substances

• Excretion differs from elimination– excretion removes substances that have been

metabolised– elimination expels unabsorbed food

Question 2:

If you compared the maximum urine concentration of a desert mammal and a rainforest mammal, you would expect to find that:

a) The maximum urine concentration would be higher for the rainforest mammal.

b) The maximum urine concentration would be higher for the desert mammal.

c) The maximum urine concentrations would be similar for the two mammals and similar to those in humans.



Excretory organs• Single-celled organisms and simple animals use

contractile vacuoles to excrete material• More complex animals possess two mechanisms

for excretion– Surface epithelial solute pumps

regulate exchange of specific ions

– Internal tubular excretory organs form liquid urine that contains a variety of materials


Epithelial excretory organs• Ion pumps on epithelia take up or excrete ions

selectively• Epithelial salt glands of brine shrimp (Artemia)

actively excrete Cl– and passively excrete Na+

• Ion pumps on gills of fish– marine species excrete Cl– and Na+

– freshwater species absorb Cl– and Na+

• Movement of ions may result in passive transport of water


Tubular excretory organs• Tubular excretory organs

– form urine– reabsorb solutes and water– secrete solutes– change osmolarity

• Excretory tubules form a filtrate of coelomic fluid or blood

• As the fluid passes along the tubule, solutes and water are reabsorbed or solutes secreted to form urine


Fig. 21.13: Tubular excretory organs


Nephridia• Nephridia

– ingrowths of body surface– coelomic fluid drawn into nephridia by cilia– excreted at nephridiopore

• Protonephridia– blind-ending flame cells– fluid enters through perforations in walls

• Metanephridia– ciliated funnel (nephridiostome) opens into coelomic

cavity


Malpighian tubules• Malpighian tubules

– open into digestive tract at junction of midgut and hindgut– K+ is actively transported into lumen of tubule– water and solutes follow– ions selectively reabsorbed– urine formed in tubules is emptied into hindgut for further

modification

• Malpighian tubules of insects and some spiders produce dry wastes


Coelomoducts• Develop from coelomic lining• Coelomic fluid

– is filtrate from blood vessels– cilia draw the fluid into funnels (coelomostomes) in

coelom– excreted at coelomopore

• Coelomoducts are present in many invertebrates, hagfishes and lampreys– blood vessels and coelomoducts combine into single

structure in higher vertebrates


Kidneys• Kidneys are principal excretory organs of

vertebrates• Each kidney is composed of nephrons (excretory

tubules)• Nephrons of higher vertebrates are modified

coelomoducts in which a cluster of capillaries (glomerulus) is associated with the tubule


Fig. 21.19: Four functions of nephron


Mammalian nephrons• At one end of a nephron tubule is an enlarged

double-walled cup– outer wall forms renal or Bowman’s capsule– inner wall of podocytes encloses glomerulus

• Proximal convoluted tubule extends from Bowman’s capsule

• Loop of Henle• Distal convoluted tubule empties into collecting

duct


Filtration• Fluid is filtered through walls of glomerular

capillaries and podocytes into lumen of Bowman’s capsule

• Erythrocytes and protein molecules are too large to pass out of capillaries

• Otherwise filtrate has similar composition to blood


Reabsorption• Approximately 180 L of filtrate is produced each

day in an adult human– 99 per cent of filtrate is reabsorbed

• Proximal convoluted tubule reabsorbs most of the– water– NaCl– glucose– amino acids

• Capillaries surrounding tubule return materials to circulation


Secretion• Peritubular capillaries secrete ions into renal

tubules– H+

– K+

– NH4+

– specific organic molecules

• H+ is secreted to regulate blood and urine pH– buffering H+ with NH3 to form NH4

+ prevents excessively low pH


Osmoconcentration• All vertebrates produce iso-osmotic or hypo-

osmotic urine• Most mammals and some birds produce

hyperosmotic urine• Hyperosmotic urine formed in juxtamedullary

nephrons– long loops of Henle in medulla


Osmoconcentration (cont.)• Osmotic concentration gradient established

between descending and ascending limbs of loop of Henle

• Countercurrent multiplication of solute and water transport– ascending limb actively removes Cl–

– Na+ follows passively– water cannot pass out of ascending limb because

membrane is not permeable to it


Osmoconcentration (cont.)• Increased solute concentration draws water out of

the descending limb• Urea from the collecting duct increases osmotic

gradient• Medullary osmotic gradient draws water from fluid

as it passes through collecting duct


Fig. 21.22: Loop of Henle


Control of kidney function• Hormones control kidney function

– antidiuretic hormone (ADH) causes water retention– renin, released in response to low blood pressure,

converts angiotensinogen into angiotensin II– angiotensin II decreases blood flow to capillary beds

and stimulates reabsorption of water and NaCl from proximal tubules

– angiotensin II causes release of aldosterone, which stimulates reabsorption of water and NaCl from distal tubules

– atrial natriuretic factor (ANF) acts antagonistically to these other hormones, inhibiting renin, aldosterone and NaCl reabsorption


Salt glands• Many marine vertebrates possess salt glands that

secrete NaCl or KCl– cartilaginous fish, coelacanth– marine reptiles– marine birds

• Excreted salt solution more concentrated than sea water

• Salt glands allow marine vertebrates to drink salt water

Summary• Homeostasis is consistency of the extracellular

fluids that provides a stable environment for the cells

• Intracellular fluid has the same osmotic concentration as extracellular fluid in animals, but solute concentration of intracellular and extracellular fluids invariably differ

• Metabolic processes produce waste products which must be excreted

• Environmental exchange occurs by passive and active processes


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