Chapter 51 Salt and Water Balance and Nitrogen Excretion Biology 102 Tri-County Technical College Pendleton, SC.

Chapter 51 Salt and Water Balance and Nitrogen Excretion

Biology 102

Tri-County Technical College

Pendleton, SC

Osmo Comparisons

• Osmoregulation describes how organism regulates solute balance and gain or loss of water

• Osmoconformers do NOT actively adjust internal osmolarity (isoosmotic with environment or able to tolerate changes)

• Osmoregulators are animals that must adjust internal osmolarity since body fluids are NOT isoosmotic with outside environment

• VERY costly energy wise since must adjust by discharging excess water/taking in water

Comparing organisms

• Most cartilaginous fishes are hyperosmotic to seawater

• Maintain internal salt [ ] lower than sea water by pumping salt out through rectal glands/kidneys

• Hyperosmotic because of large [ ]s of urea– water enters body by osmosis

– balance osmotic uptake of water by excreting urine

Comparing, cont.

• Marine bony fish are hypoosmotic to seawater

• Lose water to environment

• Drink large amounts of sea water & pump excess salt out gill epithelium

• Secrete only small amount of urine

Comparing III

• Freshwater animals are hyperosmotic to environment

• Constantly take in water by osmosis• Contractile vacuoles in protists• Many compensate by excreting large amounts of

dilute urine• Salts lost are replenished by eating substances

with high salt content or by active uptake of sodium and chlorine ions from water by gill epithelium

Comparing IV

• Terrestrial animals live in dehydrating environment & cannot survive desiccation

• Adaptations: protective outer layers (cuticles, shells, multi-layered skin with dead, keratinized cells)

• Drinking and eating moist foods• Some are nocturnal (active at night when

cooler)

Comparing V

• Some can produce large amounts of metabolic water

• Excretory system adapted to conserve water while eliminating wastes

Types of Nitrogenous Waste

• Nitrogenous waste animal secretes depends on its evolutionary history and habitat

• Most aquatic animals excrete wastes as ammonia– Very soluble in water; easily permeates

membranes– Soft-bodied invertebrates, ammonia diffuses

across body surface into surrounding water

Wastes, cont.

• Fishes excrete ammonia as ammonium ions across gill epithelium

• Ammonia excretion unsuitable for terrestrial animals because so toxic it can only be transported & excreted in very dilute solution

• Urea is nitrogenous waste excreted by most mammals & adult amphibians

Wastes III

• Urea can be [ ] in body since it is 100,000x less toxic than ammonia

• Reduces water loss for terrestrial animals• Produced in liver by combining ammonia with

carbon dioxide• Transported to kidneys via circulatory system• Sharks produce/retain urea as osmoregulatory

agent

Wastes IV• Uric acid is primary form of NW excreted

by land snails, insects, birds, & many reptiles

• Much less soluble in water than ammonia or urea

• Can be excreted as precipitate after reabsorptiion of nearly all water from urine

• Eliminated in paste-like form through cloaca in birds and reptiles

Wastes V

• Mode of reproduction important determining factor– Shelled egg embryo utilizes uric acid– Ammonia/uric acid would accumulate to toxic [

] and kill embryo

• Animal’s habitat and phylogenetic position influences type of nitrogenous waste produced and excreted (voided, so to speak)

Transport Epithelia• Blood or other body fluids exposed to filtering

device composed of transport epithelia• Membranes of transport epithelia selectively

permeable• Membranes retain proteins & other large

molecules in body fluid• Hydrostatic pressure (BP in animals) forces water

and small solutes (salts, sugars, amino acids, nitrogenous wastes) through device into excretory system

• Aqueous solution in excretory system = filtrate

Protonephridial ES

• Found in flatworms (no circulatory system and no coelom)

• Simple, tubular excretory system

• Protonephridium is network of closed tubules lacking openings that branch throughout body

• Smallest branches capped by cellular flame bulb

Protonephridial ES

• Interstitial fluid passes through flame bulb and is propelled by tuft of cilia along branched system of tubules

• Urine from system empties into external environment

• Transport epithelia lining tubules absorb salts before fluid exits body

• Also found in rotifers, some annelids, larvae of mollusks, and lancelets

Protonephridia Visual

Metanephridial Systems

• Each segment of most annelids contains pair of metanephridia…excretory tubules that have internal openings to collect body fluids

• Coelomic fluid enters funnel-shaped nephrostome which is surrounded by cilia

• Fluid passes through metanephridium and empties into storage bladder that empties outside via nephridiopore

Metanephridial, cont.

• Nephrostome collects coelomic fluid from body segment just anterior

• Network of capillaries envelopes each metanephridium

• Excretion of hypoosmotic, dilute urine offsets continual osmosis of water from damp soil across skin

Methnephridia Visual

Malpighian Tubules

• Malpighian tubules are excretory organs of insects & other terrestrial arthropods that remove nitrogenous wastes from hemolymph & function in osmoregulation

• Are outpocketings of gut that open into digestive tract at midgut-hindgut juncture

• Tubules dead-end at tips away from gut & are bathed in hemolymph

Malpighian Tubules, cont.

• Transport epithelium lining each tubule moves solutes (salts/nitrogenous wastes) from hemolymph into tubule’s lumen

• Accumulates nitrogenous wastes away from hemolymph– Water follows by osmosis

• Fluid in tubule passes through hindgut to rectum

Malpighian Tubules III• Salts/water reabsorbed across epithelium of

rectum

• Dry, nitrogenous wastes are excreted with feces

• Contributed to insect success because system CONSERVES water

Malpighian Tubules Visual

Mammalian Excretory System

• Kidneys: pair of filtering organs

• Blood enters each kidney via renal artery & exits via renal vein

• Kidneys receive 20% of all blood pumped with each heartbeat

• Urine exits kidney through duct called the ureter

Mammalian ES, cont.

• Ureters of both kidneys drain into common urinary bladder

• During urination, urine leaves body from urinary bladder through tube called the urethra

• Sphincter muscles near junction of urethra and bladder control urination

Mammalian ES Visual

Nephrons• Each nephron consists of glomerulus (capillary

cluster) surrounded by Bowman’s capsule, a proximal tubule, a loop of Henle, a distal tubule, and a collecting duct (receives filtrate from many nephrons)

• Blood Flow: Enters glomerulus via afferent arteriole & leaves via efferent arteriole which conveys blood to peritubular capillaries surrounding proximal/distal tubules, then to the vasa reca (capillaries surrounding the loop of Henle)

Nephrons, cont.

• Nephrons, collecting duct, and associated blood vessels produce urine from filtrate (water and small solutes) forced by blood pressure into Bowman’s capsule from glomerulus

• As filtrate travels from Bowman’s capsule to collecting duct, its chemical makeup is changed as substances pass via interstitial fluid between nephron and surrounding capillaries

• Filtrate processing continues in collecting duct

Three Processes

• FILTRATION: blood pressure forces fluid (water, salts, urea, & other small molecules) from glomerulus into lumen of Bowman’s capsule

• Porous capillaries and podocytes (specialized cells of the capsule) nonselectively filter out blood cells and large molecules

Processes, cont.

• Molecules small enough to be forced through capillary wall enter nephron tubule

• Filtrate at this point contains mixture of glucose, salts, vitamins, nitrogenous wastes, & small molecules in [ ]s similar to blood plasma

• Filtrate passes through proximal tubule, loop of Henle, distal tubule, and empties into collecting duct

Processes III• SECRETION: Filtrate joined by some substances

transported across tubule epithelium from surrounding interstitial fluid as it moves through nephron tubule– Adds plasma solutes to filtrate

• Proximal & distal tubules most common sites of secretion

• Secretion very selective process involving both passive/active transport

• Example: controlled secretion of H+ helps maintain constant body fluid pH

Processes IV• REABSORPTION: Selective transport of filtrate

substances across excretory tubule epithelium from filtrate back to interstitial fluid

• Reclaims small molecules essential to body• Occurs in proximal & distal tubules, loop of

Henle, and collecting duct• Nearly all sugar, vitamins, organic nutrients are

reabsorbed• In mammals & birds, water also reabsorbed

Looping the Loop…• Loop of Henle maintains interstitial gradient of

NaCl• Filtrate [ ] of this salt increases by loss of water

from the descending limb• Ascending limb then leaks the salt into interstitial

fluid• Additional salt is actively transported out of thick

segment of ascending limb• Filtrate is hypoosmotic• As filtrate descends collecting tube, increasing

osmolarity of interstitial fluid results in water loss from filtrate

Looping Visual

Hormonal Regulation• Antidiuretic hormone (ADH) enhances fluid

retention by increasing water permeability of epithelium of distal tubules and collecting duct

• Produced by hypothalamus/stored and released by posterior pituitary

• ADH release triggered when osmoreceptor cells in hypothalamus detect increased blood osmolarity due to excessive loss of water from body

Hormonal Regulation, cont.

• Juxtaglomerular apparatus (JGA) is specialized tissue near afferent arterioles which carry blood to glomeruli

• Responds to decrease in blood pressure or volume, as well as decrease in Na+ [ ] by releasing enzyme RENIN into blood

• Renin leads to conversion of inactive angiotensinogen to angiotensin II which functions as a hormone

Hormonal Regulation III

• Angiotensin II directly increases blood pressure by causing arteriole constriction

• Also stimulates adrenal glands to release aldosterone which stimulates Na+ reabsorption by distal tubules (and water follows by osmosis)

• Stimulates thirst centers in brain to induce drinking

• Inceases blood pressure and volume

Hormonal Regulation IV

• Renin-angiotensin-aldosterone system (RAAS) part of complex feedback circuit that functions in homeostasis

• Hormone Atrial Natriuretic Factor (ANF) opposes RAAS

• Released by heart’s atrial walls in response to increased blood volume/pressure

• Inhibits release of renin from JGA, reduces aldosterone release from adrenal glands, and decreases blood volume and lowers BP