Fluid Balance/Nitrogen Excretion

Kidney Function

Salt/Water Balance

• ionic composition of cytosol is maintained by osmotic interaction with intercellular fluid

• intercellular fluid is conditioned by osmotic interaction with capillary contents

• excretory organs control the osmotic composition of blood

– differentially excrete different compounds

– excrete nitrogenous wastes from terrestrial animals

Salt/Water Balance

• common mechanisms of excretory organs– filtration

• movement of water and solutes out of capillary under pressure

– secretion • active transport of additional molecules into filtrate

– resorption• active uptake of solutes from filtrate

Salt/Water Balance

• diverse challenges of different environments– osmotic potentials of aquatic environments

vary dramatically• marine: 1070 mosmol/L• fresh water: 1-10 mosmol/L

– physiological responses to different environmental osmolarities vary

Salt/Water Balance

• physiological responses to different environmental osmolarities

– osmoconformers do not regulate tissue fluid osmolarity

• ionic conformers

–same ionic composition as ambient

• ionic regulators

–modify ionic composition but not overall osmolarity

Salt/Water Balance• physiological responses to different

environmental osmolarities – osmoregulators maintain tissue fluid

osmolarity different from environmental• hypotonic osmoregulators

–marine organisms–excrete salt; conserve water

• hypertonic osmoregulators–fresh water organisms–excrete water; conserve salt

three osmoregulatory modesFigure 51.1

Salt/Water Balance• physiological responses to different

environmental osmolarities– terrestrial organisms conserve water & salt

Nitrogenous Wastes are Excreted• catabolism of amino acids & nucleotides

produces nitrogenous waste– ammonia (NH3) is quite toxic

• ammonotelic organisms lose NH3 to aqueous environment across gills

• ureotelic organisms convert NH3 to urea–highly water soluble

• uricotelic organisms covert NH3 to uric acid–slightly water soluble

Three N Excretion FormsFigure 51.3

Invertebrate Excretory Systems• protonephridia

– in flatworms– flame cell + tubule

• tissue fluid enters flame cell lumen• cilia drive fluid toward excretory pore• tubule cells modify fluid composition• urine is less concentrated than tissue fluid

protonephridia in PlanariaFigure 51.4

Invertebrate Excretory Systems• metanephridia

– annelid worms• fluid-filled coelom in each body segment• closed circulatory system

–filtration from blood into coelom–diffusion of waste products into coelom

circulatory/excretory interaction in earthwormFigure 51.5

Invertebrate Excretory Systems• metanephridia

– annelid worms• metanephridia occupy adjacent segments

–nephrostome collects coelomic fluid–tubule travels to adjacent segment–tubule cells resorb & secrete

compounds–dilute urine leaves a nephridiopore

Invertebrate Excretory Systems• Malpighian tubules - insects

– join gut between midgut & hindgut– extend into body tissues– actively transport uric acid, K+, Na+ from

hemolymph– take water into tubules by osmosis– muscular contractions propel toward gut– hindgut returns Na+, K+ to tissue fluid; water

follows– uric acid precipitates in rectum

Malpighian tubuleFigure 51.6

Vertebrate Excretory Systems

• nephron (functional unit of kidney)– an afferent arteriole branches into a dense

capillary bed = the glomerulus– the glomerulus is surrounded by Bowman’s

capsule (= renal corpuscle)– blood is filtered from the glomerulus

through podocyte “fingers” into Bowman’s capsule

nephron anatomyFigure 51/8

renal filtrationFigure 51.7

Vertebrate Excretory Systems

• nephron– glomerular capillaries combine into an

efferent arteriole – the efferent arteriole branches into a

peritubular capillary bed– the renal tubule modifies fluid composition

• resorption & secretion– peritubular capillaries

• deliver materials to be secreted into urine• take up resorbed materials

tubular modification of fluid contentsFigure 51.7

Vertebrate Excretory Systems

• nephron– peritubular capillaries combines into a renal

venule– the renal tubule delivers urine to a collecting

duct

fluid collectionFigure 51.7

vertebrate nephron

Figure 51/7

Vertebrate Excretory Systems

• nephrons of different vertebrates accomplish different tasks

– water excretion; salt conservation

– water conservation; salt excretion

Vertebrate Excretory Systems

• marine bony fishes

– secrete salts; conserve water

• hypotonic osmoregulation

• fewer glomeruli - limits volume of urine

• excrete Na+, Cl-, NH3, through renal tubules & gills

• do not absorb some ions from gut

Vertebrate Excretory Systems

• cartilaginous fishes– ionic regulating osmoconformers

• N waste retained as urea • special salt-secreting sites remove excess dietary NaCl

Vertebrate Excretory Systems

• amphibians– conserve salt; excrete water, OR– conserve both

• reduce skin permeability• estivate during hot dry periods

Vertebrate Excretory Systems

• reptiles & birds– conserve water & salt

• minimize skin evaporation• limit water loss by excreting uric acid

Vertebrate Excretory Systems

• mammals– conserve water, regulate ions

• excrete urine hypertonic to tissue fluids• kidney concentrates urine

human urinary system; kidney anatomyFigure 51.9

human kidney

• nephron components & arrangement - tubule– Bowman’s capsule - cortex– proximal convoluted tubule - cortex– loop of Henle - descending/ascending in

medulla– distal convoluted tubule - cortex– collecting duct - cortex => medulla

renal pyramidFigure 51.9

human kidney

• nephron components & arrangement - vessels– afferent arteriole supplies glomerulus– efferent arteriole branches into peritubular

capillaries– vasa recta capillary bed parallels loop of

Henle– peritubular capillaries join to form the

venule that empties into the renal vein– ~98% of filtrate leaves kidney in renal vein

human kidney

• nephron function– glomerulus filters plasma into Bowman’s

capsule– proximal convoluted tubule transports Na+,

glucose, amino acids, etc. into tissue fluid– water moves out of tubule by osmosis– peritubular venous capillaries take up water

and molecules– tubule contents enter loop of Henle at an

osmotic potential similar to plasma

human kidney• nephron function

– urine concentration in loop of Henle• thin descending limb

–permeable to water–impermeable to Na+, Cl-

thin descending

limb loses water, retains NaCl

Figure 51.10

thin ascending limb loses

NaCl, retains waterFigure 51.10

human kidney• nephron function

– urine concentration in loop of Henle• thin descending limb • thin ascending limb• thick ascending limb

–impermeable to water–actively transports Cl- out, Na+ follows

thick ascending

limb pumps out

NaCl, retains waterFigure 51.10

human kidney

• nephron function– thick ascending limb increases solute in

tissue fluid – thin ascending limb increases solute in

tissue fluid– thin descending limb contents become

increasingly concentrated– dilute fluid enters distal convoluted tubule– osmosis empties distal convoluted tubule

until osmotic potential is same as plasma

human kidney

• nephron function– the loop of Henle creates a concentration

gradient in the medulla – vasa recta removes water from medulla– collecting duct passes through the medulla

• water leaves the duct by osmosis• highly concentrated urine is produced

nephron function

in the human kidney

Figure 51.10

nephron function

• blood plasma is filtered into tubule

• ions are actively resorbed

• a concentration gradient is established in the medulla

• water is reclaimed by osmosis

Control & Regulation of Kidney Function• Glomerular Filtration Rate depends on blood

pressure and blood volume• autoregulatory renal responses

– reduced blood pressure causes afferent arteriole dilation

– continued low GFR causes release of renin which activates circulating angiotensin

Control & Regulation of Kidney Function• autoregulatory renal responses

– continued low GFR causes release of renin which activates circulating angiotensin• efferent arteriole constriction

• systemic peripheral vessel constriction

• release of aldosterone from adrenal cortex

–stimulates Na+ resorption ( & so H2O)

–stimulates thirst

Control & Regulation of Kidney Function

• Glomerular Filtration Rate depends on blood pressure and blood volume

• antidiuretic hormone (ADH) control– ADH release increases as aortic stretch

signals decrease or as osmolarity increases• increases permeability of collecting ducts to water

• increases blood volume• decreases osmolarity

control & regulation of kidney functionFigure 51/14