BIOL10002 Lecture Notes - Lecture 19: Chondrichthyes, Osmoregulation, Metabolic Waste
Lecture 19: Kidney & osmoregulation & lecture 20: tubular reabsorption
Major functions of an excretory system:
1. Maintains osmotic concentrations
2. Maintains body fluid volume
3. Maintains proper concentrations
of ions
4. Remove metabolic end products
5. Remove foreign substances
Water balance: osmoregulation
● cells need a relatively constant osmotic internal & external environment, requires constant
regulation of the internal water & salt balance
● animals that live in freshwater, seawater, and on land face different water and salt balance
problems
● osmolarity: the concentration of moles of active solutes per litre of solvent
● osmotic pressure: force associated with the movement of water
● osmolality: the concentration of solutes per kilogram of solvent
● tonicity: the concentration of non-permeable solutes per litre of solvent
Excretory organs: control osmolarity and volume of extracellular fluids, excrete water & solutes that
are in excess & conserve water & solutes that are valuable or in short supply, e.g. glucose
Urine: output of the excretory system, a waste product
How animals deal with nitrogenous waste + maintaining osmolarity
animals in water
animals on land
use a lot of water to deal with nitrogenous waste - keeping it in
the form of ammonia = metabolically favourable/not processed
- just diluted & excreted
osmoconformers: conform to the external environment, have a
concentration inside cells that matches outside environment
marine animals: have to conserve water & excrete excess salts
freshwater animals: in a very dilute environment: have to excrete
excess water and conserve salts; no freshwater
osmoconformers because there’s no salt in the environment;
would be too dilute
marine teleost fish: marine osmoregulator: drink seawater to
replace water lost to the environment, produce little urine
eliminate the salts ingested from salt water; gills balance salt
and water concentrations, chloride cells in gills actively
transport salts from the blood into the environment, gills act as
excretory organs in fish but they also have kidneys
aquatic invertebrates and bony fishes (ammonotelic animals):
excrete nitrogenous waste as ammonia; excreted by diffusion
across gill membranes directly into water; easily removed
because very soluble in water
terrestrial animals: excrete nitrogenous waste as urea (less toxic),
needs to use metabolism to create it, but means it doesn’t need to
use as much water, desert animals can excrete urea in crystalline
form to conserve water, must conserve water & salts; surrounded by
air, so we lose both to the environment, diet determines whether
salts must be conserved or excreted
osmoregulators: regulate against big changes in the environment;
can excrete salt or excess water to regulate; humans are
osmoregulators; actively control salt concentrations despite the salt
concentrations in the environment
desert animals: behavioural response: avoid daytime heat & emerge
at night; aestivation (similar to hibernation, prolonged dormancy
during a hot or dry period) - low metabolic activity so low H2O
turnover; thick keratinised cuticle (e.g. reptiles)
● cuticle: a water-impervious protective layer covering the
epidermal cells, limiting water loss
efficient kidneys, excrete highly concentrated urine, long loops of
Henle
uricotelic animals: land reptiles, birds & insects: excrete uric acid
ureotelic animals: mammals, amphibians, cartilaginous fish: urea
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Mammalian kidney
● massive blood flow through kidney tissue
● blood filtration across cortex & medulla
● filtrate collects in renal pelvis and flows down ureter
● pair of kidneys, two ureters, empty into bladder, urine
leaves by urethra
Nitrogenous waste forms ammonia in liquid/solution = toxic
therefore must be excreted
Urea synthesis: synthesized from CO2, water, asparate (a salt or
ester of aspartic acid) and ammonia in a metabolic pathway called
the urea cycle, occurs in the liver
Nephron: the functional unit of the vertebrate kidney - filtration & production of filtrate
Renal tubule
each of the long, fine, convoluted tubules conveying urine from the glomeruli to the renal pelvis
processes the filtrate into urine; local & systemic regulation, sits partly in the cortex
surrounding capillaries: two arterioles; running in and out
high surface area across which exchange of solutes can take place
glomerular filtrate flows into the renal tubule where it is modified by reabsorption of water & ions
transport of water - aquaporins (AQPs): found in the renal tubule epithelium
Glomerulus
two arterioles (blood vessels) that supply nephron, one leading in and one leading out = good control of pressure in
glomerulus:
afferent arteriole: enters short & wide = large volume of blood can come in quickly, can be constricted to speed up
flow into glomerulus
efferent arteriole: exits, maintains pressure, constricting slows down exit of blood from glomerulus, increases
pressure, increases filtration rate of nephrons
blood plasma filtered across walls of a mesh of capillaires - dense ball of capillaries - produces a fluid (renal
filtrate) lacking cells, proteins, and large molecules (filtered out)
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as blood comes in, pushing under high pressure, most of content goes out into bowman’s capsule
once enters renal tubule = filtrate
gaps in glomerulus quite wide to allow for efficient filtration, podocytes provide barrier to prevent big proteins
leaving
rate of filtration is high in the glomerulus due to high capillary blood pressure & high permeability of glomerular
capillaries & their podocytes (wrap around capillaries, and leave slits between them)
Bowman’s capsule
receives renal filtrate from glomerulus, mainly water and small molecules such as glucose, amino acids & ions
(salts)
Proximal
convoluted tubule
(PCT) (proximal =
next to glomerulus)
75% of reabsorption occurs here - concentration of solutes in blood and PCT becomes isotonic, even though more
waste products in PCT
convoluted = increases SA over which reabsorption occurs; cells packed with mitochondria near the apical surface
& have microvilli = further increases SA
PCT cells highly metabolically active: mitochondria provide ATP for active transport of Na+, glucose, and amino
acids out of filtrate back into tissue fluid, to make sure all molecules wanted are reabsorbed; water follows by
osmosis; osmosis assisted by water channel molecules called aquaporins: AQP1 regulates movement of water in
PCT
water and solutes in tissue fluid taken up by peritubular capillaries: vasa recta, all the capillaries that surround the
renal tubule, tubules through which water, solutes & tissue fluid are taken up from PCT
cells are cuboidal
Loop of Henle
creates a concentration gradient in surrounding medullary tissue - allows salt to build up in tissues so water will be
absorbed in the collecting duct - causes the urine to become more concentrated; hyperosmotic to blood
found in birds & mammals; important adaptation for terrestrial life - water conservation - desert animals have very
long loops of henle, eg. desert gerbil loop extends into ureter
thick ascending limb: filtrate becomes more dilute: actively transports NaCl from filtrate to interstitial fluid of the
renal medulla, region impermeable to water so it cannot follow so salt accumulates in the interstitial fluid, filtrate
becomes more dilute
thin descending limb: filtrate becomes more concentrated
highly permeable to water but not to Na+ or Cl-
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Document Summary
Lecture 19: kidney & osmoregulation & lecture 20: tubular reabsorption. Osmolarity: the concentration of moles of active solutes per litre of solvent. Osmotic pressure: force associated with the movement of water. Osmolality: the concentration of solutes per kilogram of solvent tonicity: the concentration of non-permeable solutes per litre of solvent. Excretory organs: control osmolarity and volume of extracellular fluids, excrete water & solutes that are in excess & conserve water & solutes that are valuable or in short supply, e. g. glucose. Urine: output of the excretory system, a waste product. How animals deal with nitrogenous waste + maintaining osmolarity animals in water animals on land use a lot of water to deal with nitrogenous waste - keeping it in the form of ammonia = metabolically favourable/not processed. Henle uricotelic animals: land reptiles, birds & insects: excrete uric acid ureotelic animals: mammals, amphibians, cartilaginous fish: urea. Nitrogenous waste forms ammonia in liquid/solution = toxic therefore must be excreted.
