Topic 11
Animal Physiology
11.3 The Kidney and Osmoregulation
Nature of Science
• Curiosity about particular phenomena:
Investigations were carried out to determine how
desert animals prevent water loss in their waste.
Excretion
• The removal from the
body of the waste
products of metabolic
pathways.
Animals are either osmoregulators or
osmoconformers.
o Osmolarity  solute concentration of a solution
o Osmoregulators – maintain a constant internal solute
concentration even when living in marine
environments with very different osmolarities.
(EXAMPLES: terrestrial animals, freshwater animals
and some marine organisms like bony fish)
o Osmoconformers – internal solute concentration
tends to be the same as the concentration of the
solutes in the environment. Example: sharks
The Malpighian tubule system in insects and the kidney carry
out osmoregulation and removal of nitrogenous wastes.
o Osmoregulation – a form of homeostasis whereby the
concentration of hemolymph (in insects) or blood (in animals) is
kept in a certain range.
o When animals break down amino acids, the nitrogenous waste
produce is toxic and needs to be excreted (waste is uric acid in
insects and urea in mammals)
o Insects have tubes that branch off from their intestinal tract
(Malpighian tubules). Cells lining the tubules actively transport
ions and uric acid from the hemolymph into the lumen of the
tubules. This draws water by osmosis from the hemolymph
through the walls of the tubules into the lumen. The tubules
empty their contents into the gut.
SKILL: Drawing the Kidney
The composition of blood in the renal artery
is different from that in the renal vein.
o Kidney function in both osmoregulation and excretion.
o The composition of blood in the renal artery (how blood enters
the kidney) is different from the renal vein (how blood leaves the
kidney).
Composition of Blood
Renal Artery
Renal Vein
• Higher amounts of toxins
• Higher amounts of excretory wastes
(urea)
• Higher amounts of salt and water.
• Slightly higher amount of glucose.
• Higher concentration of oxygen
• Lower concentration of carbon
dioxide
• SAME concentration of plasma
proteins.
• Lower amounts of toxins
• Lower amounts of excretory wastes
(urea)
• Lower amounts of salt and water.
• Slightly lower amount of glucose.
• Lower concentration of oxygen
• Higher concentration of carbon
dioxide
• SAME concentration of plasma
proteins.
SKILL Drawing and labelling a nephron (see
page 492 in text).
Structures and functions of nephron
• Bowman’s capsule – a cup-shaped structure with a highly porous
inner wall, which collects the fluid filtered from the blood.
• Proximal convoluted tubule – a highly twisted section of the
nephron, with cells in the wall having many mitochondria and
microvilli projecting into the lumen of the tube.
• Loop of Henle – a tube shaped like hairpin, consisting of a
descending limb that carries the filtrate deep into the medulla of the
kidney and an ascending limb that brings it back out to the cortex.
• Distal convoluted tubule – another highly twisted section, but with
fewer, shorter microvilli and fewer mitochondria.
Structures and functions of nephron
• Collecting Duct – a wider tube that carries the filtrate back through
the cortex and medulla to the renal pelvis.
• Blood vessels – associated with the nephron – blood flows through
them in the following sequence
• Afferent arteriole – brings blood from the renal artery.
• Glomerulus – a tight, knot-like, high pressure capillary bed that is the site of
blood filtration
• Efferent arteriole – a narrow vessel that restricts blood flow, helping to
generate high pressure in the glomerulus.
• Peritubular capillaries – a low-pressure capillary bed that runs around the
convoluted tubules, absorbing fluid from them.
• Vasa recta – unbranched capillaries that are similar in shape to the loops of
Henle.
• Venules – carry blood to the renal vein.
Ultrafiltration
• Separation of particles differing in size.
• Happens in the glomerulus.
• Blood enters the glomerulus through the afferent arteriole. And is
under high pressure.
Three parts of Ultrafiltration system
1. Fenestrations – openings between the cells in the wall of
the capillaries. Allow fluid to escape, but not blood cells.
2. Basement Membrane – covers and supports the wall of
the capillaries. Made of negatively charged
glycoproteins, which form a mesh. Prevents plasma
proteins from being filtered out, due to their size and
negative charges.
3. Podocytes – cells that form the inner wall of the Bowman’s
capsule. Have extensions that wrap around the
capillaries of the glomerulus.
**if particles pass through all three parts, they become part
of the glomerular filtrate
Proximal Convoluted Tubule
• Filtrate enters PCT from Bowman’s capsule.
• PCT selectively reabsorbs useful substances by ACTIVE
TRANSPORT.
• Most of the filtrate is reabsorbed and most of the this
reabsorption happens in the PCT. By the end of the PCT, ALL
glucose, amino acids, and 80% of the water have been
reabsorbed.
Methods used to reabsorb substances by
PCT
• Sodium ions – moved by active transport (protein pumps)
• Chloride ions – attracted from filtrate into fluid outside tubule because of
charge gradient set up by sodium ions (diffusion)
• Glucose – co-transported out of filtrate by co-transporter proteins. (cotransported with sodium ions).
• Water - osmosis
Loop of Henle
• The Loop of Henle maintains hypertonic conditions in the medulla.
• Sodium ions are pumped out of the filtrate to the interstitial fluid. (only in
ascending limb)
• The wall of the ascending limb is impermeable to water, so the interstitial fluid
is hypertonic relative to the filtrate.
• The wall of the descending limb is permeable to water, but impermeable to
sodium ions. As filtrate flows down the descending limb, the increased
concentration in the interstitial fluid in medulla causes water to be drawn out
of the filtrate.
Loop of Henle
• The length of the loop of Henle is positively
correlated with the need for water conservation in
animals.
• The longer the loop of Henle, the more water will be
reclaimed. Animals adapted to dry habitats will often
have long Loops of Henle. (must have thicker medulla in
order to accommodate longer Loops of Henle).
ADH – antidiuretic hormone
• ADH controls reabsorption of water in the collecting duct.
• If the solute concentration of blood is too low, little water is
reabsorbed as filtrate passes through distal convoluted tubule = large
volume of urine produced and low concentration.
• If the solute concentration of blood is too high, the brain detects it
and causes the pituitary gland to secrete ADH. ADH makes the
collecting duct permeable to water, so water will be reabsorbed here
= small volume of urine produced and high concentraiton.
Nitrogenous Waste
• The type of nitrogenous waste in animals is correlated with evolutionary
history and habitat.
• When animals break down amino acids, nitrogenous waste in the form of
ammonia is produced = TOXIC. So organisms must get rid of it.
• Marine or freshwater fish – ammonia
• Terrestrial organisms – urea or uric acid
• Amphibians – ammonia as larva and urea as adults
APPLICATIONS
• Consequences of dehydration and overhydration.
• Treatment of kidney failure by hemodialysis or kidney transplant.
• Blood cells, glucose, proteins, and drugs are detected by urinary tests.
***READ APPLICATIONS on pg. 496 and 497 in text for HW.