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.

oOsmolarity solute concentration of a solution

oOsmoregulators – 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)

oOsmoconformers – 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.

oKidney function in both osmoregulation and excretion. oThe 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 BloodRenal Artery• 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.

Renal Vein• 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 system1. 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. (co-

transported 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.