Evolution of the Vertebrate Kidney

Fish Nephrons

- typically have a glomerulus

- Bowman’s capsule with a ciliated neck region that connects to the rest of the tubule

- cilia may assist flow through tubule, since fish typically have low filtration pressures

- neck region usually present in amphibians, but not in amniotes

Elasmobranches nephrons

- urine is about iso-ionic with the plasma with respect to Na and Cl

- hyperionic for Mg and SO4 - slightly hyperosmotic to plasma- actively reabsorbs many solutes

– particularly urea and TMAO

dogfish Squalus - reabsorbs 90-95% of filtered urea- reabsorbs 95-98% of filtered TMAO- urea retention involves countercurrent exchange

in the kidney that is linked to Na reabsorption

Amphibians

- have two types of nephrons- collects coelomic fluid (coelomstome)- glomerulus directly interacts with nephron

- generally function like freshwater teleost nephrons- produce copious amounts of dilute urine- high glomerular filtration rates ( 25 – 100 ml/kg/hr)- high urine flow ( 10-25 ml/kg/hr)- ½ of the primary filtrate is reabsorbed- about 99% of the filtered ions are reabsorbed

- GFR and tubular reabsorptive processes can be regulated in response to dilution/ dehydration- dehydration causes release of arginine vasotocin- hormone that has both glomerular and tubular

reabsorptive effects

Reptiles and birds - generally adapted to minimize urinary water loss and excrete solutes ( nitrogenous wastes and ions)

Birds and Mammals  - can produce a very hyperosmotic urine

The mammalian kidney  humans - small about 1% of body mass

- receive about 20-25% of the cardiac output- produce about a liter of per urine per day - composition and volume reflects volume of liquid and types of food ingested

- function- maintain a more or less constant body composition

Volume urine day = (fluid ingested + metabolic water produced) - (evaporative loss from lung + sweat loss + loss in the feces)

# nephrons vary - several hundred in lower vertebrates, many thousand in a small mammal to more than a million in humans

Nephrons Two types- juxtamedulary nephrons

- glomeruli in the inner cortex- have the long loops of Henle that go deep into the medulla

- cortical nephrons- glomeruli are in the outer cortex- only have short loops of henle that only extend a short

distance into the medulla Loop of Henle - key to making a concentrated urine

- found only in birds and mammals  

Kidneys

- kidneys filter the blood plasma and then reabsorb needed substances back into the blood

- substances left behind are secreted in the urine

Three processes contribute to the final urine composition

1. Filtration - - form an ultrafiltrate in the lumen of Bowmen's capsule

2. Tubular reabsorption- reabsorb almost 99% of water and most of the salts

3. Tubular secretion - most by active transport mechanism - some (nitrogenous wastes) through synthesis in the

tubules themselves

Glomerular filtration  humans - 15-25% of the water and solutes are removed

from the plasma= 125 ml/min (180L/day)

 GFR depends on three factors

1. Hydrostatic pressure difference (glomerular capillaries to Bowman's capsule)

2. Colloid osmotic pressure of the blood plasma (opposes filtration)

3. Hydraulic permeability (sieve-like properties of the three-layered filter)

Willmer 5.10

How do we regulate this glomerular filtration rate?

- Hydrostatic pressure difference primary source of resistance to provide the hydrostatic

pressure is efferent arterioles and the capillaries of the vasa recta

- colloid osmotic pressure - tends to be constant under most conditions

- dehydration - burn victims

Regulation of GFR

1. Intrinsic myogenic response of the afferent arterioles- intrinsically restrict to an increase in blood pressure

2. Juxtaglomerular apparatus macula densa cells

- monitor osmolarity and flow of fluid in distal tubule- release a variety of factors - paracrine

granular or juxtaglomerular cells - modified smooth muscle cell in the wall of the afferent arteriole - secrete renin - locally and systematically - Ang II

3. Sympathetic innervation - induces contraction of glomerulus

- closing down filtering capillaries - thus reducing surface area available for filtration

Tubular reabsorption

Humans 

180 liters of filtrate - l liter of urine99% of filtered water reabsorbed99% of the NaCl reabsorbed (1800 g filtered, 10 g excreted)

glucose - small, freely filtered

- completely reabsorbed in normal individuals - loss is equivalent to losing chemical energy from the

organism

Transport in the proximal tubule

passive

Proximal tube

- starts the process of concentrating the urine- 70% of the Na+ is removed from the lumen - a proportional amount of Cl-, water and other solutes follow- about 75% of the filtrate is reabsorbed - fluid is iso-osmotic with respect to plasma and interstitial fluids - water transport is coupled to active sodium transport

 Filtrate that reaches the distal portion of the proximal tubule

- 1/4 its original volume- substance not taken up by active transport or do not

passively diffuse have been concentrated 4 fold ( we are iso-osmotic though)

 - all of this is due to active transport of Na+

-glucose, amino acids absorbed using the Na+ electrochemical gradient

Structure of cells in the proximal tubule

Loop of Henle Descending limb - - made up of very thin cells that have very few mitochondria

- they have no brush border, no active Na transport- have very low permeability to NaCl and urea- modest water permeability

 Thin segment of the ascending limb

- similar features as the descending limb - except it is highly permeable to NaCl- permeability to urea and water is low

 Thick ascending limb

- actively transport Na outward from the lumen to the interstitial space

- very low permeability to water - result - fluid reaching the distal tubule is hypo-osmotic

How do we make a concentrated urine?

-water is removed from the urine as it passed through the collecting duct

- only those vertebrates that have a loop of Henle can make a hyperosmotic urine

- degree to which the urine cam be concentrated is proportional to the length of the loop of Henle

desert dwellers such as the kangaroo rat have the longest loops

 - related to this ability to concentrated urine, it was observed that osmolarity of the interstitial fluids of the medulla progressively increase as it moves deeper towards the renal pelvis.

What is a counter current multiplier?- mechanism for concentrating - chemical, heat

Two major components- single effect - multiplied effect

 Primary features 1. Standing concentration gradient set up is dependent on both the single effect and the multiplier effect operating.

2. Difference from one end of the limb (loop) to the other is far greater than the difference separating the limbs of the loop

3. Requires asymmetry - net transport of Na+ in one direction.

How do marine birds maintain their osmotic balance without access to fresh water?

Knut Schmidt_Nielsen - 1957  Nasal gland capable of secreting a hypertonic solution of NaCl

Found also in reptiles , marine iguana, sea snakes , marine turtles

Crocodiles - salt gland in the tongue

Ammonotelic - excrete NH3 or NH4+ 

-teleost and aquatic invertebrates cell membranes are generally permeable to unionized

ammonia (NH3)- not very permeable to NH4+ (ammonium ions)- most ammonium excretion occurs via passive diffusion of

NH3 - teleosts

Transport of amino groups transfer amino groups to alpha-ketoglutarate to make

glutamate go to gills - reverse

go to liver - make into glutamine - less toxicmammals - glutamine is transported to kidney - deaminated in the cell of the kidney tubules- ammonia is released into lumen and excreted as NH4+

Problem with NH3 and NH4+

highly toxic (lethal at 50 mM in most mammals) - elevates pH- substitutes for K+ ions in many reactions

Positive - highly soluble - cheap  

Ureotelic - urea -excreting animals  - urea - very soluble - membranes are relatively impermeable to urea

- need a transport protein- far less toxic than ammonia- requires much less water - contains two nitrogen per molecule- made by the ornithine-urea cycle in most vertebrates (teleosts)

- occurs in the liver Two NH2 groups and one CO2 are added to ornithine to make

a molecule of arginine

Elasmobranch us the ornithine cycle

Nucleic acid catabolism

teleost and many marine invertebrates use the uricolytic pathway

-urea is produce from uric acid - uric acid comes from the transamination of aspartate or other nucleic acid  

Uricotylic excretion of nitrogenous wastes

Birds, reptiles, most terrestrial arthropods Excretory product - uric acid or guanine- carries four nitrogens per molecule- lack uricase - can't break it down - poorly soluble - readily precipitates- requires very little water to get rid of - best solution for conditions of limited water availability- expensive