This web page contains notes to accompany lectures in Vertebrate Physiology, Biology 410, taught by Dr. Peter King in the Department of Biology, Francis Marion University, Florence, South Carolina, 29502, USA.

One important homeostatic requirement is osmoregulation, i.e. the maintenance of adequate quantities of water and the maintenance of dissolved ions in that water.
Water is indispensable to life as we know it.
From the cellular to the organismal level, water is essential.
One difficulty with maintaing correct water balance is that most organisms live in environments where external environments differ in osmolarity to their internal environment.
In aquatic environments water can be gained or lost by diffusion.
Water is lost by all animals during respiration. Respiratory surfaces in air must be moist to allow oxygen to go into solution before diffusing through lungs.

Cell membranes permeable to oxygen are also permeable to water.
Animals excrete metabolic waste products which often involves loss of water. (humans must excrete a minimum of 0.9 liters of urine per day to remove toxic nitrogenous waste).
Mammals use evaporative cooling to maintain body temperature.

Many marine invertebrates have extracellular fluid close to seawater in osmolarity.
Most vertebrates have extracellular fluid with aboit a third that of seawater - suggesting freshwater origins.
Osmolarity of intracellular and extracellular fluid is very close in most animals.

So how do animals cope with their differing environments?

Possibilities?
1. Actively pump water in or out.
2. Prevent movement of water.
3. Active movement of salts.
4. Conservation of salts.

Water breathing osmoregulators.

Freshwater animals
Body fluids of freshwater animals (e.g. fish) is hyperosmotic compared to the external environment. (200-300mosm/L vs 50mosm/L).
Two big problems
1. Water moves into body
2. Continual loss of body salts
Solutions
1. Production of large quantities of urine.
2. Reabsorb salts from urine (vertebrate kidney).
3. Active ingestion and cellular transport of salts (fish have sodium pumps in their gills to take up Na⁺). A similar mechanism is found in fisg gills, frog skin, turtle bledder and mammalian kidney.
4. Don't drink freshwater!

Saltwater animals
Most have body fluids that are hyposmotic compared to the surrounding environment.
Two big problems
1. Water leaves the body
2. Salts enter the body
Solutions
Saltwter fish drink sea water to gain water.
They actively pump ions Na⁺, Cl^- and K⁺ across epithelium of gills.
Urine is isotonic to blood but rich in ions not excreted in lungs such as Ca²⁺, Mg²⁺.
Sharks are a special case: they retain urea in their body and body fluids are isotonic with sea water. They secrete salt in a special rectal gland.

Marine reptiles and birds drink seawater to replace lost water.
Kidneys cannot produce concentrated urine and excess salts are excreted using special glands.
Birds have salt glands near their eyes.
Crocodiles have salt glands on their tongues.

Terrestrial animals generally have a problem of water conservation.
Nitrogenous waste is proced as a byproduct of protein breakdown and other metabolic process. This can be toxic and must be excreted.
Excretion of nitrogenous waste generally results in loss of water.
The nitrogenous waste excreted through vertebrate kidneys varies between orders.
Fish generally excrete ammonia which is soluble but toxic. It must be reasonably dilute.
Birds and reptiles excrete uric acid which is insoluble and forms a white precipitate.
Mammals excrete soluble urea which is dilute.

The mammalian kidney has evolved the ability to concentrate urine and excrete a fluid that is higher in salt concentration that other body fluids.
Concentration of urine is correlated to some degree with water availability.

How does the vertebrate kidney work?
The kidney is basically a tube that comes into close contact with blood vessels and some fluid from the blood diffuses into the tube and passes out of the body.
We will look in detail at the mammalian kidney to understand the process.

The functional unit of the mammalian kidney is the nephron (about 1.3 million in human kidney).
Blood comes into contact with the renal tubule in athe Bowman's capsule. Each Bowman's capsule has a network of capillaries (glomerulus) surrounded by the capsule which is the end of the renal tube.
Fluid (ultrafiltrate) passes from the blood to the capsule under hydraulic pressure in a process called glomerula filtration.
More correctly ultrafiltration depends upon
1. Net hydrostatic pressure (+)
2. Osmotic pressure (-)
3. Hydraulic permeability (3 layers)
The ultrafiltrate consists of all blood constituents except blood cells and proteins.
Proteins and blood cells are too big to fit through the holes.

Glomerular filtration rate (GFR) is about 125ml per minute or 180 liters per day in humans.Why don't we dehydrate?
Because most of the water and solute are reabsorbed (we produce about 1-1.5L of urine).
How much of our blood goes to the kidneys?
About 25% (all blood filtered every 30min)
About 6% of BMR (basal metabolic rate) used in kidney function!

Reabsorption of the ultrafiltrate happens in the renal tubules.
The renal tubule can be divided into the proximal and distal tubules.
Reabsorption of virtually all organic nutrients and about 60% of ions occurs in proximal tubules.
Distal tubules have some active transport of ions in both directions and react to a number of stimuli controlling blood pH and blood volume.
What is not reabsorbed is excreted in the urine and is called clearance.
Different solutes have different rates of clearance.
Generally 100% of glucose is reabsobed - but this is limited by the active transport mechanism.
If abnormally high levels are in the blood then some may appear as clearance in the urine (diabetes?).

Mammal nephron
Between the proximal and distal part of some renal tubules in mammals is the loop of Henle.
This loop enables urine to achieve osmotic concentrations above that of plasma and conserve water.
Some birds also contain similar loops.
Mammalian kidney arranged into exterior cortex and interior medulla.
Renal cortex has an osmolarity gradient, mainly set up by active sodium and chloride excretion in the loop of Henle.
Urine is concentrated when the collecting ducts pass through the renal medulla and water is lost due to the high osmolarity.
The descending loop is permeable to water and equilibriates with the interstitial fluid.
The thick portion of the ascending loop is impermeable to water and Na⁺ and Cl^- are actively pumped out.
NaCl responsible for most of the concentration gradient. Urea also plays a role.
Collecting tubule travels through medulla and equilibriates concentrating urine.

Longer loops of Henle establish bigger osmolarity gradients and can increase the urine concentration.
Desert mammals have longer loops of Henle and have extremely concentrated urine (up to 25x blood plasma) to conserve water.

In the distal tubule some ions are pumped into the tubule from the blood to help balance pH.

How is filtration and water conservation controlled in the kidney?
Antidiuretic hormone is released by the posterior pituitary in response to high solute concentrations in the blood or low blood volume.
It increases the permeability of the collecting ducts and more water is reabsorbed, and urine is more concentrated.

A second controlling system is the renin/angiotensin system. (page 599)
This involves autoregulation of glomerular filtration.

A second controlling system is the renin/angiotensin system. (page 599)
This involves autoregulation of glomerular filtration.
Low blood pressure in the afferent arterioles (juxtaglomerula cells) or low Na⁺ in macula densa of distal tubules causes release of renin from juxtaglomerula cells.
Collectively these make up the juxtaglomerular apparatus.
Angiotensin I is converted to angiotensin II by converting enzyme released by capillary endothelium in lungs.
Angiotensin II causes release of aldosterone from adrenal cortex.
Aldosterone causes more sodium to be reabsorbed in the distal tubules.
Due to osmotic pressure, water follows Na⁺ out of distal tubule into blood, increasing blood volume and pressure.
Angiotensin II also acts as a general sytemic vaso constrictor, increasing blood pressure.
Some studies suggest that it has a particular constriction on the efferent arteriole (so increasing glomerular filtration, GFR).

Yet another hormone effects water balance (page 336).
In response to increased venous pressure in the heart certain atrial cells release atrial natriuretic peptide (ANP). This counteracts aldosterone and causes secretion of salt in the urine and greater quantities of urine production.

These are some of the major controllers of GFR and water conservation. There are many other factors that can play a role including the sympathetic nervous system.

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This page was created by Peter King. Please contact the author at pking@fmarion.edu with comments.
http://people.fmarion.edu/pking/vertphys/osmoregulation.html
Last edit January 10, 2011.
Copyright Peter King.