Chapter 44OSMOREGULATION AND EXCRETION

Osmoregulation is the management of water content in the body and solute composition.

Excretion is the elimination of nitrogenous waste product of metabolism.

OSMOREGULATION

Osmoregulation controls the movement of solutes between tissues and their external environment.

This process also regulates water because water follows solutes by osmosis.

OSMOSIS

Diffusion is the movement of solutes from the region of higher concentration to the region of lower concentration.

Osmosis is the movement of wateracross a membrane from the area of higher concentration to area of lower concentration.

Osmolarity is the concentration of a substance expressed in moles per liter, mol/l.

The unit of osmolarity often used in physiology is milliosmoles per liter, mosm/L.

  • 1 mosm/L = 10-3 M.
  • The osmolarity of the human blood is 300 mosm/L.
  • The osmolarity of seawater is 1000 mosm/L.

When two solutions separated by a selectively permeable membrane have the same osmolarity are said to be isoosmotic. The terms hyperosmotic and hypoosmotic are also used.

OSMOTIC CHALLENGES

The ability of animals to regulate their internal environment is called homeostasis.

A regulator is an animal that uses mechanisms of homeostasis to maintain an internal environment when the external environment fluctuates.

A conformer is an animal that allows the internal environment to fluctuate in agreement with the changes in the external environment. Conformers usually live in stable environments.

No organism is a perfect regulator or conformer. Organisms use a combination of mechanisms when faced with environmental changes.

Homeostasis requires a careful balance of materials and energy: gains versus losses.

Osmoconformers are isoosmotic to their surroundings.

Osmoregulators are animals that must control their internal osmolarity.

  • Stenohaline animals cannot tolerate substantial changes in external osmolarity.
  • Euryhaline animals can survive large fluctuations of external osmolarity.

Most organisms are stenohaline.

MARINE ANIMALS

  • Most marine invertebrates and hagfishes are osmotic conformers. Their body fluids vary with changes in the seawater.
  • The cells are hypoosmotic relative to the surrounding seawater.
  • Water tends to flow out of the gill cells.
  • The cells run the risk of plasmolysis, which is shriveling and dying.
  • Saltwater fish secrete large amounts of salt and drink lots of water.

Marine bony fish must replace lost fluid.

  • They lose water osmotically through their skin and gills.
  • Drink large amounts of water and take in salt.
  • Excrete excess salt through their gills
  • Excrete little urine in order to conserve water.

Chondrichthyes accumulate and tolerate urea and their tissues are hypertonic to seawater.

  • Water diffuses into their body.
  • They maintain a high concentration of urea and trimethylamine oxide (TMAO), which protects proteins from damage by urea.
  • Concentration of body salts, urea, TMAO and other compounds is greater than 1,000 mosm/L and therefore slightly hyperosmotic to seawater. This decreases the water loss through the skin.
  • Water slowly enters the body of sharks and relatives.
  • Kidneys excrete large volume of urine.
  • Excess salt is excreted by the kidney and in some by the rectal gland.

FRESHWATER ANIMALS

In freshwater fish...

  • Freshwater is hypotonic to the cell.
  • They constantly gain water.
  • Ions tend to move out of the cell into the surrounding water.
  • Electrolytes lost must be replaced by eating and by active transport from the surrounding water.
  • The gill cells are hypertonic relative to the surrounding water; therefore, the cells gain water through osmosis.
  • Cells and tissues that are gaining water are under osmotic stress.
  • Freshwater fish excrete large amounts of water in the urine and do not drink water.
  • Some protists have contractile vacuoles that pump out excess water.

ANIMALS THAT LIVE IN TEMPORARY WATER

Some animals that live in temporary ponds or films of water around soil particles can lose almost all their body water and survive. This ability is called anhydrobiosis.

  • Tardigrades (water bears) contain 85% water in their body; in a dehydrated state they have less than 2% water in their bodies.

These animals can live in this desiccated state for years. The mechanism is not understood.

The disaccharide trehalose seems to protect the cells by replacing the water that is normally associated with membranes and proteins.

Anhydrobiosis is not well understood by scientists and research is being done in this area.

Land animals...

  • Land animals constantly lose water to the environment through evaporation.
  • Gas exchange occurs through the wet surfaces of the lung epithelium.
  • Sweating and panting in order to keep their body cool also loses water.
  • They have adaptations that minimize water loss, e. g. exoskeleton, shells, and keratinized dead skin cell layer.

TRANSPORT EPITHELIA

Water balance and waste disposal depend on transport epithelia.

Transport epithelia have the ability to move specific substances in controlled amounts in particular directions.

In most animals, transport epithelia are arranged into complex tubular networks with extensive surface areas.

The secretory cells of the transport epithelium actively secrete salts from the blood into the tubules.

NITROGENOUS WASTE

Principal metabolic wastes are water, carbon dioxide and nitrogenous wastes (ammonia. urea and uric acid).

Nitrogenous wastes are the products of deamination of amino acids and breakdown of nucleic acids.

Ammonia is highly toxic and it is usually converted to uric acid or urea.

  • Ammonia excretion is most common in aquatic species.
  • Ammonia is converted to ammonium, NH4+.
  • Ammonium ions are excreted through the gills.
  • In many invertebrates, ammonia is excreted across the whole body surface.

Urea is formed in the liver by combining ammonia and carbon dioxide.

  • Urea is soluble and less toxic than ammonia.
  • It requires less water than the same amount of ammonia.
  • Mammals, most adult amphibians and many marine fishes and turtles excrete urea.
  • The animal must spend energy to produce urea.

Uric acid is the product of nucleic acid and amino acid breakdown.

  • It is excreted in the form of a crystalline paste with little water loss.
  • It is relatively non-toxic.
  • Uric acid production requires more energy than urea production.
  • Birds, reptiles, land snails, and some insects secrete uric acid.

The kind of nitrogenous waste excreted depends on the animal’s evolutionary history and habitat.

Uric acid precipitates out of solution and can be stored inside the amniotic egg.

Soluble wastes can diffuse out of the shell-less egg of amphibians.

The amount of nitrogenous waste produced is coupled to the animal’s energy budget.

DIVERSE EXCRETORY SYSTEMS

1. Sponges and cnidarians use diffusion from cells to environment.

2. Protonephridia are found flatworms, nemerteans, rotifers, lancelets and some annelids.

  • Internal tubules with no openings.
  • Blind ends called flame bulbs have flame cells.
  • Fluid enters the lumen of the tubule through selectively permeable membranes of the folding tubule cells.
  • The beating of the cilia of the flame cells keep the fluid moving towards the nephridiopore.
  • Excrete through nephridiopores.
  • It functions mostly in osmoregulation; wastes diffuse through the skin or are excrete through the lining of the gastrovascular cavity.
  • In some parasitic worms that are isoosmotic to their environment, the protonephridia are used to expel wastes.

3. Metanephridia are found in most annelids, in mollusks.

  • Metanephridia are found in most annelids.
  • Each metanephridium is a tubule open at both ends.
  • The inner ends open into the coelom as a ciliated funnel called nephrostome, which collects fluid from the coelom.
  • The outer end is a nephridiopore.
  • As coelomic fluids pass through the tubule, needed material is reabsorbed.
  • The urine is much diluted and balances the uptake of water through the skin.
  • Metanephridia have an osmoregulatory and excretory function.

4. Malpighian tubules are found insects and spiders.

  • Insects and other terrestrial arthropods have Malpighian tubules.
  • Blind end tubules of the digestive tract that stretch into the hemocoel.
  • Their cells transfer wastes and salts from the hemolymph to the lumen of the tubule by diffusion and active transport. Water follows.
  • They empty into the intestine.
  • Water and some salts are reabsorbed in the rectum and almost dry nitrogenous wastes are eliminated with the feces.

MAMMALIAN URINARY SYSTEM

Kidney produces urine.

Ureter brings urine to the urinary bladder.

Urinary bladder stores urine temporarily.

Urethra leads the urine to the outside.

The outer region of the kidney is called the renal cortex and the inner region the renal medulla.

The renal medulla contains a number of cone-shaped structures called renal pyramids.

At the tip of each renal pyramid is a renal papilla into which the collecting ducts open.

The renal pelvis is a pyramidal chamber that collects and leads the urine to the ureter.

The nephron is the functional unit of the kidney.

KIDNEY STRUCTURE

The outer region of the kidney is called the renal cortex and the inner region the renal medulla.

The renal medulla contains a number of cone-shaped structures called renal pyramids.

At the tip of each renal pyramid is a renal papilla into which the collecting ducts open.

The renal pelvis is a pyramidal chamber that collects and leads the urine to the ureter.

The nephron is the functional unit of the kidney. It consists of a single long tubule and a ball of capillaries. The blind end of the tubule forms the Bowman's capsule, which surrounds the glomerulus.

The filtrate consists of water, salts, HCO3–, H+, urea, glucose, amino acids, some drugs and other foreign chemicals.

1. The filtrate passes from capillaries  Bowman's capsule  proximal convoluted tubule  loop of Henle  distal convoluted tubule  collecting duct  renal pelvis.

About 80% of the nephrons in human are cortical nephrons with a short loop, and 20% are juxtamedullary nephrons with a long loop of Henle.

Mammals and birds are the only animals with juxtamedullary nephrons. The nephrons of all other animals lack the loop of Henle.

The tubules of the nephron are lined with a transport epithelium whose function is to reabsorb water and solutes.

  • From 1,000 to 2,000 liters of blood flows through a pair of human kidneys each day.
  • Nearly all of the sugar, and other organic nutrients and most of the water is reabsorbed.
  • Only about 1.5 liter urine is discarded.

2. Blood circulates through the kidney in the following sequence:

Renal artery  afferent arteriole  capillaries of glomerulus  efferent arteriole 

peritubular capillaries  vasa recta  small veins  renal veins.

3. Filtration, reabsorption and secretion produce urine.

Bowman's capsule

  • Filtration is not selective with regard to ions and small molecules.
  • Reabsorption is highly selective.
  • Some substances are actively secreted from the blood.

Hydrostatic pressure in glomerular capillaries is higher than in other capillaries. Efferent arteriole is smaller than the afferent arteriole.

The high pressure forces about 10% of the plasma out of the capillaries into Bowman's capsule.

Glomerular capillaries are highly permeable with numerous small pores (fenestration) present between the endothelial cells.

There is a large permeable surface provided by the highly coiled capillaries.

Glucose, amino acids, ions and urea pass through and become part of the filtrate.

Reabsorption is highly selective.

Proximal tubule

  • H+ and NH3 are filtered into the lumen of the Bowman's capsule and the proximal tubule.
  • NH3 neutralizes the acid and maintains a constant pH.
  • HCO3- is a blood buffer and about 90% is reabsorbed here through active transport.
  • Toxins and foreign substances also pass into the filtrate.
  • K+, glucose and amino acids are reabsorbed.
  • NaCl and water are reabsorbed. Na+ is actively transported from the filtrate into the interstitial fluid of the kidney; water follows by osmosis.
  • The osmolarity of the filtrate in the proximal tubule is about 300 mosm/L.

Descending limb of the loop of Henle

  • The transport epithelium in this section of the tubule is permeable to water but not to salts and small solutes.
  • The interstitial fluid in this area, the outer medulla, is hyperosmotic to the filtrate.
  • The osmolarity of the interstitial fluid gradually increases from the outer cortex to the inner medulla of the kidney.
  • As the filtrate descends, it loses water to the interstitial fluid and it becomes more concentrated.
  • The osmolarity in the descending arm of the loop of Henle changes from 300 mosm/L at the beginning to 1,200 mosm/L at the tip of the loop.

Ascending limb of the loop of Henle

  • The filtrate reaches the tip of the loop deep into the inner medulla in the case of the juxtamedullary nephron, and then moves back to the cortex.
  • The transport epithelium of the ascending limb is permeable to salts but not water (in contrast with the descending limb).
  • The filtrate passes first through a thin section of the ascending limb and NaCl, which had become concentrated in the descending tube, now passes out by diffusion into the interstitial fluid.
  • This addition of salt contributes to the high osmolarity of the inner medulla.
  • In the following thick section of the tubule, NaCl is excreted into the medulla by active transport.
  • The filtrate passes out salts but no water, and becomes more diluted.
  • In the ascending arm, the osmolarity changes from 1,200 mosm/L to 100 mosm/L in the distal tubule.

Distal tubule

  • K+ and H+ are actively secreted into the filtrate.
  • NaCl and HCO3- are actively reabsorbed, and water diffuses out by osmosis.
  • The control secretion of H+ and reabsorption of HCO3- contribute to the pH regulation of the blood and interstitial fluids.
  • Osmolarity here is 100 mosm/L.

Collecting duct

  • The collecting duct brings the filtrate from the cortex to the inner medulla.
  • The filtrate is hypoosmotic to the interstitial fluid as it enters the collecting duct.
  • NaCl is actively reabsorbed here and determines how much salt is actually excreted in the urine.
  • The transport epithelium here is permeable to water and urea (in the inner medulla) but not to salt.
  • The filtrate becomes more concentrate as it loses more and more water to the hyperosmotic interstitial fluid of the medulla.
  • NaCl and urea are the major contributors to the high osmolarity of the interstitial fluid in the medulla.
  • By reabsorbing water, the urine becomes hyperosmotic to the general body fluids, but is isoosmotic to the interstitial fluid of the medulla.
  • In the inner medulla, the duct becomes permeable to urea; but most of the urea in the filtrate remains in the collecting tubule.
  • As the filtrate flows in the collecting duct passes interstitial fluid of increasing osmolarity, more water moves out of the duct by osmosis, thereby concentrating the solutes, including urea, that are left behind in the filtrate.
  • The high osmolarity of the interstitial fluid allows solutes to remain in the urine and be eliminated with minimal water loss.
  • In the collecting tubule, the osmolarity changes from an initial 100 moms/L to 1,200 mosm/L.

MAMMALIAN ADAPTATION TO CONSERVE WATER

The ability of the kidney to conserve water is a an adaptation to terrestrial life.

The nephron especially in the area of the loop of Henle uses energy in order to produce a region of high osmolarity in the outer and inner medulla of the kidney, which can then be used to extract water from the filtrate and urine in the collecting duct.

The principal solutes in this osmolarity gradient are NaCl, which is excreted by the loop of Henle, and urea, which leaks across the epithelium of the collecting duct in the inner medulla.

Summary of osmolarity change in the filtrate:

Bowman's capsule: 300 mosm/L

Proximal tubule: 300 mosm/L

Descending loop of Henle: 300 to 1,200 mosm/L.

Ascending loop of Henle: 1,200 to 100 mosm/L

Distal tubule: 100 mosm/L.

Collecting duct: 100 to 1,200 mosm/L.

Urine: 1,200 mosm/L

The process depends on the salt concentration in the interstitial fluid in the kidney medulla.

The interstitial fluid has higher salt concentration around the loop of Henle.

There is a counterflow of fluid through the two limbs of the loop of Henle.

Water is drawn by osmosis from the filtrate as it passes through the collecting ducts and it concentrates the filtrate.

  • As the filtrate flows in the collecting duct passes interstitial fluid of increasing osmolarity, more water moves out of the duct by osmosis, thereby concentrating the solutes, including urea, that are left behind in the filtrate.

Capillaries known as the vasa recta remove some of the water that diffuses from the filtrate into the interstitial fluid.

The vasa recta are extensions of the efferent arteriole that extend deeply into the medulla and then return fluid to the veins draining the kidney.

Urine is about 96% water, 2.5% urea, 1.5% salts and traces of other substances.

Urinalysis is the physical, chemical and microscopic examination of urine.

HORMONE REGULATION OF KIDNEY FUNCTIONS

Urine volume is regulated by the hormone ADH (antidiuretic hormone), which is produced by the hypothalamus, and stored and released by the posterior lobe of the pituitary gland in response to an increase in osmotic concentration of the blood, caused by dehydration.

An increase above the set point is 300 mosm/L releases ADH.

  • Low fluid intake decreases blood volume and increase osmotic pressure of blood.
  • ADH increases the permeability of collecting ducts to water, increasing reabsorption of water and decreasing water excretion.
  • An increase in fluid uptake decreases the osmolarity of the blood below 300 mosm/L.
  • Little ADH is released and the permeability to water of the distal tubule and collecting duct is reduced and more water is excrete.

A second regulatory mechanism involves the tissue called juxtaglomerular apparatus or JGA.