OSMOREGULATORY FUNCTIONS IN VERTEBRATE

Prepared by: Pradip Subedi

ISOTONIC , HYPOTONIC AND HYPERTONIC ANIMALS

Isotonic animals

Body fluid concentration=surrounding medium concentration, eg-most marine animals.

This type of animals never face the problem of osmoregulation.

Hypotonic animalsAnimals lives in lower salt

concentration are called hypotonic animals,

In this animals, the water continuously enters into their bodies and dilute their body fluids, they have to develop special mechanisms to get rid of the excess water that enters into their bodies.

Eg- fresh water fishes

Hypertonic animals

Animals that lives in a medium of higher salt concentration is called hypertonic animals.

In this animals, the water continuously loss from their body and face a danger of dehydration. , they have to develop special mechanisms to get rid of the dehydration.

Eg- marine bony fishes

Introduction

Osmo regulation is the process which regulates the concentration and osmotic pressure of blood by regulating the water contents of blood plasma. It is an important process as excessive loss of water may cause dehydration whereas excess of water intake may dilute the body fluids.

There is always an osmotic challenge in animals due to different medium they live, temperature, diet and weather conditions, to cope with it some osmoregulatory mechanism is intiated to maintain the optimum concentration.

Role Of Kidney In Osmoregulation

The vertebrate kidney is extremely flexible in its working. It excretes large amount of hypotonic urine when water intake is very high, while it excretes small amount of hypertonic urine when water is deficient and needs to be conserved.

Fresh water fishes

All animals that live in fresh water must cope with a continual inflow of water from their hypotonic environment. In order to maintain homeostasis of its fluid, the freshwater fish develops some special osmoregulatory mechancism to excrete excess amount of water.

Freshwater teleosts:active

passive

H2O

Na+, Cl-

Na+, Cl-

Drink less

waterkidneys

Ion exchangepumps; beta chloride cells

Marine water fishes

Unlike of fresh water animals, marine animals a continual loss of water to their hypertonic environment. In order to maintain homeostasis of its fluid, the marine animals develops some special osmoregulatory mechancism to prevent from dehydration

Saltwater teleosts:

drink

active tran.

passive diff.

Na+, Cl-

Mg++, SO4=

H2O

Na+, Cl-

Na+, Cl-

chloride cells

Mg++, SO4=

kidneys

Marine sharks and most other cartilaginous fishes (chondrichthyans) use a different osmoregulatory “strategy.”

Unlike bony fishes, marine sharks do not experience a continuous osmotic loss because high concentrations of urea and trimethylamine oxide (TMAO) in body fluids leads to an osmolarity slightly higher than seawater.

TMAO protects proteins from damage by urea. Consequently, water slowly enters the shark’s

body by osmosis and in food, and is removed in urine

Amphibia

Fresh water which contains a small percentage of salts diffuses into the body through smooth skin of amphibians. The well developed glomerular kidneys serves to remove the excess water which is excreted as dilute urine. The salts lost through the urine are replenish by reabsorption of ions from the urine.

When in water, the frog's bladder quickly fills up with a hypotonic urine.

On land, this water is reabsorbed into the blood helping to replace water lost through evaporation through the skin.

The reabsorption is controlled by a hormone similar to mammalian ADH.

Lizards and Snakes

Many reptiles live in dry environments (e.g., rattlesnakes in the desert). Among the many adaptations to such environments is their ability to convert waste nitrogen compounds into uric acid.

Uric acid is quite insoluble and so can be excreted using only a small amount of water. Thus we find that reptile glomeruli are quite small and, in fact, some reptiles have no glomeruli at all.

Birds

Bird kidneys function like those of reptiles (from which they are descended). Uric acid is also their chief nitrogenous waste.

Most birds have a limited intake of fresh water. However, they need filter only enough to wash a slurry of uric acid into the cloaca where enough additional water is reclaimed to convert the uric acid into a semisolid paste. (It is the whitish material that pigeons leave on statues.)

Mammals

Mammals consists of highly developed kidney for osmoregulatory purposes.

Nephrons and associated blood vessels are the functional units of the mammalian kidney

The mammalian kidney’s ability to conserve water is a key terrestrial adaptation

Mechanism and functions of kidney in vertebrates

Excess water in body fluids

When this condition arises, the urine passed out of the body is more dilute or hypotonic than the body fluids to expel the excess of water. This is because excess of water in the body fluids lowers the osmotic pressure of the blood and increases the blood volume. These changes disturbs the exchange of materials between the blood pressure and causes cardiovascular dysfunctioning respectively

It involves 2 processes. They are:

a) Ultra filtrationb) Decreased reabsorption.

a) ultrafiltration

Ultra filtration is increased due to the increase in hydrostatic pressure due to the excess of water. So more nephric filtrate is filtered out from the glomerular capillaries into Bowman's capsule.

b). Decreased reabsorption

Water is mainly reabsorbed through the collecting tubules. The permeability of the wall of the DCT and collecting tubules is controlled by anti - diuretic hormone or ADH or vasopressin hormone which is released from the posterior lobe of the pituitary gland. Excess of water in the body fluids signals to posterior pituitary to stop the release of the hormone vasopressin. Deficiency of this hormone lowers the permeability of the cells of the distal convoluted tubule and the collecting duct, decreasing the reabsorption of Na+ from the filtrate continues in these regions of the nephrons.

More filtration combined with less reabsorption of water produces abundant dilute urine and this brings down the volume of body fluids to normal.

Shortage of water in body fluids When the volume of body fluids decreases below

normal due to profuse sweating during heavy exercise or high temperature or excessive bleeding or a prolonged delay in fluid intake, the rate of ultra filtration is decreased due to decreased blood volume and low hydrostatic pressure of blood in the glomerular capillaries and rate of reabsorption of water is increased by increasing the permeability of the wall of DCT and collecting tubules due to increased release of ADH from the posterior pituitary.

Less ultrafiltration and more reabsorption produce small amount of hypertonic urine which increases body fluid volume to normal.

Achieving osmoregulation in vertebrates

Four process occurs:-a) filtration — fluid portion of blood

(plasma) is filtered from a nephron (functional unit of vertebrate kidney) structure known as the glomerulus into Bowman's capsule or glomerular capsule (in the kidney's cortex) and flows down the proximal convoluted tubule to a "u-turn" called the Loop of Henle (loop of the nephron) in the medulla portion of the kidney.

Achieving osmoregulation in vertebrates

b) Reabsorption — most of the viscous glomerular filtrate is returned to blood vessels that surround the convoluted tubules.

c) Secretion — the remaining fluid becomes urine, which travels down collecting ducts to the medullary region of the kidney.

d) Excretion — the urine (in mammals) is stored in the urinary bladder and exits via the urethra; in other vertebrates, the urine mixes with other wastes in the cloaca before leaving the body (frogs also have a urinary bladder).

Key

Activetranspor

tPassivetranspor

t

INNERMEDULL

A

OUTERMEDULL

A

H2O

CORTEX

Filtrate

Loop of

Henle

H2O K+HCO3–

H+NH

3

Proximal tubuleNaC

lNutrient

s

Distal tubule

K+ H+

HCO3–

H2O

H2O

NaCl

NaCl

NaCl

NaCl

Urea

Collecting

duct

NaCl

Juxt

aglo

mel

ula

r ap

para

tus

•Smooth muscle cells of afferent and efferent arterioles are swoolen and glanural called juxtaglomerular cells. • epithelial cells of DCT in contact with afferant and efferent artiole: Macula densa•Macula densa + juxtaglomerular cells= juxtaglomerular apparatus which secretes renin •Renin is associated with the concersion of angiotensinogen into angiotensin influences the reabsorption of sodium ion from DCT and water from collecting tubules

Early proximal tubule

Reabsorbed nutrient

Glucose (100%), amino acids (100%), bicarbonate (90%), Na+ (65%), Cl−, phosphate and H2O (65%)

Notes PTH will inhibit

phosphate excretion AT II stimulates Na+, H2O

and HCO3− reabsorption.

Thin descending loop of Henle

Reabsorbed nutrient

H2O

Notes

Reabsorbtion makes urine hypertonic.

Thick ascending loop of Henle

Reabsorbed nutrients

Na+ (10–20%), K+, Cl−; indirectly induces para cellular reabsorption of Mg2+, Ca2+

Notes This region is

impermeable to H2O and the urine becomes less concentrated as it ascends.

Early distal convoluted tubule

Reabsorbed nutrients

Na+, Cl−

Notes

PTH causes Ca2+ reabsorption.

Collecting tubules

Nutrients reabsorbed

Na+(3–5%), H2O

Notes

Na+ is reabsorbed in exchange for K+, and H+, which is regulated by aldosterone.

ADH acts