Homeostasis is defined as the maintenance of a constant internal environment within an organism.

Internal environment means the immediate surroundings of the cells. In mammalian tissues the cells are generally surrounded by tiny channels and spaces filled with fluid.

The most important features of the internal environment that must be kept constant are:

  • Chemical constituents, for example, glucose ions, etc.
  • Heart rate
  • Its osmotic pressure, determined by the relative amounts of water and solutes.
  • The level of carbon dioxide
  • Its temperature

The importance of a constant internal environment to the well-being of cells can be shown by removing tissues from the body. If they are subjected to conditions markedly different from those prevailing in the body they will die, but if maintained under the correct conditions they will survive.


The Homeostatic control of glucose. The normal value of glucose in the human blood stream is approximately 90mg per 100cm3 and even after the heaviest carbohydrate meal rarely exceeds 150mg per 100cm3.

After entering the hepatic portal vein, it is conveyed to the liver. In the liver three main things may happen to it.

  • It may be broken down into CO2 and water (cell respiration).
  • It may be built up into glycogen and stored.
  • It may be converted into fat and sent to the body’s fat deposits for storage.
  • Instead of being metabolized or stored, it may pass on from the liver to the general circulation. In fact under certain circumstances the glycogen stores in the liver may be broken down so as to add the level of glucose in the body.
  • The level of glucose in the blood and tissue fluids at any given moment is mainly determined by the relative extent to which these different processes occur in the liver if there is too much glucose, as for example, after a heavy meal rich in carbohydrate, the liver metabolizes what it can, and stores the rest as glycogen. If there is a deficiency of glucose, the liver breaks down glycogen, into glucose, thereby raising the glucose level in the body.
  • In cases of prolonged deficiency; glucose may be formed from the non-carbohydrate sources, including proteins. This is called gluconeogenesis. The wasting away of the tissues, which occurs in extreme starvation is because the body resorts to converting its tissue protein into carbohydrate.

The role of Pancreas

The liver cannot perform this homeostatic function unaided. It has to receive information instructing it what to do. This is provided by the hormone insulin which is secreted into the blood stream by special group of cells, the islets of langerhans in the pancreas.

On reaching the liver, insulin exerts its effect increasing the oxidative breakdown of glucose the formation of glucose from glycogen and non-carbohydrate sources. Insulin thus achieves the overall effect of lowering the level of glucose in the body.

In the absence of insulin the reverse takes place, oxidative breakdown of glucose is inhibited, and additional glucose is formed from storage compounds.

As a result the glucose level raises an effect which is enhanced by another hormone from the islets of langerhans called glucagon.

Clearly insulin plays a vital role in the regulation of glucose without it the liver cannot respond appropriately to the needs of the body. This can be illustrated considering what happens if the pancreas is surgically removed from an animal. The result is a drastic increase in general level of the glucose in the blood, accompanied by a decrease in the glycogen content of the liver and muscles content.

Certain individuals have islets of langerhans which for one reason or another are unable to produce as much insulin as they should. The result is a condition known as diabetes mellitus, the symptoms of which are similar to those seen in an animal deprived of its pancreas. There is an increase in the blood glucose level condition known as (hyperglycaemia), and glucose appears in the urine (glycosuria). If untreated the condition is fatal. Diabetes can be controlled by regular injections of insulin. Unfortunately the hormone cannot be taken by mouth as it is a protein and is digested in the alimentary canal, though mild cases can be controlled by means of other chemical agents taken orally.

It is amount of glucose in the blood itself which is the effective agent for the control of secretion of insulin. If the blood glucose level is abnormally high, this stimulates the islets cells to produce correspondingly more insulin. On other hand if the glucose level is low, less insulin is secreted. In other words the glucose itself switches on the mechanism by which it is itself regulated, an excess of glucose setting into motion the physiological processes which return the glucose level to its normal value.


  • Reference point/set point

This is the set level at which the system operates. Only be deviating from this norm or set-point then that the homeostatic mechanism is brought into play.

  • Reception/Detectors

Signals the extent of any deviation from the reference point and are capable of detecting the change.

  • Controller/control mechanism

Capable of initiating the appropriate corrective measures/coordinates the information from various detectors and sends out instructions which will correct the deviation.

  • Effectors

Bring about a response to a certain change.

Feedback loop

Change in the system as a result of action by the effectors


  • The efficiency of the control system is measured in terms of how little displacement from the reference point (optimal level) occurs and the speed with which the level is restored.
  • Homeostatic mechanisms must be free to fluctuate, as it is the fluctuations themselves which activate the control systems and return its parameter towards its optimal level.
  • Such control systems rely upon their components being linked together so that the output can be regulated in terms of the input, a concept known as feedback mechanism.
  • Feedback mechanisms require the action of the system to be referred back to a reference point, which is the optimal level of the parameter, so that subsequent action may be modified to restore the set point.
  • There are two forms of feedback, negative and positive, the former being most common in homeostatic mechanisms of organisms.
  • Negative feedback is associated with increasing stability of systems. If the system is disturbed, the disturbance or error sets in motion a sequence of events which counteract the disturbance and tend to restore the system to its original state. This serves as an advantage.

Whenever there is an increase in normal state the response causes to decrease or whenever there is a decrease in normal state the response causes an increase.


The directions of the lines of the diagram indicate the directions of stimulus and response.

Examples of biological negative feedback mechanisms include the control of gas tensions in the blood, heart, rate, arterial blood pressure, hormone and metabolite levels, water and ionic balances, the regulation of pH and body temperature.

The below figure illustrates the rate of negative feedback in the control of thyroxin release by the thyroid gland.


TRF, thyroid releasing factor; TSH Thyroid stimulating hormone.

Positive feedback is rare in biological systems since it leads to an unstable situation and extreme states. In these situations a disturbance leads to events which increase the disturbance even further. This acts as a disadvantage. For example, depolarization of the neuronal membrane produces an increase in its sodium permeability and sodium ions pass into the axon through the membrane and produce a further depolarization which leads to the production of an action potential. In this case, positive feedback acts as an amplifier of the response whose extent is limited by other mechanisms.


It is a well known fact that irrespective of fluctuations in the environmental temperature, the body temperature of humans is approximately 36.90C. Many of the body’s structures and physiological processes contribute towards the maintenance of this constant temperature.

  • Death occurs if our temperature raises much above 420C.
  • A constant body temperature of about 36.90C is necessary because it is the optimum temperature for the action of enzymes, upon which the organized functioning of the cells depends. Enzymes are proteins. If the temperature raises much above 400C the proteins are denatured and enzyme activity ceases.
  • Thermoregulation is the maintenance of constant internal temperature in the body of an organism.
  • Mammals and aves are endotherms, i.e. generate heat from within the body and conserve it they maintain constant temperature independent of the environmental temperature by using physiological mechanism.
  • Amphibians, reptiles and Pisces are ectotherms. I.e. their body temperature fluctuates with the environment temperature therefore rely on heat delivered from the environment than metabolic heat.

Endortherms may loose heat through

Evaporative cooling through sweating – this depends on various factors such as temperature, humidity and air currents but it can account for a substantial loss heat.

Conduction – Transfer of heat from the body to the nearest surrounding things which are in contact OR transfer of energy in form of heat in solids by vibration of its particles.

Convection-is the movement of air resulting from local currents of warm air being adjacent to each other replaced by cooler air and vice versa. These air movements speed up loss of heat by radiation and evaporation.


Convection– is the transfer of energy in fluids in form of heat due to density difference of the molecules where they are moving and carrying the heat energy.

Radiation – Movement of energy in form of electromagnetic waves from a warm body to relatively colder objects. Via the air/vacuum can be a major source of heat loss.

Heat may be gained in two main ways

  • Metabolism. Release of heat as a result of chemical reactions within the cell.
  • Absorption of solar energy (external environment)


When reference is made to body temperature in animal studies, it usually refers to the core temperature. This is the temperature of tissues below a level of 2.5cm beneath the surface of the skin. Temperature near the surface of the body can vary tremendously depending upon position and external temperature.


Endothermic organisms which live permanently in warm climates have developed a range of adaptations to help them maintain a constant body temperature. These adaptations may be anatomical, physiological or behavioral and include the following.

1. Vasodilation

  • Blood in the network of capillaries in the skin may take three alternative routes. It can pass through capillaries close to the skin surface, through others deeper in the dermis or it may pass beneath the layer of subcutaneous fat.
  • Many of the capillaries form loops and have shunts which enable the body to vary the amount of blood flowing through them.
  • The hypothalamus detects the change in blood temperature (thermoregulatory centre) and send nerve impulses to the vasomotor centre in the medulla oblongata of the hind brain
  • The vasomotor control of superficial arterioles causes them to dilate encouraging blood flow through the capillary beds. The shunt veins and connecting veins are constricted. Therefore more blood flows closer to the skin surface and heat from this blood is lost through the epidermis by conduction, convection and radiation.

2. Evaporative cooling through sweating

  • Sweat consists mainly of: water, mineral salts and urea. It is less concentrated than blood plasma and is secreted from tissue fluid by activity of the sweat glands i.e. the sweat gland cells absorb fluid (water, dissolved mineral salts and urea) from the surrounding capillaries and secrete it into sweat ducts which lead to the surface of the skin
  • When sweat evaporates from the skin, energy (heat) is lost from the body as latent heat of evaporation and this reduces body temperature.
  • Being furless humans have sweat glands over the whole body, making them efficient at cooling by this means, animals with fur generally have sweat glands confined to areas of the skin where fur is absent e.g. pads of the feet of dogs and cats and ears of rats (sweating beneath a covering of thick fur is in efficient as the fur prevents air movements which would otherwise evaporate the sweat)

NB: the evaporation of each gram of water requires 2.5 KJ of energy.

3. Flattening of hair/fur

  • At high environmental temperatures, the hair erector muscles are relaxed and the elasticity of the skin causes the fur/hair to lie closer to its surface. The thickness of insulatory warm air trapped is thus reduced and therefore the body looses heat by

4. Panting and licking

  • Where animals have few or no sweat glands cooling by evaporation of water nonetheless takes place from the mouth and nose.
  • Painting in dogs may result in the breathing rate increasing from 30 to 300 breaths per min. This result in excessive removal of CO2 from the blood which is partly offset by a reduction in the depth of breathing. Even so, dogs are able to tolerate depletion of CO2 which would prove fatal to other organisms.

Panting is also common in birds

  • Licking while not as effective as sweating may help cool the body. It has been reported in Kangaroos, cats and rabbits. Licking cools the body as heat from the body is required to evaporate the wet fur.

5.  Increase in metabolic rate

The metabolic rate decreases in order to decrease heat production. The metabolic rate is controlled by the hormone thyroxine by negative feedback mechanisms involving the hypothalamus and anterior pituitary.


1. Vasoconstriction

The hypothalamus detects the change in blood temperature and sends nerve impulses to the vasomotor centre in the medulla oblongata of the hind brain – bulb of Krause.

The vasomotor control of superficial arterioles causes them to constrict so reducing the quantity of blood reaching the skin surface.

The shunt vein and connecting vein are dilated and therefore less blood flows close to the skin surface and hence less heat is lost through the epidermis by conduction, convection and radiation.

2. Shivering

When the body’s temperatures fall below the core, the skeletal muscles of the body may undergo rhythmic; involuntary contracting which produce metabolic heat. This shivering may be proceeded by asynchronous twitching of groups of muscles.

3. Erection of hair (increase in thickness of the air layer)

At low environmental temperatures the hair erector muscles contract and the elasticity the skin causes a pull in the hair.

The hair becomes erect; goose bumps/pimples develop due to skin being pushed up by hair and so increase the thickness of the layer of air trapped. Therefore less heat is lost from the body.

4. Increased Metabolic Rate

The liver may increase its metabolic rate during cold conditions. Low temperature induces increased activity of the adrenal, thyroid and pituitary glands. All these produce hormones which help to increase the body’s metabolic rate and so produce additional heat.

(Adrenal produce cortisol and adrenaline, thyroid produce thyroxine and pituitary produce somatotrophin).

This requires increased consumption of food; arctic animals consume more food per gram of body weight than their tropical relatives. Rats kept at 30C take in 50% more food than those at 200C.


Excretion is a process whereby waste products from the metabolic activities of the body are eliminated from the body.


Excretion is the removal from the body of the waste products of metabolism.

Excretory products in animals:

  • Carbon dioxide

Source ;( from carbohydrate metabolism and is produced during respiration in the Krebs cycle) when and alpha – ketoglutarate changes to succinate. CO2 is formed in the body tissues. It diffuses out of the body through gaseous exchange surface e.g. Lungs in man, gills in fish and trachea in arthropods.

The concentration gradient will make the CO2 move out of the body through the lungs, alveoli, alveoli duct, trachea and the nose.

  • Excess water and mineral salts.

 Source; from carbohydrate metabolism and mineral metabolism removed from the body trough special organs e.g.: as sweat from sweating glands, kidneys in man.

  • Bile pigment.

 Source; from the breakdown of haemoglobin in the liver spleen or bone marrow. Excreted by the liver in bile and eliminated with faces.

  • Nitrogenous waste.

Source: from the breakdown of amino acids.

Forms of nitrogenous wastes.

Ammonia (gas)

Source: from the breakdown of amino acids

  • Its nature is highly toxic and highly soluble in water. Thus must be removed from the body quickly i.e. must not be allowed to accumulate in the tissue
  • It diffuses quickly from the body where there is plenty of water.
  • Oesteichythes, marine invertebrates and all fresh water animal can afford to excrete it unchanged
  • The animals excreting ammonia are referred to as ammoniotelic.
  • Urea
  • Made from the molecule of CO2 and 2 molecules of NH3.It’s soluble and relatively non toxic and harmless(less harmful) in moderate concentrations. Thus animals can retain it for some time in their bodies before being excreted.
  • Mammals can afford to lose some amount of water because they can replace it’s by drinking, thus every time urea is excreted some water is lost since urea is highly soluble in water thus mammals always drink water.
  • Formed in the liver through cyclical reactions referred to as ornithine cycle. Marine and cartilegenous fish and animals excrete urea and TMO and are known as ureoteric.

Role of river in protein metabolism

  • Formation of urea
  • Transamination
  • Production of plasma protein.

Formation of urea.

  • Excess amino acids cannot be stored
  • In the liver they undergo a process called deamination which involves splitting amino acids to amine and ketogroups. Amines are converted to ammonia (NH3) and ketogroups are converted to carbohydrates.
  • Mammals are ureotelic hence NH3 is converted to urea in series of reaction which constitute ornithine cycle whose steps are as follows:


  • The molecule of NH3 from deamination and CO2 enter the cycle and react with ornithine to form H2O and citruline.
  • Another molecule of NH3 is fed into the cycle to read with citruline forming water and arginine.
  • Reaction of arginine with water forms ornithine and urea and it is catalyzed by arginase enzyme.




Transamination is the process whereby amino acids are synthesized by transferring an amine group from amino acids to another organic; glutamic acid is synthesized from alanine.

The process occurs in the liver.

ii. Uric acid.

  • It is non toxic and relatively insoluble.
  • For this reason it can be stored in the body for longer periods before excreting it.
  • It is excreted as pallets or thick paste hence little water is lost e.g.; arthropods (insects) and aves and reptiles.
  • Suitable for excretions.
  • Animals which excrete uric acid are called uricotelic.

 Fate of amino acid

They are deaminated to urea, uric acid or ammonia if not they are transaminated.


  • Found towards the back of the lower part of the abdominal cavity.
  • Held in position by a thin layer of tissue called the peritoneum and are usually surrounded by fat.
  • Each kidney is supplied with blood from the renal artery and drained by a renal vein. The urine which is produced by the kidney is removed by a ureter for temporary storage in the urinary bladder
  • Within each kidney there are a number of clearly defined regions
  • The outer region is cortex. It mainly comprises of collagen fibre, fibrous connective tissues, glomeruli, bowman’s capsules and convoluted tubules with their associated blood supply.
  • Medulla contains blood vessels, loop of henle and collecting duct. These structures are in group known as renal pyramids.
  • Pelvis is almost empty with only colleting ducts empting there.
  • The whole kidney is nephron. Each kidney has 1 million nephrons; each of 3cm. Total length of tubules is 120 km.

General functions of the kidney

  • Removal of metabolic waste especially urea.
  • Osmoregulation ( maintenance of constant osmotic conditions in the body osmotic involves regulation of water content and solute concentration of body fluids i.e. Na+, K+,Cl
  • Maintains the acid- base balance of the body effected by either excreting or retaining H+/ HCO3, thus varying the pH of the urine between 4.5 – 8 (regulates pH)
  • Maintains the ionic concentration in the body i.e. Na+, K+, Cl, Mg2+, HCO3,.
  • Regulate blood pressure


Mammalian nephron (human beings)

Nephron is the structural and functional unit of the kidney i.e. it is an independent urine making machine.

Two types of nephron

  • Cortical nephron– found in cortex and have short loop of henle.
  • Juxta medullary nephron – renal corpuscle in junction of cortex and medulla long descending and ascending loop of Henle

Structure of the nephron

Have 6 main regions;

  • Renal corpuscle (bowman’s capsule & glomerulus)
  • Proximal convoluted tubule
  • Descending limb of loop of Henle
  • Ascending limb of loop of Henle
  • Distal convoluted tubule
  • Collecting tubule(ducts)

Renal corpuscle (malphigian body)

  • Ultra filtration is a passive process, of filtration under pressure involving the passage of materials from the blood to the Bowman’s capsule.

Adaptations of renal corpuscle to Ultra filtration.

  • The afferent arteriole is thick which later on breaks down to small blood capillaries.
  • The endothelial cells contain pores so that materials can leak
  • The inner layer of the Bowman’s capsule is made up of special cells called podocyte cells which have many infoldings to allow materials to enter the Bowman’s capsule.
  • The capillaries have a single layer of endothelial cells which are perforated to allow easy diffusion.
  • The arterioles break into capillaries which are one cell thick.
  • They are special podocyte cells to increase surface area for filtration and have got filtration slits.
  • The endothelial cells press against basement membrane to allow rapid diffusion.
  • Afferent arteriole has larger diameter than the efferent arteriole, this sets up high pressure in the glomerulus and the whole kidney.
  • On the outside of the capsule they have got squamous epithelium for easy passage of glomeruli filtrate into the tubule.

Cuboid epithelium line the proximal and distal convoluted tubule and collecting ducts.


The ultra filtration process.

  • Blood enters the kidney by the renal artery.
  • Renal artery branches further to form the afferent arteriole which then branches to a mass of capillaries, the glomerulus.
  • The glomerular capillaries then join to form the efferent arteriole which has a small diameter.
  • This sets up high pressure in the glomerulus which in turn forces substances such as glucose, amino acids, vitamins, some hormones, urea, trace of uric acid, ions, water through endothelial pores of capillaries across basement membrane into bowman’s capsule by ultra filtration.
  • Blood cells, RBCs, proteins molecules, WBCs, platelets are too large to pass through however large molecule can pass through due to hormonal and nervous signals which cause further constriction of the efferent arteriole.


Proximal convoluted tubule:

Function; Selective reabsorption of materials. The process is both active and passive.

Adaptations of the proximal convoluted tubule.

  • Longest region of nephron to allow materials to be reabsorbed.
  • Single layer of cells (one cells Thick on either side) to allow rapid diffusion.
  • The cells are cuboids and have lumen with brush border (microvillus) hence increases surface area to volume ratio for rapid re-absorption of materials.
  • The other ends of cells adjacent to blood capillaries have their bases convoluted (infoldings) with many intercellular spaces (channels) to reabsorb substances.
  • Cells have many mitochondria which provide energy (ATP) for active re-absorption process (active transport of glomerular) selective inward re-absorption of materials from glomerular filtrate.
  • The basement membrane of epithelium is likely to allow materials to pass through.
  • Immediately after basement membrane there is blood capillary reducing the diffusion distance.
  • The other end of proximal cells resting on basement membrane has infoldings known as basal channels to increase surface area.
  • Many blood vessels to maintain the concentration (diffusion) gradient

Mechanism of the re-absorption process

  • As many glomerular filtrate is passing through proximal convoluted tubule (PCT) all the food substances as mentioned are reabsorbed i.e. amino acids, glucose, ions, diffuse into the proximal convoluted cells where they are then actively transported into the intercellular spaces (the channels) then diffuse into the surrounding blood vessels.
  • Na+ ions & others raise osmotic pressure in the cells and water enters by osmosis.
  • Small proteins which may not be able to diffuse are taken up at the base of microvilli by phagocytosis.
  • All glucose, amino acids, vitamins and hormones are reabsorbed
  • 80%,of H2O, Na +, Cl, are reabsorbed
  • 40 – 50% urea is reabsorbed.
  • The active transport is out into the space in basal channel then it is diffused into the blood capillaries.


The loop of henle (Counter current multiplier i.e. fluids move in difference directions and effects is accumulative)

  • Consists of the longer thinner (descending limb) and shorter wider (ascending limb)
  • The limbs are slightly apart
  • A blood vessel, the vasa recta runs parallel through them.
  • The descending limb is permeable to water (the ascending limb is impermeable to water)
  • The Na pump operates in the ascending limb i.e. Na and Cl ions are actively removed from the glomerular filtrate in the ascending limb into the interstitial region of the medulla thus raising the local concentration and the concentration of the vasa recta vessels,
  • Water moves by osmosis from the descending limb into the vasa recta vessels which are permeable to water, ions and urea.
  • As vasa recta capillaries enter a high concentration in medulla they lose water from the plasma by osmosis and gain Na ions, Cl ions and urea, But as the vasa recta capillaries leaves the medulla to enter cortex which is under low concentration, they gain water by osmosis from the cortex and lose Na, Cl  ions and urea from the plasma. Thus operate under – counter – current mechanism.

Below the diagram shows the movement of ions and H2O from the loop of henle into the medulla of the kidney.



Distal convoluted tubule (DCT)

  • Absorption in the distal convoluted tubule is active
  • The cells have brush border and numerous mitochondria. The permeability of their membranes affected by hormones and so precise control of the salt and water balance of the blood is possible.
  • Re-absorption of H2O, ions depends on the body needs thus it is controlled by ADH/ vasopressin.
  • High osmotic potential (less H2O in the blood) is detected by osmoreceptors by hypothalamus which sends nerve impulses in the posterior pituitary gland and make it produce ADH.
  • In the presence of ADH (which moves through blood) the epithelium of the distal convoluted tubule become permeable to H2O hence more re-absorption by blood vessels.
  • This result in production of small amounts of concentrated urine summary: – less H2O, ADH released, epithelium permeable and urine.
  • Low osmotic potential (more water in blood) is detected by hypothalamus which stops the sending of nerve impulse to the posterior pituitary gland and thus ADH release is inhibited.
  • The epithelium of the distal convoluted tubule become impermeable to H2O hence less H2O (water is not) reabsorbed by the blood vessels (filtrate is dilute)
  • This result in production of large volume of diluted urine

Summary: – more than H2O, ADH inhibited, epithelium impermeable and urine is dilute.

NOTE: – failure to release sufficient ADH leads to a condition known as diabetes insipidus in which large quantities of dilute urine are produced (diuresis) and is replaced by lots of drinking.

The role of kidney in osmoregulation (feedback mechanism)


Low blood volume results from loss of Na+ ions because less water enters the blood by osmosis and this leads to decrease in blood pressure.

The low blood pressure is detected by juxtraglomerular apparatus (JGA (mass secretory cells)) lying between the afferent arteriole and the distal convoluted tubule.

Aldosterone also stimulates Na+ absorption in the gut decreases loss Na+ through the sweat.

Effect: more water enters the blood by osmosis  the blood volume and blood pressure rises.


Maintenance of the solute potential of blood (water and salts) at steady state by balancing water up take from the water lost in evaporation, sweating, egesting urine.

Regulation in fresh water fish

In relation to their environment, they are faced with two major conditions.

Are hypertonic to the environment. There is continuous flooding water into their bodies due to the osmosis gradient.

Aching of salt out of their bodies through the highly permeable gills to the surroundings.


  • They have large and many glomeruli thus producing large volume of glomerular filtrate.

Salts are selectively reabsorbed into the capillaries surrounding the tubule and hence produce large volume of dilute (cupious) urine

  • Their gills have got specialized NaCl cells which actively uptake salts from the water passing through them.
  • Some salts are replaced by absorption of food they take.
  • They do not drinking water.

Osmoregulation in marine cartilagenous fish.

  • The body fluids of marine are hypotonic to the environment. There is excessive loss of water to the environment thus leading to dehydration.
  • High osmotic pressure of the blood (because of loosing H2O & retaining the salts)


  • They synthesize and retain urea within their tissue and body fluids together with TMO (trimethly amine oxide).

The above makes their bodies concentration higher than their environment so take in water by osmosis through their gills.

  • The kidney has long tubules for selective reabsorption of urea and not for the elimination of salts.

Not salts because if salts are absorbed then water will follow by osmosis making blood dilute and loss of H2O to the surrounding as the surrounding is more concentrated than the blood.

  • Excess NaCl ions are removed from the body fluids by active secretion into the rectum by the cells of the rectal gland and through the faces they are out.
  • The gills are impermeable to nitrogenous waste and thus their removal is controlled by the kidneys.

If the gills were permeable to urea then the concentration of the urea in blood would be low and thus blood would lose water to surrounding as we know that urea has to be retained in the body.


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