This is the maintenance of a steady internal environment. The internal environment must be kept constant for the health, growth and efficient functioning of the body cells. This is done by the body constantly adjusting to changes in the physical and chemical condition of its body fluids.


Factors to be kept constant ( in equilibrium).

Internal conditions such as glucose level, salt concentration, osmotic pressure, body temperature and hydrogen ion concentration are put in check.


Parts involved in homeostasis

The main organs and substances like in homeostasis are

  • Kidneys
  • Liver
  • Skin
  • Hormones (substances secreted by endocrine glands).
  • Brain which has overall control over the homeostatic processes in the body




The structure and excretory functions of the kidney has been discussed under excretion in mammals. We shall discuss kidney and how it regulates the internal environment and the important diseases that affects it.


Functions of the kidney

The kidneys

  1. Removes nitrogenous wastes (the excretory functions) and
  2. Control of the amount of the water, salts (sodium, potassium and chloride ions) and acids (hydrogen and hydrogencarbonate ions) – osmoregulatory function


As an osmoregulator (osmoregulation)

This is the process by which the osmotic pressure of the blood and tissue fluid is kept constant. The solute concentration of the blood rises when we

  • Eat too much salt
  • Drink very little water or
  • Sweat profusely

This results in high osmotic pressure or concentration.

In the brain there are groups of osmoreceptor cells sensitive to the osmotic pressure. These cells are located in the hypothalamus in the brain. Immediately they send impulses to stimulate the release of an anti-diuretic hormone (ADH) from the posterior pituitary gland.when blood with high osmotic pressure or concentration gets to the brain,the osmoregulator cell are stimulated and impulses are sent to the pituitary gland. This gland releases a hormone know as anti-diuretic hormone (ADH).

ADH is then transported by the blood to the kidneys and the kidney tubules are made to reabsorb more water into the blood stream, less water is therefore passed to the bladder and the urine becomes concentrated. This process goes on until the osmotic pressure of the blood reaches it’s normal level. The secretion of ADH is then stopped.

On the other hand, if the osmotic pressure of the blood and tissue fluid falls because of a low concentration of salts, the secretion of ADH ceases and less water is reabsorbed from the kidneys and watery urine is produced. The osmotic pressure of the blood is thus kept constant. This mechanism is known as a negative feedback mechanism.


Diseases of the kidney.

  1. Nephritis: inflammation of the nephron caused by bacteria glomeruli are unable to filter blood completely. Useful substances like protein pass out with urine (proteinuria).
  2. Dropsy: water is retained in the blood since reabsorption by kidney has failed. There is a serious infiltration of the cellular tissues which result to oedema.
  3. Diuresis: there is increased secretion of can be caused by accident. The urine is watery because the tubules fails to reabsorb water.
  4. Kidney stone: due to one abnormality or the other, amino acids and calcium phosphate settle in the kidney to form kidney stone. This obstruct the free flow of urine.

Kidney failure occurs at the advanced stage of kidney diseases like chronic nephritis. When this happen, the patient is only kept alive by dialysis or kidney transplant.


Effects of kidney diseases

The following are the effects of kidney disease.

  1. High blood urea level.
  2. Presence of protein in urine.
  3. Impaired ability to produce sufficient urine.
  4. Diuresis.
  5. Oedema.
  6. Acidosis.
  7. High blood pressure.


Ways to remedy the effects of kidney diseases.

  1. Promit treatment by a kidney specialist using drugs.
  2. Rest and warmth.
  3. Dietary control.
  4. Increase of a failed or a failing kidney, dialysis (using machine in removing the waste) should be recommended.
  5. Surgery might be carried out involving outright remover or replacement with another through kidney transplant.


It is large reddish-brown organ with two lobes. It lies between the diaphragm to the right side parthy overlapping the stomach.

The Diagram Of The Liver.

Homestatic function

The liver’s homeostatic functions are as follows:

  1. The regulation of nutrient level in blood especially blood glucose level which is controlled by the homeonwns insulin, glucagon and glucocorticoid.
  2. Deamination of excess amino acids to form urea.
  3. Storage of excess glucose as glycogen.
  4. Bile formation. The liver makes bile which is stored in the gall bladder and secreted into the duodenum when needed.

The bile which is 98% water contains

  • Bile salt which aid in digestion.
  • A yellow pigment, bilirubin,an excretory product formed during the breakdown of haemoglobin.
  • Inorganic salt and
  1. Storage of vitamins (fat soluble vitamins A, D, E, and K) and water soluble vitamins (B and C)and certain minerals (zinc, iron, copper and potassium).
  2. Storage of blood. The liver is full of blood vessels and can store up to 1,500cm³ of blood.
  3. Formation and breakdown of red blood cells.
  4. Detoxification of chemical substances or foreign substances in the body.
  5. Inactivation of hormones and production of heat.

The various metabolic activities of the liver generates as a by-product.

The liver screens the food-laden blood from the small intestine and adjust its composition to suit the immediate need of the blood before releasing into the circulatory system.

Carbohydrate metabolism (glucose level in blood).

  1. The normal glucose level is 90mg/100cm³ blood. If it falls below 40mg/100cm³ blood, the brain cell is affected resulting to coma.
  2. If the level rises to 160mg/100cm³ blood, the kidney excreted the excess glucose.


Homestatic process of liver to control blood glucose level.

  1. The liver converts any excess glucose in the blood that reaches it after a meal to glycogen by the insulin (hormone) secreted by the pancreas.
  2. When blood glucose level falls, below normal, the liver converts glycogen into glucose and release the required amount of it into the blood by another pancreatic hormone called glucagon. But if the glycogen level is depleted as can be detected by the hypothalamus which stimulates the secretion of the various hormones to cause the liver to convert amino acids and glycerol into glucose.


Protein metabolism

The liver releases the required amount of amino acids into circulation while the excess that cannot be stored is broken down into

  • Nitrogen containing amino parts which the liver converts to urea for excretion by the kidney.
  • It breaks down carbon compounds (keto-acids) into glucose, glucagon or fats by the process known

The liver manufactures plasma proteins from dietary amino acids eg. albumins, globulins and those for clotting such as fibrinogen.


Diseases of the liver and bile duct.

  1. Gall stones
  2. Viral hepatitis
  3. Cirrhosis
  4. Amoebic liver abscess
  5. Jaundice may be due to liver diseases or causes


—  Gall stones: the stones are formed in the gall bladder or bile ducts . Gall stones are made of cholesterol. Gall stones are side to abnormal Boller production by the liver. Gall stones obstruct the for of b bile and increase infection of the gall bladder. They can be solved by oral feeding of bile acid or by surgery.


— Jaundice: is caused by an increase in the blood bilirubin (bile) level, thereby giving a yellow colour to the skin and whites one the eyes. Jaundice may be due to three causes:

  • Excessive breakdown of red blood cells.
  • Obstruction of the bile duct.
  • Diseases of the liver.


— Diabetes mellitus: this disease is experienced when the liver is unable to convert unused sugar into glycogen. This situation arises when there is an insufficient hormone (insulin) from the islets of langerhans located in the pancreas. Consequently, excess sugar is left in the blood and lost through urine.


— Viral hepatitis: there are two types:

  • Infective hepatitis caused by virus type A
  • Serum hepatitis caused by virus type B

The virus causes inflammation and destruction of the liver cells. The liver cells turn red, become swollen and painful. The disease results in liver cells being unable to store glycogen or to convert stored glycogen to glucose. This is for infective hepatitis. If it subsides, the liver recovers and work normal again.


— Amoebic liver abscess: the parasitic amoeba, Entamoeba histolytica which causes amoebic dysentery is also responsible for this infection. It gets into the liver from the large via the hepatic portal vein – producing an enzyme that destroy liver tissues and causes abscess to form.


— Cirrhosis of the liver: this is a serious liver disease in which the damaged liver cells become replaced by useless fibrous tissue. Cirrhosis may be caused by excessive drinking of alcohol over a prolonged period and hepatitis.


Effects Of The Diseases Of The Liver.

The effects of the liver are due to the failure of liver cells to function properly. Some symptoms and signs include weakness, tiredness, jaundice, slight fever, etc.


Treatment Of Liver Diseases.

The liver has high capacity to divide and produce new cells (regenerate). This means that it is able to replace damaged cells and function normally. It can be treated as follows:

  • Rest in bed until the jaundice subsides and after a long period of recovering from illness.
  • Eat balanced diet especially food containing low fat.
  • Stop taking alchohols especially one suffering from cirrhosis.
  • Remove the cause of the liver disorder eg. in amoebic abscess, the pus can be drained from the abscess. The person now uses anti-amoebic drugs to remove the parasitic infection.

A person can donate a lobe of his or her liver for transplant and the remaining liver can regenerate.



The skin is the largest sense organ in the mammalian body. It is the outer covering of vertebrates and usually bears a dense outgrowth of hairs.

Diagram Of The Mammalian Skin

The Layers Of The Mammalian Skin

The Epidermis

  1. Cornified layer (surface/ outer layer of epidermis): this is made up of dead cells containing keratin. Keratin makes cornified layer tough, flexible and waterproof. The cells of cornified layer are constantly rubbed off and replaced by the cells of the granular layer.
  2. Granular layer (middle layer of epidermis): it consists of living cells that are constantly being pushed up from the malpighian layer. The pushed up cells eventually move up to the surface of the skin, become flattened, accumulating keratin (a fibrous protein).
  3. Malpighian layer (innermost layer of epidermis): Malpighian layer is made up of melanin (a pigment) that gives the skin its colour. The blood in the capillaries of the dermis supplies nutrients and oxygen to the cells of the malpighian layer by the process of diffusion.


The Dermis

  1. Blood capillaries: the blood capillaries supply nutrients and oxygen to both the dermal and epidermal cells and remove their waste products. Capillary network supply hair follicle and sweat glands. The capillary loop that lies close to the surface of the body regulate the body’s temperature.
  2. Hair follicle: it is a deep pit formed by the infolding of the malpighian layer. The nerve endings in the follicle respond when the hair is touched. The hair follicle has a hair erector muscle which contracts to pull the hair to a  more upright position.
  3. Sebaceous glands: They secrete sebum (an oily substance) into the hair follicles. The sebum keeps the hair and the epidermis waterproof and supple. It keeps out dust and micro-organisms from the skin.
  4. Sweat glands: they are coiled tubes which continue as sweat ducts and open at the surface of the skin as sweat pore. The cells of the sweat glands absorbs water from the capillaries around them and secrete them into the tubes. The absorbed water contains small quantities of waste eg. mineral salts and urea called sweat. The sweat evaporates through the surface of the skin and thus cools the body.
  5. Sensory nerve ending: the skin is a sense organ that consists of sensory cells (receptors) which can receive stimuli from the environment and send them to the right parts of the body for interpretation and action.

Types of skin receptors

  • Touch receptors (meissner’s corpusules)
  • Pressure receptors (pacinian corpusules)
  • Pain receptors (free nerve ending)
  • Therm


Functions of the skin

  1. Protection
  2. Sensitivity
  3. Temperature control
  4. The skin excrets waste products of metabolism eg. sweat.
  5. Skin produces/ manufactures vitamin D when exposed to the ultraviolet rays of the sun.



  • The dead layer of epidermal cells form a continuous barrier that stops micro-organisms from invading the living cells.
  • The waterproof nature of the epidermis reduces water loss by evaporation and prevent water from entering into the body cells by osmosis. Therefore, the epidermal cells maintain constant fluid volume eg. during swimming.
  • The melanin in the malpighian layer absorbs the ultraviolet rays of the sun thereby preventing it from being damaged eg. living cells.



The skin responds to external stimuli for necessary actions for survival eg. heat, touch, cold, pressure and pain.


Temperature Control

The mammalian skin regulates the body temperature through vasodilation and Vasoconstriction.

  • Vasodilation: this is obtained when the body is overheated. The blood vessels/ capillaries under the epidermis of the skin are dilated or widened. This causes an increase in the amount of blood flowing near the surface. The sweat glands become more active and extract large quantities of sweat from the blood which soon evaporates. The evaporation of sweat needs heat. The heat is taken away from the body hence the body’s temperature is reduced.
  • Vasoconstriction: during cold weather, the blood vessels under the skin constrict or become thinner thereby reducing the blood flowing near the surface of the skin. The sweat glands become less active and heat loss by convection and radiation is also reduced. Sweat production is reduced and heat loss by evaporation is reduced.


Differences between Vasodilation and Vasoconstriction

Vasodilation Vasoconstriction
  1. Heat loss: A rise in body temperature get rid of excess heat
Heat gain: A fall in body temperature produce and conserve heat.
  1. Sweating: In humans, sweat glands become active and produce large amount of sweat that flows out onto the surface of the skin
In humans, the sweat glands become inactive and produce very little sweat that flows out onto the skin surface.
  1. Decreasing metabolic rate: the body slows down its activities to reduce heat production.
Increasing metabolic rate: the body increases metabolic rate using especially the liver to produce more heat. Shivering is also aimed at increasing the body’s metabolic rate.
  1. Lowering the hairs: In a furry mammal, the hair erector muscles relax causing the hairs to live flat on the skin surface. This forces out the air trapped among the hairs thereby making the animal to loose more heat from the body.
Raising the hairs: In a furry mammal, the hair erector muscles contract to raise the hairs and trap more air. Air is a bad conductor of heat. The layer of air next to the skin insulates the body and prevents heat loss.
  1. Behavior changes: Many mammals stay under shade. Humans wear thin light coloured absorbent clothing.
Many mammals keep warm by staying in nests and huddling in a ball to reduce surface area. Humans wear woolen clothing.


Importance Of Temperature Control

It is important to maintain a constant body temperature for normal activity and health. It also makes mammals fairly independent of the temperature of their habitats.

Care of the Skin

A balanced diet with vitamin A and B2 (Riboflavin) and regular exercise maintain a healthy and supple skin.

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