This blog post provides readers with the following objectives. The reader will be able to:

·             Explain excretion and identify organs of the mammalian excretory system.
·             Describe the structure of mammalian kidney, skin and lungs.
·              Describe the formation and elimination of excretory products by kidney, skin, lungs and liver.
·             Explain the term homeostasis and outline the role of lungs, skin, liver and the kidney in homeostasis.


Excretion is the removal of the metabolic wastes from the body of an organism. Wastes that are removed include carbon dioxide, water, salt, urea and uric acid.

Excretory Organs in Mammals

1.      Liver as an Excretory Organ

1. It excretes bile pigment or bilirubin

2. It disposes worn-out red blood cells or hormones

3. It detoxifies poisonous substances and drug

4. Foreign particles in the blood stream are removed by phagocytic cells in the blood vessels of the liver.

5. It converts excess amino acid or ammonia into urea in a process known as deamination. The urea is eliminated from blood stream through the skin or the kidney.

2.      Skin as an Excretory Organ

It removes excess water, salt, urea and uric acid

The sweat glands absorb water, salts and small quantities of urea from the blood. These substances form sweat in the sweat gland which eliminates through the sweat pore.

3.      Lung as Excretory Organ 

 It eliminates carbon dioxide and water vapor, a waste product given off by cellular respiration. These products are removed from the body when expired air diffuses through the nostrils into the atmosphere.

4.      Kidneys as Excretory Organ

 It filters the blood to form urine, (excess water, salt, urea and uric acid)

Urinary System

It consists of two kidneys, two ureters, the urinary bladder and the urethra.

drawing of Urinary system in human with labeling

Functions of the kidney

1.       It regulates the volume of blood plasma
2.       It regulates the concentration of electrolytes (Na+, K+, and other ions) in the plasma
3.       It controls the pH of plasma
4.       Elimination of waste products in the blood
5.       Production of enzyme renin which helps rise blood pressure

Structure of the Kidney

 Kidneys are reddish-brown, bean-shaped organs located behind the peritoneum. Each kidney is cover by outer fatty fibrous layers called capsuleA cone-shaped, adrenal gland lies on top each kidney. Blood vessels, renal artery enters the kidney and renal vein leaves the kidney through a depression termed the hilum.

The hilum also give rise to a long narrow tube called ureter that connects the kidney to an oval, transparent sac called urinary bladderThe longitudinal section of the kidney shows two distinct regions.

1.   The outer cortex is reddish brown and granular in appearance.

2.   The inner medulla is darker in color, made up of collecting tubules and blood vessels. 

The medulla is made up of a number of cone-shaped tissues called pyramids 

Each pyramid projects into a large, funnel-like cavity called pelvis. The pelvis collects urine and transports it to the ureter.

Structure of the kidney

Structure of the Nephron

The nephron is responsible for the formation of urine. It’s the basic functional unit of the kidney. Each kidney contains more than a million nephrons. A nephron consists of small tubules and associated blood vessels. The tubular portion of a nephron consists of a Bowman’s capsule, a proximal convoluted tubule, loop of Henle and a distal convoluted tubule.

The Bowman’s capsule cup-shaped structure containing network of blood capillaries called the glomerulus. The Bowman’s capsule and glomerulus are located in the cortex of the kidney and together constitute the renal corpuscle.

The capsule gives rise to a tubule that coils up to form proximal convoluted tubule. The wall of the tubule consists of microvilli which increase the surface area for absorption.

The tubules assume a U-shape to form loop of Henle.

The tubule again becomes coiled and is called the distal convoluted tubule. The distal convoluted tubule is shorter and has few microvilli. It is the last segment of the nephron and it opens into the collecting duct.

structure of the nephron and function of the nephron

Formation of Urine

There are three phases in the formation of urine: Ultrafiltration, Selective reabsorption and Tubular Secretion.

Blood enters the kidney by renal artery. Afferent vessels enter Bowman’s capsule and form a network of capillaries called glomerulus. The afferent arterioles of the glomerulus have a greater diameter than the efferent arterioles. This creates high hydrostatic pressure in the glomerulus which forces water and certain substance to pass (or filter) into the Bowman’s capsule.  This process is called ultrafiltration or glomerular filtration. The filtrate contains small substances such as water, mineral salts, glucose, amino acids, hormones, urea, uric acid, toxins, drugs etc.

A malpiighian corpuscle

The non-filterable components such as blood cells, and proteins leave the glomerulus by way of the efferent arteriole. Filtrate that enters the glomerular capsule passes into the lumen of the proximal convoluted tubule.

Selective reabsorption occurs in the proximal convoluted tubule. Glucose, vitamins, important ions and most amino acids are reabsorbed from the tubule back into the capillaries surrounding the tubule. At the loop of Henle, reabsorption of water, sodium ions and chloride ions take place. At the distal convoluted tubule, there is reabsorption of salts and water into the surrounding network capillaries. Some wastes are actively secreted into the fluid by a process called tubular secretion. Some of these are H+, K+, NH4+ toxic substances and foreign substances (drugs, penicillin, and uric acid). Secretion of H+ adjusts the pH of the blood.

As fluid passes through the collecting ducts, much of the water reabsorbed by osmosis. The permeability of the collecting duct to water is regulated by Antidiuretic hormone (ADH).

drawing of the nephron with labelings

Composition of Urine

Urine contains: water – 96-97%, Urea – 2% and remaining 2% constitute uric acid, creatine, ammonia sodium, potassium, chloride, phosphate, sulphate and oxalates.

Integumentary System

The integumentary system consists of the skin, hair, nails and cutaneous glands.

Structure of mammalian skin

The skin or integument is the largest organ of the body. It consists of two layers:

1.  Epithelial layer called the epidermis.

2.  Connective tissue layer called the dermis

Structure of mammalian Skin with labeling


The epidermis is the superficial protective layer of the skin. It is composed of stratified squamous epithelium. It consists of three regions: Malpighian, granular and cornified layer.

1. Malpighian or basal layer: is composed of a single layer of cells in contact with the dermis. This layer composed specialized cells which constantly divide mitotically and replaced those lost from the skin surface. These specialized cells include;

a.  Keratinocytes: are cells that produce keratin. Keratin is responsible for the toughness and waterproof covering of the skin.

b.  Melanocytesare epithelial cells that synthesize the pigment called melanin. Melanin provides a protective barrier to the ultraviolent radiation in sunlight. It’s also responsible for skin color. 

c.  Langerhans cells: are microphages that protect the skin against infection.

2. Granular layer: The granular layer consists of only three or four flattened rows of cells. These cells are produced by the Malpighian layers.

3. Horn-like layer or Cornified layer: This layer is composed of several layers of flattened, cornified, dead cells. These cells constantly shed from the skin surface and replaced by new ones from granular layer. It is the layer that actually protects the skin. 


The dermis is deeper and thicker than the epidermis. The dermis contains many sweat glands, oil secreting glands, nerve ending, blood vessels, erector muscles, pigment cells and hair follicles.

1.  Hairs: These are formed in tiny pits in the skin called hair follicles. At the base of the follicle there is a cluster of cells called the bulb. The hair is formed by the multiplication of cells of the bulb. The part of the hair above the skin is the shaft and the remainder the root. The hair is responsible for temperature regulation of the skin.

2.  Erector muscles: are little bundles of involuntary muscle fibres attached to the hair follicles. Contraction makes the hair stand erect and raise the skin around the hair.

3.  Sebaceous glands: are oil-producing glands that empty their secretions into hair follicles. This oily secretion is called sebum. Sebum lubricates and keeps the hair and skin soft. It also inhibits the growth of some bacteria.

4.   Sweat glands: secrete sweat onto the surface of the skin. Sweat is composed of water, salts, urea and uric acid. It serves not only for evaporative, but also excretion of wastes.

5.  Mammary glands: are found in the breast of female mammals.  They are specialized sweat glands that secrete milk during lactation.  

Hypodermis or subcutaneous

It is not actually a part of the skin but it binds the dermis to underlying organs. It is connective tissues composed of adipose cells. This layer stores fats, insulates and cushions the body and regulates temperature.

Functions of the skin

1.       Protection

a.  It prevents invasion of micro-organisms  
b.   It protects the body from excessive water loss
c.   Bacterial are killed by bactericidal substances in sebum
d.   Melanin absorbs ultraviolent light and protects underlying structures

2.      Excretion: the sweat glands excrete salt, water, urea and uric acid in sweat

3. Synthesis: the skin synthesizes vitamin D, which helps regulate calcium and phosphorous metabolism.

4.      Sensory Detection: it contains sensory receptors that detect heat, cold, touch and pain.

5.      Regulation of Body Temperature

a.  The thin layer of fat cells in the dermis insulates the body.  

b.   Sweat produced by sweat glands evaporate using excess body heat, which cools the body.


­­­Homeostasis is the maintenance of a constant internal environment in a living organism. It involves a series of automatic control mechanisms.

Examples of ­­­homeostasis

1.       Regulation of water                    
2.       Regulation of temperature
3.       Regulation of glucose                 
4.       Regulation of salt, amino acids, toxins etc. 

Significance of homeostasis

1.     Maintains a constant internal environment
2.     It enables the biochemical process in the body to take place.
3.     It ensures independence from environment strains

 Thermoregulation (Temperature Regulation)

Increase in Body Temperature: An increase in body temperature, for example due to vigorous exercise or being in a warm place, causes the blood and the skin temperature to increase. Thermo-receptors in the skin detect this increase and transmitted a signal in the form of nerve impulse to the hypothalamus in the brain. When the hypothalamus is stimulated, it then transmits nerve impulses to the relevant body parts.

1.   The arterioles in the skin dilate (a process called vasodilation). More blood is transported to the blood capillaries and this removes more heat.

2.   Sweat glands become active and more sweating occurs and hence more heat removed.

3.  The hair erector muscles in the skin relax causing the hair to lie flat. Therefore, less heat is trapped by the hair.

4.  Metabolic rate decreases and hence heat production decreases.

5.   Breathing rate increases so as to remove more heat in exhaled air.

All these mechanisms lower down the blood temperature (Negative feedback) and eventually the temperature will return to normal.

Decrease in Body Temperature: A decrease in body temperature, for example due to starvation, fasting or being in cold places, causes the blood and the skin temperature to decrease. The temperature receptors in the skin detect this decrease. A signal in the form of nerve impulse is then transmitted to the hypothalamus in the brain. When the hypothalamus is stimulated, it then transmits nerve impulses to the relevant body parts.

1.   The arterioles in the skin constrict (a process called vasoconstriction). Less blood is transported to the blood capillaries and hence less heat is removed.

2.    Sweat glands become inactive and no sweating occurs. Therefore, no heat is removed.

3.   The hair erector muscles in the skin contract causing the hair to stand erect. Therefore, more heat is trapped by the hair.  

4.    Metabolic rate increases and hence heat production increases.

5.   Breathing rate decreases so as to conserve more heat.

6.    Shivering due to muscle contractions occurs if very cold to produce more heat.

All these mechanisms increase the blood temperature (Negative feedback) and eventually the temperature will return to normal.

Heat production

Heat is gained by the body cell in following ways

1.    The muscle contraction produces large amount of heat. Shivering involves muscles contraction and produces heat.

2.      The liver is very chemically active, and heat is produced as a byproduct.

3.     The skin absorbs heat directly from the sun.

4.      The metabolic reactions of the body produce heat.

Heat loss

1.     Most of the heat is loss from the blood through the skin in the form of sweat

2.     Small amounts are lost in expire air, urine and faces

3.  Heat lost through the skin can only be regulated to maintain a constant body temperature.

Water Regulation (Osmoregulation)

Regulation of the amount of water in the body is controlled by the kidneys and hormone called Antidiuretic Hormone (ADH)

ADH increases the permeability of the distal convoluted tubule and collecting duct. At high osmotic blood pressure (i.e. more salty, not enough water), the posterior pituitary releases ADH. ADH increases the permeability of the collecting ducts, allowing water to leave by osmosis into the blood.

If osmotic pressure of blood decreases, pituitary does not release ADH and more water is lost in urine due to decreased permeability of the collecting duct.

Alcohol inhibits the secretion of ADH, thus increases water loss. The absence of secretion of ADH result in a condition called Diabetes insipidus.

Regulation of Body Sugar

The liver regulates glucose level of the blood. The liver converts excess glucose to glycogen, using insulin produced by pancreatic cells (islet of langerhans), for storage in the muscle or liver (i.e. decreasing blood sugar level).

The hormone glucagon produced by islet of langerhans, promotes the breakdown of glycogen in the liver into glucose in the blood. It also promotes gluconeogenesis, the synthesis of glucose from amino acids or fatty acids (i.e. increasing blood sugar level).

Role of Skin in Homeostasis

The thermoreceptors in the hypothalamus of the brain detect changes in the body temperature and initiate mechanisms to bring the temperature to normal by sending impulses to the erector muscles, sweat gland, blood vessels (arterioles) in the surface of the skin and skeletal muscles.

When the body temperature is low (cold weather),

1.   Erector muscles contract, thereby raising the hair and trapping a thick insulating layer of air which reduces heat loss.

2.   Sweat glands produces no or little sweat to reduce or conserve the heat through sweat.

3.   Arterioles in the surface of the skin constrict, to reduce the amount of warm blood flowing near to the skin surface, thereby reducing heat loss through radiation.

4.  Shivering caused by spontaneous skeletal muscles contraction produces heat which raises the body temperature.

When the body temperature is high (hot weather)

1.         Erector muscles of the hair relax making the hair to lie flat, leaving a thin blanket of air so that heat is lost readily from the body.

2.         Sweat glands produce sweat and heat is lost through evaporation of the sweat.

4.        Arterioles in the surface of the skin dilates, causing more warm blood to be brought near the skin surface for heat to be lost through radiation.

5.        Shivering stops and the muscles generate less heat.

Role of Kidney in Homeostasis

When the amount of water in the body is low, the osmotic pressure of the blood rises due to high salt concentration. This change is detected by osmoreceptors in the hypothalamus of the brain. The hypothalamus sends nerve impulse to the pituitary gland to release more ADH (antidiuretic hormones or vasopressin) into circulation. The ADH increase the permeability of the collecting ducts and more water is reabsorbed. On the other hand, when the amount of water in the body is more, the osmotic pressure of the blood reduces, the hypothalamus detects this change and less ADH is released. The collecting ducts become less permeable and less water is reabsorbed.   

Role of Lung in Homeostasis

The lung maintained homeostasis in two ways: gas exchange and regulation of blood pH. The lungs eliminate carbon dioxide, a waste product given off by cellular respiration. As carbon dioxide exits the body, oxygen needed for cellular respiration enters the body through the lungs. The removal of carbon dioxide gives the blood a more neutral pH, allowing hemoglobin to bind up more oxygen.

Role of Liver in Homeostasis

1.   The liver controls the blood sugar levels. (N/B: refer to sugar regulation for detail)

2.   It detoxifies many poisonous substances, such as some drugs, ammonia, alcohol etc.

3.   It removes foreign particles in the blood stream by phagocytic cells in its blood vessels.

4.   It converts excess amino acid or ammonia into urea in a process known as deamination.

5.   It stores irons from breakdown of red blood cells which is reused to from hemoglobin.

6.  The liver also produces heat resulting from its excessive metabolic activities.

7.   It stores fats which is released when the need arises.

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