EXCRETION

Learning Objectives.
  After studying or reading this, you should be able to:

1. Explain excretion and identify organs of the mammalian excretory system.

2. Describe the structure of mammalian kldney, skin and lungs.

3. Describe the formation and elimination of excretory products by the kidney, skin and lungs.

4. Explain the term homeostasis and outline the role of lungs, skin and the kidney in homeostasis.








EXCRETION

Excretion is the removal of metabolic waste products from the body of living organisms. The common excretory product formed in the bodies of animals are: water, carbon (IV) oxide, mineral salts, bile pigments and nitrogenous waste products; such as urea, uric acid and ammonium compounds. In plants, metabolic waste products include: water, carbon (IV) oxide, Oxygen, salts crystals, resins, latex alkaloids, anthocyanin and gums. The importance of excretion is .to remove substances that could be toxic or poisonous from the body of living organisms. If waste substances are allowed to accummlate in the body, they could prevent the maintenance of constant internal environment and this could lead to death of the organism.

The maintenance of a constant internal environment, irrespective of changes in the external environment, is known as homeostasis. Processes which contribute to homeostatsis, such as osomoregulation and excretaion, are referred to as homeostatic mechanisms.

Secretion and egestion should not be with excretion. Secretion is the release of useful substances or products, including some products of metabolism from a cell. Examples include: secretion of saliva by the salivary glands and gastric juice by gastric glands. Egestion, on the other hand, is the removal of undigested food materials, i.e. faeces, from the alimentary canal. The egested materials have not been involved in any metabolic process, since they have not been absorbed into the cells.

Lower organisms, such as, Amoeba and Paramecuim, can get rid of excretory products simply by diffusion through their entire body surface, due to their large surface to volume ratios and the small quantity of metabolic wastes, they produce. They also have contractile vacuoles to assist in both excretion and osmoregulation. This method of excretion is, however, insufficient to eliminate metabolic wastes from larger and more complex organisms, such as, worms, insects, amphibians, fishes, reptiles and mammals. This is due to their smaller surface to volume ratios and the large amounts of metabolic wastes they produce. These organisms therefore, need special tissues or structures to help them eliminate their metabolic wastes. These structures, concerned with excretion. are known as excretory organs.


In mammals, the four excretory organs are: kidneys, for eliminating water, salts and large quantities of urea; skin, for eliminating water, salts and small quantities of urea; lungs, for eliminating carbon (IV) and alcohol and liver, for eliminating bile pigments.






THE STRUCTURE OF MAMMALIAN KIDNEY, SKIN AND LUNGS

THE KIDNEY
There are two reddish-brown bean-shaped kidneys in the body of mammals. The kidneys lie asymmetrically on the dorsal wall of the upper abdomen, one on each side of the vertebral column or backbone. They are held in position by a mass of fatty tissue. Lying on top of each kidney is a coneshaped, adrenal ' gland, which is an endocrine gland that secretes the hormone, adrenalin.

Each kidney is covered with a tough, transparent membrane called capsule. The outer edge of the kidney is convex while the inner edge is concave, At the inner or concave edge is a depression called hilum, from which arises the ureters. The hilum is also the point at which the renal artery enters the kidney and renal vein leaves the kidney.

The ureter is a long narrow tube that connects the kidney to an oval, transparent sac-like urinary bladder, situated at the base of the abdomen. The urinary bladder, with its elastic walls, is used for the temporary storage of urine. From the bladder arises a muscular, narrow tube, the urethra, which opens to the outside as a small aperture or hole. The urethra traverses or passes through the penis in male mammals but opens behind the clitoris in females, in whom it is only urinary in function. In male humans, however, urethra urinogenital in function as it is used for the passage of both gametes and urine.
Urinary system of a mammal



A longitudinal section of a kidney shows that it consists of two distinct regions. The outer, darker and thinner region is known as cortex and an inner, lighter and thick medulla. Several thousands of fine tubules called urinary tubules or nephrons, which traverse both the cortex and medulla. The urinary tubules open into larger tubes called collecting ducts all of which open into the cavity at the hilum called pelvis, which Is continuous with the ureter.


The pelvis is boarded from the medulla by several cone-like masses of tissues called pyramids. The kidney tissue also contains a vast number of capillaries which are branches of the renal artery and renal vein called afferent arterioles and efferent arterioles respectively.

The urinary tubules or nephrons are the functional units of the kidneys. Each nephron begins in the cortex as the Malpighian body or Malpighian corpuscle, which consists of a cup-shaped Bowman’s capsule and a knot of blood capillaries called Glomerulus, which fits into the Bowman's capsule. From the. capsule arises a tubule which coils on itself several times, in the cortex, to form the proximal convoluted tubule. The tubule leaves the cortex into the medulla and returns back into the cortex, forming a U-shaped loop of Henle or Henle's loop. In the cortex, the tubule coils several times on itself again, forming the distal convoluted tubule, which opens into a relatively larger, collecting duct. All the collecting ducts in a kidney traverse the pyramids and open into the pelvis, which leads to the ureter.

Each urinary tubule receives blood from a branch of the renal artery called the afferent arteriole, which divides into several capillaries, in the Bowman’s capsule, to form the Glomerulus. The capillaries of the Glomerulus reunite to form the efferent arteriole, which drains blood from the Bowman's capsule. The efferent arteriole which is narrower than the afferent arteriole coils back and forms a network of blood capillaries, that envelopes the whole nephron except the Malpighian body.




THE SKIN
      The mammalian skin consists of two main layers: an outer epidermis and an inner dermis, which is much thicker than the epidemis. The epidermis varies in thickness at different parts of the body; being thickest at exposed and frequently usedparts like, the soles of the feet andpalms of the hand.

The epidermis consists of three layers: cornified layer or stratum corneum, granular layer or stratum granulosum and Malpighian layer or stratum germinisum. The Malpighian layer is pigmented and very thin, one or two cells thick. It consists of cells which are constantly dividing to form new cells. As more cells are formed in the Malpighian layer, older cells are pushed out towards the surface of the skin, which results in these older cells becoming flattened.

The outer cornified layer consists of flattened dead cells, which peel or flake off from time to time. They are, however, replaced by new ones from the Malpighian layer. The cornified layer forms a tough waterproof covering, which protects the body against mechanical injuries, excessive water loss or desiccation and prevents entry of some pathogenic microbes into the body. Unlike cells in the cornified layer, those in the granular laryer are living and are deposited with the protein, keratin, a highly insoluble substance. Keratin gives the cornified layer a horny consistency, since the granular layer cells eventually replace the cells in the cornified layer. The Malpighian layer is pigmented because of the presence of the skin pigment called melanin. This pigment absorbs ultra-violet rays from the sun and prevents the rays from harming inner tissues of the body.

The dermis consists of: blood vessels, lymphatic vessels, muscles, nerve endings, connective tissue and a few cells. It also contains hair follicles, sweat glands and sebaceous glands.


The hair follicles, sweat glands and sebaceous glands are all formed in the Malpighian layer of the epidermis but descend into the dermis. Hairs are made up of dead cells impregnated with keratin. At the base of each pit or follicle is a conical structure of dermal tissue known as hair papilla. This contains blood capilliaries and nervers associated with the growth of the hair. Each hair is made up of a hair root and a hair shaft. The hair root is embedded in the skin, while the hair shaft projects out of the skin through the hair follicle. Each hair follicle is associated with hair erector muscle; which helps to regulate the temperature of the body. The sebaceous gland opens into the hair follicle and secrets an oily substance called sebum; which makes the hair and epidermis supple and waterproof. It also has antiseptic properties.

The sweat gland consists of a slender tubule, which coils several times on itself in the dermis. it continues as the sweat duct, which traverses the skin and opens on the surface of the epidermis as the sweat pore. A network of blood capillaries surrounds each sweat gland.




There is a layer of subcutaneous connective tissue, which contains fat cells. These fat cells constitute the subcutaneous fatty layer called adipose tissue, which is used for storage of fat and insulates the body against excessive heat loss.




THE LUNGS

Gaseous exchange, also known as external respiration or breathing, is a process of gas exchange between the tissues of the body and the external environment. It involves the intake of oxygen and the release of carbon (IV) oxide and water. Two processes, therefore, involved are inspiration and expiration.

Inspiration, which is also known as inhalation or breathing in. involves the intake of oxygen in mammals and other terrestrial animals; the oxygen is obtained from atmospheric air. The atmostpheric air diffuses into the nostrils. The inhaled air then diffuses through nasal passages, pharynx, larynx and into the trachea. From the trachea, it passes through the bronchi and bronchioles into the alveoli in the lungs. The oxygen in the air diffuses into the blood in the alveolar capillaries. Carbon (lV) oxide and water, evolved during tissue respiration, also diffuses from the blood in the alveolar capillaries into the alveolar air.

The air, now containing larger amount of carbon (IV) oxide and water vapour, passes through the same passages by which the inhaled air entered the lungs, but in opposite direction, and eventually through the nostrils into the atmosphere. This constitutes expiration, which is also known as exhalation or breathing out. It involves the removal of carbon (IV) oxide and water (vapour) from the tissues of the body.

The surface area of the lungs is greatly increased by the presence of numerous alveoli in the lungs. The alveoli are thin-walled, one cell thick and highly vascularised, as they are supplied a complex network of blood capillaries. The surfaces of the alveoli are kept constantly moistened with mucus, secreted by mucus-producing glands. The above characteristics of the lungs make excellent respiratory surfaces for the exchange of respiratory gases during gaseous exchange or external respiration.








FORMATION AND ELIMINATION OF EXCRETORY PRODUCTS

FORMATION AND ELIMINATION OF URINE
Formation of urine involves two main processes: ultrafiltration and selective reabsorption. The afferent arteriole, supplying the glomerulus with blood, is wider than the efferent arteriole which drains blood from the glomerulus. This creates a high blood pressure in the glomerulus. causing some substances to tilter through the thin-walled barrier between the blood in glomerulus and capsular space. The barrier is Just two cells thick; which makes it easy for substances such as. amino acids, water. salts. vitamins, glucose, hormones urea and other nitrogenous wastes to filter through. This process is known as ultra-filtration. The substances iiitered out form the glomerular filtrate. Blood cells and plasma proteins do not form part of the filtrate. The glomerular liltrate is similar in composition to blood plasma.

As the filtrate moves down the tubules, useful substances are reabsorbed back into the blood. This constitutes selective reabsorption. The substances are selectively reabsorbed by osmosis, diffusion or active transport. The cilia lining the tubules help to move the filtrate along the tubules. In the proximal convoluted tubules, glucose, amino acids, some salts and some water are reabsorbed into the blood. In the loop of Henle, the reabsorption of salts is regulated by the sodium pump mechanism. The pH of the blood is also regulated in the loop of Henle. More water is reabsorbed by osmosis in the distal convoluted tubule. The filtrate remaining in the tubule constitutes urine which flows into the collecting duct, where the final adjustment of its concentration takes place. This adjustment is controlled by the hormone, Antidiuretic hormone, ADH. The urine formed, flows into the pelvis, from where it enters the urinary bladder through the ureter. The urine is temporarily stored in the bladder and voided out of the body through the urethra. This is possible by the conscious contraction of the muscles of the bladder and the relaxation of the sphincter muscles at the exit of the bladder.




THE SKIN AS AN EXCRETORY ORGAN The sweat glands in the skin absorb water, salts and small quanlities of urea from blood in the capillaries surrounding them. These substances in the sweat gland constitute sweat. The water is absorbed by osmosis and the urea by diffusion. Through diffusion and active transport. salts are absorbed from blood by the sweat glands.

The sweat passes through the sweat ducts onto the surface of the skin through the sweat pores. It absorbs latent heat from the skin and evaporates, thus cooling the body.




The Lungs as Excretory Organs

During gaseous exchange in the alveoli of the lungs, carbon (IV) oxide and water, produced by metabolic activities in cells, are removed from the blood. These are eventually removed frcm the body when expired air diffuses through the nostrils into the atmosphere. This constitutes excretion; since water and carbon (IV) oxide are metabolic waste. The lungs thus, flinction as excretory organs.






HOMEOSTASIS

Homeostasis is the maintenance of a constant internal environment. The cells in the body of multicellular organisms are able to carry out their metabolic functions perfectly. They can, however, do this only when the conditions in their external environmental are kept constant. A change in the external conditions of the cells can greatly affect the proper functioning of the cells. The external environment of the cells is. however, the internal environment of the organism as a whole. The maintenance of the constancy of this internal environment constitutes homeostasis. It involves the maintenance of constant (or changes kept within very narrow limits) temperature, pH, concentration of hood glucose, carbon [IV] oxide, Oxygen and many other factors.




The Role of the Lungs in Homeostasis

The lungs regulate the concentration of carbon (IV) oxide in the body. High concentrations of carbon (IV) oxide in the blood are detected by the chemoreceptors in the wall of the aortic arch.


Through the stimulation of the hypothalamus, the breathing rate or the rate of expiration increases. The lungs remove (excess) carbon (IV) oxide from the body during expiration.

Low concentrations of carbon (IV) oxide in the blood do not stimulate the chemoreceptors in the aortic arch. The rate of expiration therefore remains normal.




The Role of the Skin in Homeostasis

Thermoreceptors in the brain detect low or high temperature in the body. When the body temperature is high, superficial blood capillaries dilate. or vasodilation takes place, to allow heat loss by convection and radiation. Erector muscles relax, hairs lie flat and heat is lost by radiation and convection through the skin. Sweating increases to effect heat loss through evaporation of sweat.

When the body temperature is low, superficial blood capillaries constrict, or vasoconstriction takes place, thereby conserving heat by preventing heat loss by radiation and convection. Erector muscles contract, hairs are raised to stand on end. This traps air around the skin to prevent heat loss by radiation and convection. Spontaneous contraction of muscles, called shivering occurs. This generates heat which raises the body temperature.





The Role of the Kidneys in Homeostasis

The kidneys regulate the water and salt balance in the body. Antidiuretic hormone, ADH, increases the permeability of the walls of the collecting duct. This results in the production of more concentrated or hypertonic urine. in the absence of ADH, the permeability of the walls of the collecting duct decreases. Dilute or more hypotonic urine is, therefore, produced.

When the solute concentration of the blood increases. secretion of ADH increases. This results in the production 0f concentrated or hypertonic urine.






Did you understand what you have just studied or read?

To make sure you have UNDERSTOOD, ANSWER the QUESTIONS below:

EXCRETION

1. What is meant by the term homeostasis? Mention two examples of the process.

2. a. How does the skin function as a homeostatic organ?

b. How does the kidney function as a homeostatic organ?

3. Explain how each of the following enables mammals to maintain a constant body temperature
i. Blood capillaries in the skin
ii. Shivering
iii. The hair.
iv. Sweating

4. a. What is excretion?
b. List the excretory products of mammals and indicate where each product is formed.

5. A.D. Make a labelled drawing of a nephron of a mammal.
b. How does the structure of a nephron suit its function?

6. a. List the main excretory products produced in the body of a mammal.

b. Describe how urine is formed in the kidney of a mammal.

7. Support your answer with a well labelled diagram.

a. What is the significance of homeostasis?

b. Indicate the role played by each of the following organs in the homeostatic mechanism of mammals:
i. The kidney ii. The skin iii. The liver

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