Excretion

1.0Introduction

Metabolic reactions occurring in animal body produce certain nitrogen-containing materials which are of no use in the cells. These are called nitrogenous waste materials. They are mainly formed from breakdown of proteins and nucleic acids. They become toxic if allowed to accumulate in the body. Therefore, these materials must be removed from the body for healthy life. The elimination of nitrogenous waste products from the animal body to regulate the composition of the body fluids and tissues is called excretion. Various metabolic activities generate nitrogenous materials which need to be removed. It is an essential process in all forms of life.

2.0Excretory organs/structures in animals:

The organs which remove the waste products of metabolism from the animal body are called excretory organs. In single-celled organisms, waste products are discharged directly by diffusion through the surface of the cell. Multicellular organisms utilize more complex excretory methods.

• Ammonotelic organisms are those which excrete ammonia e.g., most aquatic animals like bony fishes, tadpole stage in frog.
• Ureotelic organisms are those which excrete urea e.g., cartilaginous fishes like sharks, frogs and mammals.
• Uricotelic organism are those which excrete uric acid e.g., reptiles, birds and insects.
• There are some other organs, such as lungs, liver, intestine and skin, that remove certain metabolic waste e.g., ${\mathrm{CO}}_2$, sweat, oil etc. They are known as accessory or additional excretory organs.

How does skin help in the regulation of body temperature?

• Explanation: The thin layer of fat cells in the dermis of the skin insulates the body. The contraction of smooth muscles attached to hairs forms goosebumps and creates an insulating blanket of warm air. Also, sweat produced by sweat glands uses excess body heat to evaporate, providing a cooling effect.

3.0Excretory system in human

Human excretory system consists of: (1) A pair of kidneys (2) A pair of ureters (3) Urinary bladder (4) Urethra Rigra Kidney

(1) Kidney The main excretory organs of our body are kidneys.

Function : To remove the poisonous substances like urea, other waste salts and excess water from the blood and excrete them in the form of a yellowish liquid called urine.

External structure : Each kidney is surrounded and covered by a tough, fibrous, capsule of connective tissues. This capsule is called renal capsule. The lateral surface of kidney is convex while medial surface is concave. On the inner border of each kidney is a depression called hilum/hilus.

Internal structure : The internal structure of kidneys can be divided into two parts. Its outer part is called cortex and inner part is called medulla. A kidney is composed of numerous microscopic, coiled tubules called nephrons or uriniferous tubules.

(2) Ureters They are tubes which carry urine to urinary bladder. Each ureter originates from hilum of kidney. The anterior part of the ureter is broad, like a funnel and called pelvis and its posterior part is in the form of long tubule.

(3) Urinary bladder Each ureter opens into the urinary bladder. It is muscular pear-shaped sac. It has thick distensible wall lined by transitional epithelium that allows expansion. The contraction of bladder muscles squirts urine out.

(4) Urethra It is a tubular structure which extends from the urinary bladder to the outside. It carries the urine to the outside.

• Pale yellow colour of urine is due to the urochrome pigment. It is formed in the liver due to the breakdown of haemoglobin.
• Urea (main nitrogenous waste in our body) is formed in liver through ornithine cycle.
• Micturition is the term used for urination (Passing out of urine).

Why is egestion not a part of excretion?

• Explanation: Egestion is the elimination of undigested material in solid or semisolid form from the digestive tract through the anus. Excretion on the other hand is getting rid of metabolic waste formed from the chemical reactions of the body such as in urine, sweat etc.

4.0Structure of Nephron :

The nephron is the structural and functional unit of the kidney. A nephron consists of a twisted tubule closed at one end, open at the other with a network of associated blood vessels. Each kidney of man is formed of about one million nephrons. Each nephron has a length of about 3 cm . It is differentiated into 4 regions having different anatomical features and different physiological roles.

The nephron cleans all your body blood in 45 minutes. Everyday nephron sends about six cups of urine to urinary bladder. Main component of urine is water and urea.

The 4 regions of nephron are: (i) Bowman's capsule (ii) Proximal convoluted tubule (PCT) (iii) Loop of Henle (iv) Distal convoluted tubule (DCT)

(i) Bowman's capsule It is a large double walled cup. It lies in the renal cortex. It contains a tuft of capillaries called glomerulus and the outer wall is continuous with the rest of the nephron. The space between the two walls of the Bowman's capsule is continuous with the lumen of the next part of the nephron. The bowman's capsule and the glomerulus together constitute the renal corpuscle or Malpighian body.

(ii) PCT It starts from the back of the Bowman's capsule and it is highly convoluted. It lies in the renal cortex. The wall consists of a single layer of cuboidal cells bearing a lot of microvilli on the surface.

(iii) Loop of Henle It is a U-shaped segment of the nephron located in the renal medulla. It consists of two straight parallel limbs, a descending limb which is a continuation of the PCT and enters into the renal medulla and an ascending limb which re-enters the renal cortex and joins the DCT.

(iv) DCT It is greatly twisted like the PCT and lies in the renal cortex. The terminal relatively short part of the DCT is called the collecting tubule. It opens into the collecting duct. The collecting ducts receive the collecting tubules of several nephrons.

• The process of maintaining the right amount of water and proper ionic balance in the body is called osmoregulation. The hormones that regulate the function of kidney are ADH (Anti diuretic hormone) and aldosterone. ADH or vasopressin is released from the pituitary gland and aldosterone is released from adrenal gland.
  

5.0Physiology of excretion:

Kidney filters all of the blood that comes from the heart forming urine. The process of cleaning takes place in the nephrons present in kidneys in three stages. They are ultrafiltration, reabsorption and tubular secretion. (1) Ultrafiltration : The first step in clearing the blood is filtration under pressure, the passage of a liquid through a filter to remove impurities. Filtration occurs in the glomeruli. Blood pressure helps plasma (the liquid portion of the blood) to pass through the capillary walls in the glomerulus. Glomerular capillaries are more permeable than other capillaries. The filtrate contains water, glucose, amino acids, ions and urea.

• Normally in a healthy adult, the initial filtrate in the kidneys is about 180L daily. However, the volume actually excreted is only a liter or two a day because the remaining filtrate is reabsorbed in the renal tubules.

(2) Reabsorption: During reabsorption, substances considered necessary in the filtrate are reabsorbed by renal tubules which then travel back into the bloodstream. Maximum reabsorption occurs in PCT because of the presence of microvilli.

(3) Tubular secretion : Passage of unwanted substances out of the capillaries directly into the renal tubules is called as tubular secretion. This is an additional way of getting waste materials into the urine.

• Kidney stones (Nephrolithiasis) are formed due to crystallization or precipitation of uric acid or oxalate of minerals and waste materials found in urine. They grow & become a painful irritant that may require surgery or ultrasound treatments.

How does kidney help in regulating the pH of our body?

• Explanation: The kidneys provide a slower but more powerful means to regulate pH . They excrete or absorb hydrogen ions $\left({\mathrm{H}}^{+}\right)$and bicarbonate ions ( ${\mathrm{HCO}}_3^{-}$) as necessary for adjusting pH of blood.

6.0Artificial Kidney or Hemodialysis

The blood of a person having both kidney failure can be cleaned regularly by using kidney machine (or dialysis machine). The procedure used for cleaning the blood of person by separating the nitrogenous waste substance (urea) from it is called dialysis.

Artificial kidney called hemodialyzer, is a machine that is used to filter the blood of a person whose kidneys are damaged. The process is called haemodialysis.

The blood from an artery in the patient's arm is made to flow into the dialyzer or dialysis machine made of long tubes of semipermeable membrane (like cellophane which are coiled in a tank containing dialyzing solution). The dialyzing solution contains water, glucose and salts in similar concentrations to those in normal blood. It does not have urea. As the patient's blood passes through the dialyzing solution, most of the wastes like urea present in it pass through the semipermeable cellphone tubes into the dialyzing solution by diffusion. The clean blood is pumped back into a vein of the patient's arm.

Drawbacks of artificial kidney

(i) It involves a great deal of discomfort and a risk of formation of blood clots. (ii) It may cause fever, cardiovascular problems and hemorrhage.

What do you mean by organ donation?

• Explanation Organ donation is a generous act of donating an organ to a person who suffers from nonfunction of organ(s). Donation of an organ may be done by the consent of the donor and his/her family. Anyone regardless of age or gender can become an organ and tissue donor. Organ transplants can save or transform the life of a person.

Transplantation is required because the recipient's organ has been damaged or has failed by disease or injury. In organ transplantation the organ is surgically removed from one person (organ donor) and transplanted to another person (the recipient). Common transplantations include corneas, kidneys, heart, liver, pancreas, lungs, intestines and bone marrow. Most organ and tissue donations occur just after the donor has died or when the doctor declares a person brain dead. But some organs such as kidney, part of a liver, lung, etc., and tissues can be donated while the donor is alive.

The plant get rid of stored solid and liquid wastes by the shedding of leaves, peeling of bark and felling of fruits.

7.0Excretion in plants

In plants no definite excretory system or organ is present for removal of wastes. The excretory products which are easily diffusible are removed by way of diffusion. In higher plants waste materials are deposited in various body parts, which are later eliminated. The main waste products produced by plants are carbon dioxide, water vapour and oxygen. ${\mathrm{CO}}_2$ and water are produced as wastes during respiration by plants. ${\mathrm{CO}}_2$ produced during respiration in day time is all used by the plant itself in photosynthesis. Plants excrete oxygen as a waste only during day time. The gaseous wastes of respiration and photosynthesis in plants are removed through the stomata in leaves and lenticels in woody stem into the atmosphere. Oxygen is produced as a waste during photosynthesis. Plants get rid of excess water by transpiration. Many plant waste products are stored in cellular vacuole. Plants also store some of the waste products in their body parts (leaves, bark and fruits). e.g., Tannins, essential oils, latex, gums, resins. Aquatic plants lose most of their metabolic wastes by direct diffusion into water surrounding them.

8.0Biology Diagrams made Easy

9.0Chapter At a Glance

10.0SOME BASIC TERMS

• Metabolic reaction : Metabolic reaction are the chemical processes that occur in all living organisms, which help them to complete their life cycle.
• Elimination : Removal of waste matter from the body.
• Diffusion : Diffusion is the net movement of anything from a region of higher concentration to region of lower concentration.
• Extends : To make something larger.
• Egestion : The process of discharging undigested material from the cell or organism.
• Restricted : Limited in extent.
• Reabsorbed : Being absorbed again.
• Transpiration : Loss of water in the form of water vapour from the aerial parts of plants.
• Deposited : Set down in a specific place.
• Get rid : To remove something that you do not want any longer.