ICSE Class 10 Biology Chapter 08 The Excretory System

The Excretory System [Elimination Of Body Wastes]

Syllabus: Excretory System: Elementary treatment of the structure and function of the kidneys; the kidneys treated as comprising cortex and medulla and consisting of a branched system of tubules well supplied with blood vessels leading to the ureter (details of the courses of the tubules and their blood vessels not required).

Scope of Syllabus: External and internal structure of the kidney; parts of the excretory system along with the blood vessels entering and leaving it should be taught with the help of charts or models. Students should be able to draw the diagrams with correct labelling and know the functions of various parts. A general idea of the structure of a kidney tubule nephron should be given. A brief idea of ultra filtration, selective reabsorption and tubular secretion in relation to the composition of blood plasma and urine formed.

A large number of waste products are formed during metabolic activities in the body. Large amounts of \(\text{CO}_2\) and \(\text{H}_2\text{O}\) are produced by metabolism of carbohydrates, fats and proteins. Nitrogenous wastes such as ammonia, urea, uric acid, etc., are formed from proteins and other complex nitrogenous compounds. These nitrogenous products become toxic or harmful if retained inside and hence are sent out with the help of the excretory system.

8.1 Excretion

The process of removal of chemical wastes (mainly nitrogenous) from the body is known as excretion. Excretion plays an important role in maintaining the homeostatic (steady state) condition of the body.

Organs which are concerned with the formation, storage, and elimination of urine constitute the Excretory system.

Excretion should not be confused with defaecation (meaning passing out faeces, i.e. the undigested food from the intestine). Similarly, passing out \(\text{CO}_2\) through the lungs is a part of respiration and not excretion. In humans, the term urinary system is more appropriate than the excretory system.

8.2 Substances To Be Got Rid Off

There are a number of chemical substances which are regularly formed in our body or which are absorbed through the food that must be got rid of—otherwise they become harmful. Some such substances are:

1. Carbon dioxide and water (respiratory products)

2. Nitrogenous metabolic wastes

3. Excess salts and vitamins

4. Water

5. Bile pigments.

1. Carbon dioxide and water: Every living cell liberates energy by oxidizing glucose with the production of carbon dioxide and water. Carbon dioxide is eliminated through the lungs. The water becomes a part of the rest of the water in the body.

2. Nitrogenous metabolic wastes: These include urea, uric acid and ammonia. These are produced mainly in the liver from the dead protein–remains of the other tissues that are brought to it. Any extra amino acids (digestion products of protein) cannot be stored in the body. They are broken down in the liver to produce usable (also storable) glucose, and the urea that has to be excreted out. Urea is highly poisonous; if allowed to accumulate in the blood to a certain level, it causes death. Urea is excreted out through the kidneys.

3. Excess salts such as common salt (NaCl) and even some excess water-soluble vitamins (B & C) need to be eliminated. Salts are mainly given out by the kidneys.

4. Water is taken in with food and beverages, in large quantities. The excess quantity of water is removed which, incidentally, also serves a useful purpose of dissolving the harmful materials to carry them out.

5. Bile pigments (chiefly yellow bilirubin) are the breakdown products of the haemoglobin of the dead RBCs. The liver cells extract it from the circulation and secrete it into the bile juice poured into the duodenum through the common bile duct. These pigments are modified in the intestine to pigments that give faeces their yellowish brown colour. Some of these pigments are excreted in urine.

Excretion is the removal of all harmful and unwanted products from the body, especially the nitrogenous wastes.

Teacher's Note

Just like your body produces waste when you eat food and exercise, a city produces garbage that must be removed daily to keep things clean and healthy.

8.3 The Excretory Organs

Excretion in humans (mammals) is brought about by the following organs:

1. Kidneys: These are the primary excretory organs throwing out excretory products (chiefly urea) in the form of urine.

2. Sweat glands: Excretion by sweat glands is incidental. These glands are primarily concerned with cooling (thermoregulation). The sweat secreted carries with it small amounts of nitrogenous wastes. Sweat glands pass out sweat only when required for cooling, so truly they are not excretory.

3. Lungs: Excretion by lungs in the form of carbon dioxide released in the expired air.

1. Given below is a list of substances—select the ones that need to be got rid off from the body. Glucose, excess water, amino acids, urea, carbon dioxide, excess common salt, glycogen, uric acid.

2. (i) Undigested and unabsorbed food which passes out is termed as excreta in popular language. Is it a kind of excretion? Yes/No (ii) Give reason in support of your answer.

Teacher's Note

When you sweat during exercise, your body is cooling itself while getting rid of extra salt—a dual purpose system that keeps you comfortable and healthy.

8.4 Kidneys (Urinary System)

The kidneys (Fig. 8.1) are two bean-shaped organs about 10 cm long and 6 cm wide, located on either side of the backbone and protected by the last two ribs. The right side kidney is at a slightly lower level than the left one. A tube, the ureter, arises from the notch (hilum) in the median surface of each kidney and connects behind with the urinary bladder in the lower part of the abdomen. The front end of the ureter is somewhat expanded into the kidney and is called the pelvis (Fig. 8.2) (Lat. pelvis: basin/cup). The urine produced in the kidneys constantly flows through the ureters and collects in the urinary bladder. The urine is intermittently emptied from the urinary bladder to the outside of the body through the urethra. The openings of the ureters (into the bladder) are somewhat projecting and act like valves to prevent the backflow of urine when the bladder contracts to pass out the urine. A sphincter (circular muscle) guards the opening of the bladder into the urethra and relaxes only at the time of urination (micturition) under an impulse from the brain.

8.4.1 Internal Structure Of The Kidney

A longitudinal section of the kidney (Fig. 8.2) shows two main regions—an outer dark cortex and an inner lighter medulla. The medulla is composed of a finely striped substance arranged in several conical pyramids. The apex of each pyramid (papilla) projects into the pelvis of the kidney. The kidney is composed of an enormous number of minute tubules called uriniferous tubules or nephrons or renal tubules or just kidney tubules. These are the structural as well as functional units of the kidney.

8.4.2 Structure Of A Kidney Tubule

Each kidney tubule (Fig. 8.3) has the following parts:

Bowman's capsule: It is a thin-walled (single-cell thick epithelium) cup, something like a hollow ball pressed deep on one side. Its hollow internal space continues into the tubule. The outer concavity of the cup lodges a knot-like mass of blood capillaries, called glomerulus. The Bowman's capsule and the glomerulus together are called Malpighian capsule or just renal capsule.

Proximal or first convoluted tubule (PCT) is the starting convoluted region of the tubule ("proximal" means nearer, i.e. nearer to the Bowman's capsule). Both the Bowman's capsule and the proximal convoluted part lie in the cortex giving it a dotted appearance in sectional view (Fig. 8.2A).

Middle U-shaped part (Loop of Henle) is shaped like a hair-pin; it is not convoluted. It runs in medulla to turn back and to re-enter the cortex to continue into the next convoluted region of the tubule.

Distal convoluted tubule (DCT) (again lying in the cortex) is the end part of the kidney tubule ("distal" means farther, i.e. farther or away from the Bowman's capsule). It opens into a collecting duct. The collecting duct receives the contents of many kidney tubules and pours it as urine in the pelvis of the kidney.

Teacher's Note

The kidney's filtration system works like a coffee filter, slowly separating what your body needs from what it doesn't, producing the waste product urine that must be eliminated daily.

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