ICSE Class 7 Biology Chapter 07 Excretion in Animals and Plants

Excretion In Animals And Plants

Syllabus

Excretion in human beings - what is excreted and through which organs.

The renal excretory system - drawing of a simple labelled diagram.

Getting rid of wastes in plants - resins and gums.

What Is Excretion?

During different activities taking place in our body, the body produces many substances of which some are useful and some are useless (not required by the body). If retained in the body, they even act as poison and cause much harm and in severe cases, even death. The organs which remove these useless substances are called excretory organs and the process of removal of these substances is called excretion.

Substances To Be Excreted

1. Urea and uric acid (nitrogenous wastes). These are produced as breakdown products of excess amino acids. If allowed to accumulate in the body, these are harmful.

2. Urine pigment (bile pigments). You usually find a yellowish tinge in the urine. This is even more prominent in summers when you urinate in less quantities. This is a harmless substance and is therefore not required in the body.

3. Water. Water is taken in with food and beverages in large quantities. Plentiful water in the body helps in easily "washing out", the nitrogenous wastes. The body retains some water required as a normal constituent while the excess water leaves the body in different ways.

4. Extra Salts. Such as the common salt (NaCl). Sodium and chlorine ions or any other ions are needed in the body only in certain proportions. Any extra quantity must be removed. Extra salts are expelled alongwith the urine.

5. Extra Vitamins. The vitamins absorbed from the food may be in excess. The fat-soluble ones are stored in the body to some extent, but the extra water-soluble ones such as vitamin B and C are passed out in urine. Similarly, certain medicines including antibiotics, if taken in extra quantity are passed out along with urine.

The Urinary System

The urinary system consists of a pair of kidneys, two ureters, a urinary bladder and a urethra.

1. Kidneys. The two bean-shaped kidneys are situated towards the back of the abdomen, one on either side of the back-bone at the level of the last two ribs. The right kidney is located slightly lower than the left one (Fig. 7.1).

2. Ureters. A narrow tube called the ureter runs from the inner border of each kidney up to the urinary bladder.

3. Urinary bladder. It is a muscular bag situated towards the bottom of the abdomen.

4. Urethra. Leading from the bladder is a single median tube called the urethra. The urethra runs through the penis in the male, but in females it opens very close to and above the vaginal opening.

How The Bladder Empties

The base of the urethra is surrounded by a circular muscle. Normally, this muscle is tightly contracted, to close the passage. When there is an urge to empty the bladder, the circular muscle releases to open the passage. Simultaneously, the muscles in the wall of the bladder contract to force out the urine. This process is called urination.

Structure Of The Kidney

Internally, each kidney is composed of an outer darker region (cortex) and an inner lighter region (medulla). The medulla drains the urine into a funnel-like pelvis which continues as a narrow ureter (Fig. 7.2).

Inside the kidney, there are millions of microscopic tubes called renal tubules or nephrons (Fig. 7.3). Each nephron starts as a cup-like Bowman's capsule which continues behind as a narrow tubule. The tubule is convoluted (twisted) and opens into a collecting duct. All the collecting ducts then open into the ureter. Nephron is the structural and functional unit of kidney.

Formation Of Urine In The Nephron

There are three distinct processes involved in urine formation - filtration, reabsorption and secretion.

(i) Filtration: Look at the Fig. 7.3. A narrow arterial branch (arteriole) is entering the Bowman's capsule. It divides into a cluster of minute capillaries called glomerulus. Filtration occurs in the glomerulus.

When blood enters the glomerulus under pressure, some of the liquid plasma of the blood is forced out of the walls of the blood capillaries into the Bowman's capsule and then through the thin wall of the capsule into the tubule. This filtrate contains no blood cells or proteins but has water, sodium, glucose, amino acids, urea and uric acid in it.

(ii) Reabsorption: As the filtrate moves down the tubule, the useful substances such as glucose, amino acids, water, sodium, etc. are reabsorbed by the cells of the walls of the tubule in as much quantity as is needed and the rest passes downward to be passed out as urine.

(iii) Secretion: As the filtrate is passing down through the last part of the tubule, most of the potassium is secreted into it in exchange of reabsorbed sodium (Fig. 7.4).

The remaining fluid passes down through the collecting duct as urine, then into the pelvis of the kidney and next through the ureters into the urinary bladder.

Composition Of Urine

The normal human urine mainly consists of water, urea, uric acid and some amount of mineral salts. Their normal percentage in urine is given in the Table 7.1 and the same has been compared with that in the normal blood.

Note down the constituents showing higher quantities in urine. The higher quantities are due to their removal from the blood so as to keep it in its normal composition. The composition of the blood will always be the same whereas that of the urine may change depending on your food.

Excretion is the removal of all toxic and unwanted products (specially the nitrogenous products) from the body.

Teacher's Note

When you notice your urine is pale in winter and dark yellow in summer, that is osmotic regulation in action - your kidneys are balancing water loss from sweating and urine concentration.

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