Life Processes: Excretion
Removal of harmful waste from the body is called excretion. Many wastes are produced during various metabolic activities. These need to be removed in time because their accumulation in the body can be harmful and even lethal for an organism.
These notes are based on the chapter life processes from class 10 science NCERT book and CBSE syllabus.
Human Excretory System
The human excretory system is composed of a pair of kidneys. A tube, called ureter comes out of each kidney and goes to the urinary bladder. Urine is collected in the urinary bladder, from where it is expelled out through urethra as and when required.
Kidney
Kidney is a bean-shaped organ which lies near the vertebral column in the abdominal cavity. The kidney is composed of many filtering units; called nephrons. Nephron is called the functional unit of kidney.
Nephron
It is composed of a tangled mess of tubes and a filtering part, called glomerulus. Glomerulus is a network of blood capillaries to which renal artery is attached. The artery which takes blood to the glomerulus is called afferent arteriole and the one receiving blood from the glomerulus is called efferent arteriole.
Glomerulus is enclosed in a capsule like portion; called Bowman’s capsule. The Bowman’s capsule extends into a fine tube which is highly coiled. Tubes from various nephrons converge into collecting duct; which finally goes to the ureter.
Filtration in Glomerulus
Filtration happens because of very high pressure inside the glomerulus. The lumen of efferent arteriole is smaller than that of afferent arteriole. Due to this, the blood entering the glomerulus experiences very high pressure and due to this, the waste products are filtered out through the thin membrane of capillaries in the glomerulus.
The filtered blood is sent to the systemic circulation through efferent arteriole and the filtrate goes to the Bowman’s capsule. That is how urine is formed inside the kidneys.
Reabsorption of water and some other filtrates takes place in the tubular part of the nephron. This increases the concentration of urine. The human urine is mainly composed of water and urea.
Excretory products in animals
Nitrogenous compounds such as urea, amino acid and uric acid are the main excretory product in animals. Based on the main excretory product, there are four types of animals, viz. ammonotelic, ureotelic and uricotelic.
Ammonotelic Animals
Animals in which ammonia is the main excretory product are called ammonotelic animals. Ammonia is directly released into water from the body of an animal through osmosis. This mode of excretion is present in most of the aquatic animals because lot of water is needed for removing ammonia from the body. Fish are examples of ammonotelic animal.
Ureotelic Animals
Animals in which urea is the main excretory product are called ureotelic animals. There is moderate need of water for removal of urea from the body. Ureotelism is present in most of the terrestrial animals. Mammals are examples of ureotelic animals.
Uricotelic Animals
Animals in which uric acid is the main excretory product are called uricotelic animals. There is negligible need of water for removal of uric acid from the body. Uric acid is released as white pellets. Birds and lizards are examples of uricotelic animals.