Revision: Excretion and Osmoregulation Biology HSC Science (General) 11th Standard Maharashtra State Board

Excretion is the process that biological organisms use to expel or eliminate the waste products produced by their metabolism.

Micturition is the process of expelling urine out of the body through the urethra by opening the sphincter muscles and passing urine, involving the relaxation of the sphincter muscles between the urinary bladder and urethra. 

The kidney while removing wastes like urea from the blood also regulates its composition, i.e., the percentage of water and salts. This function is called osmoregulation.

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

The process of removal of chemical wastes (mainly nitrogenous wastes) from the body is known as 'excretion' (ex: out, crete: flow).

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Excretion is the process of removal of harmful and unwanted nitrogenous waste products from the body.

During different metabolic activities taking place in our body, the body produces many substances, of which some are useful and some are useless.
The process of removing useless and harmful metabolic waste substances is called excretion.

The process of removal of chemical wastes (Mainly Nitrogenous) from the body is known as ''excretion''. It plays an important role in maintaining the homeostatic (steady-state) condition of the body.

Kidneys are the primary excretory organs, eliminating nitrogenous wastes (chiefly urea) from the blood and throwing it out in the form of urine.

During different metabolic activities taking place in our body, the body produces many substances of which some are useful and some are useless.
If retained in the body the unwanted substances may become poisonous and cause much harm and in severe cases, even death. The organs which remove these unwanted and toxic substances from the body are called excretory organs.

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 and functional units of the kidney. 

The apex of each pyramid in the medulla of the kidney that projects into the pelvis is called the papilla.

Bowman's capsule is a thin-walled, cup-shaped structure in the nephron that surrounds the glomerulus and collects the filtrate from the blood.

The glomerulus is a knot-like network of blood capillaries located inside the Bowman's capsule, where blood filtration occurs.

The Malpighian capsule is the combined structure of the Bowman's capsule and glomerulus, forming the filtration unit of the nephron.

Tubular secretion is the active transfer of certain substances, such as ions and drugs, from the blood into the renal tubule during urine formation.

Glomerular filtration rate is the amount of glomerular filtrate formed in all the nephrons of both kidneys per minute. In a healthy individual, it is about 125 ml/minute.

Inside the kidney, there are millions of microscopic tubes called renal tubules or nephrons. It is the structural and functional unit of the kidney.

The blood flows through the glomerulus under great pressure which causes the liquid part of the blood to filter out from the glomerulus into the renal tubule. This filtration under high force is called Ultrafiltration.

The artificial process which cleans and filters the blood in a person where one or both the kidney may stop working properly is called dialysis.

Ultrafiltration is the process in which blood is filtered under high pressure in the glomerulus, allowing water and small solutes to pass into the Bowman’s capsule.

Glomerular filtrate is the fluid formed after ultrafiltration that enters the renal tubule and contains water, urea, salts, glucose, and other small molecules.

Selective absorption is the process by which only useful substances like glucose, some salts, and water are absorbed from the renal tubule back into the blood, without disturbing its normal concentration.

1. Humans have two bean-shaped kidneys, located on either side of the backbone and protected by the last two ribs.
2. The right kidney is slightly lower than the left due to the position of the liver.
3. Each kidney has a hilum where the ureter, renal artery, and renal vein are connected.
4. Internally, the kidney has an outer cortex and an inner medulla made up of conical structures called renal pyramids.
5. The kidney contains numerous nephrons (uriniferous tubules), which are the structural and functional units responsible for urine formation.

1. Each kidney contains about 2 million uriniferous tubules, each 4–5 cm long, with a total length of over 60 km aiding reabsorption.
2. The Malpighian capsule is formed by Bowman’s capsule and glomerulus, where filtration begins.
3. Proximal convoluted tubule (PCT) in the cortex is the first site of reabsorption; Loop of Henle extends into medulla and helps in water conservation.
4. Distal convoluted tubule (DCT) opens into the collecting duct, which transports urine to the pelvis.
5. Blood flows through the kidneys about 350 times a day, filtering approximately 160 litres of fluid daily, most of which is reabsorbed.
6. The kidneys produce around 1.2 litres of urine per day, and their blood supply involves afferent and efferent arterioles along with capillary networks.

1. Kidneys are the main excretory organs that remove urea and other nitrogenous wastes from the blood in the form of urine.
2. Skin eliminates small amounts of urea, salts, and water through sweat, mainly for body cooling.
3. Lungs excrete carbon dioxide through exhaled air, playing a key role in gaseous waste removal.
4. Liver helps in detoxification by converting ammonia to urea and breaking down toxins like alcohol and drugs.