The process of removing harmful metabolic waste such as urea, uric acid and salts from the body is called excretion.
Unicellular organisms, like the amoeba, remove waste by simple diffusion from the body surface into the surrounding water.
Lower multi-cellular organisms, like flat worms, use flame cells, while earthworms use nephridia for excretion.
Higher multi-cellular organisms like fish, frogs, lizards, birds and humans use kidneys for excretion.
Homeostasis is maintenance of the internal environment of the body.
The excretory system in human beings includes a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra. This is also known as the urinary system.
The kidneys are located in the abdomen, one on either side of the backbone. They act as excretory organs, and also control the balance of water and mineral ions in the body.
Each kidney divided into the outer cortex and the inner medulla. The medulla is composed of conical masses of tissue that lead to the pelvis.
The cortex has a random arrangement of tiny tubules called nephrons, which are the functional units of the kidney.
Each nephron consists of a corpuscle and a renal tubule.
A corpuscle is composed of tangled clusters of tiny blood capillaries called “glomerulus”. This is called the filtration unit. A thin-walled, sac-like structure, called "Bowman's capsule," surrounds the glomerulus.
The renal tubule leads away from Bowman's capsule, and becomes highly coiled to form the "proximal convoluted tubule." The tubule makes a hairpin loop, called the loop of Henle, and joins the distal convoluted tubule. The distal convoluted tubule opens into a collecting duct, which passes into the renal medulla.
From each kidney, a ureter arises and opens into the urinary bladder.
The urethra carries urine to the outside of the body. The urethra emerges through the penis in males, and close to the vagina in females.
Excretory pathway in kidney – blood carried by the renal artery to the nephron, to the renal pyramid, to the pelvis, to the ureter, to the urinary bladder, to the urethra, to outside.
The kidney performs three functions leading to urine formation - ultra-filtration, selective re-absorption and tubular secretion.
Nephrons filter minerals, waste and water, but retain red cells, proteins and large molecules. This process is known as ultra-filtration.
Some substances in the initial filtrate, such as glucose, amino acids, salts, and a major amount of water, are selectively re-absorbed as urine flows along the tube.
Substances not required by the body may be secreted into the filtrate by the cells of the distal convoluted tubule before it leaves the kidney. This process is known as tubular secretion.
Dialysis is a device used to remove nitrogenous wastes from blood. An artificial kidney contains a number of tubes with a semi-permeable lining, suspended in a tank filled with dialysing fluid.
The patient’s blood is passed through these tubes. During this passage, waste products from the blood pass into the dialysing fluid by diffusion. The purified blood is pumped back into the patient.
Resins, gums and dead leaves are some excretory products of plants.