Human Excretion and Kidney Physiology

ADH (Anti diuretic hormone) has this function which gives it its name.

Most fish are ammonotelic, meaning they excrete nitrogen primarily as ammonia. This is because they live in water and can easily dilute and eliminate toxic ammonia through their gills. Ammonia requires minimal energy to produce. Terrestrial organisms convert ammonia to less toxic urea or uric acid, which need less water for elimination but require more energy to synthesize.

The cup-shaped part of nephron which receives the glomerular filtrate is called Bowman’s capsule.

The blood in kidneys is filtered by nephrons, the functional units of the kidney. Each nephron begins in a renal corpuscle, which is composed of a glomerulus enclosed in a Bowman's capsule.

Several collecting ducts unite to form larger ducts that empty urine into a calyx at the tip of the renal pyramid. 

The outlet of urine from the body is through urethra. Urine produced in the kidneys passes through the ureters into the urinary bladder where it is released through the urethra. Ureters are the long tubes, which connect the kidneys with the urinary bladder. Urinary bladder is muscular sac where urine is stored until the pressure of the expanded bladder leads to urge to pass it out through the urethra. Anus is the excretory opening at the end of the digestive tract, it controls the excretion of faeces produced during digestion.

Nitrogenous waste substances such as ammonia, urea and uric acid are produced during protein metabolism according to the species. Amino acids are the breakdown products of proteins in the body. Ammonia is the most toxic and requires large amount of water for its excretion. It is excreted as waste product in protozoans, e.g. amoeba, paramecium, sponges, hydra, liver fluke, earthworm, etc. Uric acid is also excreted as waste product by the most insects (e.g. cockroach), land snails (e.g. helix), land reptiles (e.g. lizards), snakes and birds. 

Nephron is the basic functional unit of kidneys. Each human kidney possesses about one million structural and functional units called nephrons.

Along with the removal of waste, kidneys also maintain constant blood pressure. One of the major roles of the kidneys is maintaining the proper levels of electrolytes (such as sodium and potassium) in the body. The amount of electrolytes in the body influences the amount of fluid in the body. When electrolyte levels are high, the body retains more water, which in turn increases the volume of the blood. More blood volume results in higher blood pressure. Thus, the kidneys maintain blood pressure by indirectly controlling the amount of blood in the body. 

Kidneys are multicellular organs. They filter approximately 1100-1200 ml of blood per minute. The amount of filtrate formed by the kidney per minute is called Glomerular filtrate rate (GFR). GFR, in healthy individual, is approximately 125 ml per minute, i.e. 180 litres per day. Thus, kidney needs to have million nephrons.

Urine produced in the kidneys pass through ureters into urinary bladder. Kidneys filter out waste products from the blood. Just as CO₂ is removed from the blood in the lungs, Nitrogenous wastes like urea or uric acid are removed from blood in the kidneys. This process is called excretion. Excretory system of human beings includes a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Kidneys are located in the abdomen, one on either side of the backbone. Urine produced in the kidneys passes through the ureters into the urinary bladder where it is released through the urethra. Ureters are the long tubes, which connect the kidneys with the urinary bladder. Urinary bladder is a muscular sac where urine is stored until the pressure of the expanded bladder leads to urge to pass it out through the urethra. Vena cava is a large vein, which brings blood from the various parts of body to the heart. Aorta is the largest artery and it arises from the left ventricle and supplies oxygenated blood (not in case of pulmonary artery) to various parts of body.

Kidneys filter out nitrogenous waste from blood. Also, they help in removal of excess of water from the body. Kidneys are the vital organs for survival. Several factors like infections, injury or restricted blood flow to kidneys, reduce the activity of kidneys. This leads to accumulation of poisonous wastes in the body. The composition of blood is disrupted. It may even lead to death of an organism.

Apart from kidneys, liver and skin also help in elimination of nitrogenous excretion. Liver, the largest gland in our body secretes bile containing substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs. Formation of urea also takes place in liver. Most of these substances eliminate through digestive wastes. The sweat and oil glands in the skin also eliminate substances through their secretions, e.g. sweat produced by the sweat glands comprises water, NaCl, small amounts of urea, lactic acid etc. Similarly, oil glands in the skin eliminate certain sterols, hydrocarbons and waxes. Lungs, however, do not excrete nitrogenous wastes, but remove large amounts of carbon dioxide and significant amounts of water every day.

The shape of human kidney is bean shaped. Kidneys are reddish brown, bean shaped structures situated in the abdomen. Each kidney of an adult human measures 10-12 cm in length, 5-7 cm in width, 2-3 cm in thickness with an average weight of 120-170 grams.

Hemodialysis is the separation of certain substances from blood by use of a selectively permeable membrane.