Life Sciences Glossary: Internal Structure of Kidney and Physiology of Excretion

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Internal Structure of Kidney

Nephrons

Nephrons (uriniferous tubules) are morphological and physiological units of kidney. Man has 10 - 12 laces () nephrons in each kidney.

A nephron is made of two parts:

Malpighian corpuscle and table Malpighi and corpuscle (renal corpuscle) consists of Bowman՚s capsule and glomerulus. They are placed in renal cortex. The wall of the Bowman՚s capsule is made of a single layer of squamous Glomerulus epithelial cells. Afferent arteriole carry blood to glomerulus while efferent arteriole collect blood from it. The diameter of afferent arteriole is much more than that of efferent arteriole. Tubule consists of proximal convoluted tumble (PCT) , loop of Henley (U-shaped with descending and ascending limbs) and distal convoluted tubule (DCT) . The wall of the tubules is made of cuboidal cells which become ciliated in the neck region. Tubules open to collecting tubules.

Two types of nephrons present in kidney are:

  • cortical and juxtamedullary nephrons. Cortical nephrons (15 - 35%) close to kidney surface, have a shorter loop of Henley and per tubular capillary network. Juxtamedullary nephrons at the junction of renal cortex and medulla, have a longer loop of Henley and vasa recta.
  • Inside the kidney, the blood vessels run along the junction of cortex and medulla. Pelvis of each kidney is directly continued into ureter.

Physiology of Excretion

Nitrogenous excretory products are formed during proteins catabolism.

The main excretory products are:

  • Amino Acids: The animal which excrete amino acids are called amino telic.
  • Ammonia: Ammonia is highly toxic, readily soluble in water and is diffusible. Ammonia can thus be lost through the skin and gills of aquatic organisms. Excretion of ammonia requires a large amount of water, so carried out only by aquatic organisms. Organism in which ammonia is the chief excretory product are called ammoniotelic.
  • Urea: Urea is derived from ammonia and carbon dioxide in liver. It is transported in the blood by blood plasma. Urea is the breakdown product of amino acids. It is less toxic, less soluble in water than ammonia and can be present in higher concentrations in the blood of animals without toxic effects, for example, in man 18 to 38 mg per 100 mill Animals that excrete principally urea are ureotelic. Mammalia (man, dog and camel) are ureotelic. A person has taken a large amount of protein in his diet. He will excrete more of urea. A person who is starving, that is not having food, water and beverages will have more urea in his blood.
  • Uric Acid: Terrestrial reptiles (lizards and snakes) , birds, most infects have to conserve water in their body. Uric cad has a low toxicity and is insoluble in water. Cu acid cable stored or accord in cryslline from. The animals which excrete mainly uric acid are ureotelic. Waste products of adenine and guanine metabolism are excreted by man as uric acid. Uric acid is produced by purine catabolism (adenine and guanine) and protein metabolism in birds and reptiles. Muscular dystrophy is a disease which breaks up muscular tissues and increases endogenous metabolism and it will eliminate in urine great amount of creatinine.

Bilirubin and Biliverdin

  • The bile pigments are formed in the liver due to breakdown of hemoglobin of worn out RBCs. Bilirubin and biliverdin are excreted through bile. Jaundice is a disease characterized by high level of bilirubin in blood resulting in yellowness of skin, white of eyes, mucous membranes, and body fluids.
  • Physiology of excretion includes two parts: Chemical and mechanical parts.
  • The chemical part includes deamination and ornithine cycle. The mechanical part comprises the formation of urine.

Deamination

Deamination takes place in a living cells especially in the liver cells. It is a process to make use of excess of amino acids which cannot be incorporated into the protoplasm. It is the removal of surplus proteins or amino acids or removal of amino group from carboxyl group with the formation of ammonia and keno acid. These are two types of deamination: Oxidative and hydrolytic. In man, nearly all ammonia comes from glutamic acid. Transamination is the transfer of an amino group from an alpha-amino acid to an alpha-keno acid.

Ornithine Cycle

Ornithine cycle is also called as urea cycle or kerb-Henseleit cycle (discovered by Hans՚s kerns and Kurt Hensley in 1932) . Ornithine cycle is the detoxification of ammonia. Urea cycle takes place in liver cells. It involves the union of two molecules of ammonia and a carbon dioxide. Ornithine, a non-protein amino acid, reacts with one molecule of ammonia and carbon dioxide to give coralline. Coralline reacts with a second molecule of ammonia to give arginine. With the hydrolytic enzyme arginine, arginine is splatted into urea and ornithine with the elimination of a water molecule. Three participating amino acids in ornithine cycle are orthenine, coralline and arginine. In addition, two molecules of ammonia enter ornithine cycle. The enzyme taking in ornithine cycle is arginine.

Formation of Urine

Three processes involved in the urine formation are:

  • Ultrafiltration
  • Selective reabsorption and
  • Tubular or active secretion