Human Excretion and Kidney Physiology

The organ that temporarily stores urine before it passes out of the body is urinary bladder. 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 each 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 muscular sac where urine is stored until the pressure of the expanded bladder leads to urge to pass it out through the urethra. While gall bladder is a pear shaped sac like structure and is attached to the liver. It stores bile secreted by the liver. Uterus is part of human female reproductive system. It is hollow muscular and pear shaped structure.

Renal artery brings the dirty blood containing waste substances into the kidney.  The renal artery, after entering the kidney, divides and redivides forming various afferent renal arterioles. This renal artery brings dirty blood containing waste substances. 

The component of glomerular filtrate not reabsorbed in the tubular part of nephron is urea. Glucose and amino acids are reabsorbed to minimize the loss from the body. Also, much water is reabsorbed in the tubular part of the nephron so as to produce concentrated urine. Urea, being the chief excretory product, is not reabsorbed and is excreted out of the body.

Filtration of blood occurs in Bowman's capsule. The filtration of blood is carried out by blood capillaries (glomerulus). On an average, 1100-1200 ml of blood is filtered by the kidneys per minute. The glomerular capillary blood pressure causes filtration of blood through membranes and almost all the constituents of the plasma, except proteins, are passed through the lumen of Bowman's capsule (cup-shaped part of nephron, which receives the glomerular filtrate). Tissue fluid is the fluid that surrounds the cells in an organism. It is aqueous in nature and is composed of amino acids, sugars, fatty acids, hormones, salts as well as waste products from the cells. Lymph is also a part of tissue fluid. Cerebrospinal fluid is the fluid that surrounds our brain. It acts like cushion and protects our brain.

Antidiuretic hormone promotes the reabsorption of water from the nephron during urine formation. It acts on the distal convoluted tubule, collecting ducts and increases the reabsorption of water from it. Calcitonin is the hormone produced from the thyroid glands. 

Ureters are the long tubes which connect the kidneys with the urinary bladder. 

The structural and functional unit in kidneys is nephron. Each nephron is made up of two main parts: Malpighian body and renal tubule. Renal tubule is the tubular part of the nephron. Malpighian body comprises cup shaped Bowman's capsule and a network of blood capillaries (glomerulus). The Bowman's capsule leads into the renal tubule, which leads to proximal convoluted tubule (PCT) and then to descending limb of loop of Henle, further to ascending limb and then to distal convoluted tubul, which further leads to collecting duct . Urine formation involves the filtration of blood, which is carried out by blood capillaries (glomerulus). On an average, 1100-1200 ml of blood is filtered by the kidneys per minute. The glomerular capillary blood pressure causes filtration of blood through membranes, and almost all the constituents of the plasma, except the proteins, are passed through the lumen of Bowman's capsule (cup shaped part of nephron, which receives the glomerular filtrate). From here, the glomerular filtrate enters the tubular part of nephron, i.e. renal tubule.

Ureters are 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.

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. Diaphragm is a tissue of internal muscle that separates thorax and abdomen of mammals. Abdomen comprises digestive organs like stomach, small intestine, large intestine with caecum and appendix, liver, gall bladder, pancreas and spleen; organs of urinary system like kidneys and ureters.

The final urine is prepared from the primary urine by reabsorption. After the glomerular filtration, absorption of selected materials occurs from the filtrate into the blood. About 65-80% filtrate is reabsorbed from here. Solutes like glucose, amino acids are absorbed so that these do not appear in urine. Also, considerable amount of water is absorbed. Hence, concentrated urine is produced. Ultra filtration is the process that takes place in the Bowman’s capsule. On an average, 1100-1200 ml of blood is filtered by the kidneys per minute. The blood pressure of glomerular capillary causes filtration of blood through membranes and almost all the constituents of the plasma, except the proteins, are passed through the lumen of Bowman’s capsule (Cup shaped part of nephron, which receives the glomerular filtrate, is called Bowman’s capsule). Therefore, it is considered as a process of ultrafiltration. Dialysis is the separation of certain substances from blood by use of a selectively permeable membrane. The pores in the membrane allow some substances to pass through, however, prevent others.

Filtration of blood occurs in the kidneys in a cup shaped part of nephron, which receives the glomerular filtrate is called Bowman’s capsule.  

 In ornithine cycle, CO₂ and ammonia wastes are removed from the blood. In mammals, this cycle takes place primarily in the liver, and to a lesser extent in the kidney.

 The net pressure gradient that causes the fluid to filter out of the glomeruli into the capsule is 20 mm Hg. A glomerular hydrostatic pressure of 70 mm Hg develops in glomerulus. It is opposed by blood osmotic pressure (30 mm Hg) and capsular hydrostatic pressure (20 mm Hg ) inside the Bowman's capsule.

  A person is undergoing prolonged fasting. His urine will be found to contain abnormal quantities of ketones. Ketones are metabolic end-products of fatty acid metabolism. In healthy individuals, ketones are formed in the liver and are completely metabolized so that only negligible amounts appear in the urine. However, when carbohydrates are unavailable or unable to be used as an energy source, fat becomes the predominant body fuel instead of carbohydrates and excessive amounts of ketones are formed as a metabolic byproduct. Higher levels of ketones in the urine indicate that the body is using fat as the major source of energy.

Urea cycle disorders lead to buildup of ammonia. The mainstay of current treatment is to use substances such as sodium benzoate and sodium phenylacetate to complex with ammonia, forming water soluble compounds that can be excreted in the urine. In some cases, liver transplantation is used successfully as well.