Glomerular and tubular function:Glomeruli and renal tubule are involved in the formation of urine. Urine actually originates as a result of filtration of selected components of blood plasma. Each glomerulus serves as a special filter or sieve which permits only selective material to filter into Bowman's capsule. The membrane of glomeruli and Bowman's capsule permits water and all dissolved smaller molecules to pass through and enter Bowman's capsule. Blood cells and plasma proteins are not filtered owing to their larger size. The fluid filtered in Bowman’s capsule is termed as Glomerular filtrate. In adult human about 110-120 ml of glomerular filtrate is produced every minute from glomeruli of both kidneys. This filtrate contains both useful as well as waste products. Glomerular filtrate is then subjected to selective process of secretion and reabsorption while passing through subsequent portions of renal tubule.
Tubular Secretion applies to passage of metabolic wastes from blood to the renal tubules.
Tubular reabsorption applies to passage to useful metabolites from renal tubules to the blood.
Conservation of water & Electrolytes: Various segments of renal tubule (PCT, LH, DCT and CT) are permeable to varying degree towards different solutes and metabolites. By tubular reabsorption, useful substances like glucose, amino-acids, sodium, chloride, bicarbonates etc. are returned to blood. By tubular secretion, substances like organic acids, urea, surplus minerals and other waste solutes are added to renal tubule for excretion into urine. Reabsorption also accounts for returning 99.5% of water appearing in glomerular filtrate. Thus only 0.5% of filtrate is allowed to be excreted as urine. Renal tubular reabsorption and secretion process is also involved in maintenance of water and electrolyte homeostasis in the body. Because of this special feature of renal tubule, the kidneys are regarded as vital organs through renal tubules, the kidneys adjust the amount of water and electrolytes to be excreted in urine and this adjustment occurs 24 hours a day and for whole life continuously. Some hormones plays important role in regulation of water and electrolyte excretion through urine and these are listed below:
Antidiuretic Hormone (ADH) is involved in retention of water in body. By acting on DCT and CT, it conserves water and thus decreases volume of urine. Deficiency of this hormone results in excretion of enormous volume of urine thus leading to dehydration. The disease is termed as diabetes insipidus.
Aldosteroneis a steroid hormone secreted from adrenal cortex and it is involved in renal conservation of sodium. The hormone acts on DCT segment.
From the above discussion it has become apparent that kidney is a wonderful organ, which serves to eliminate selective waste products from blood and retain the useful ones. Apart from this kidneys are involved in maintaining uniform environment in the body for optimum cellular functioning.
Storage And Discharge Of Urine:
Formation of urine in the renal tubule is a continuous process. Occurring throughout life but discharge or expulsion of urine from body occurs at one's desire. Synthesized urine is stored in urinary bladder. Urinary bladder is muscular pouch like organs which undergo stretch relaxation to accommodate urine. Contracted urinary sphincters do not allow urine to pass into urethra. When the bladder is full, a pressure is exerted in bladder wall and when pressure exceeds its there hold limit, a physiological reflex called "micturition reflex" is initiated with the involvement of nervous system. Micturition reflex results in contraction of bladder muscle and relaxation of urethral sphincters thus ultimately expulsion of urine.