Kidneys form urine, which passes to bladder for storage prior to excretion.
Steps involved in the process of urine formation:
As glomerular ﬁltrate enters renal tubules, it ﬂows sequentially through successive parts of tubules which includes proximal tubule, loop of Henle, distal tubule, collecting tubule, and, ﬁnally, collecting duct before it is excreted as urine. Along this course, some substances are selectively reabsorbed from tubules back into the blood, whereas others are secreted from blood into tubular lumen. Most reabsorption from filtrate takes place in proximal convoluted tubule, whose walls are lined with microvilli to increase surface area for absorption. This process is selective reabsorption. Reabsorption of some substance is passive, while some substances are actively transported.
Active transport can move a solute against an electrochemical gradient and requires energy derived from metabolism. Transport that is coupled directly to an energy source, such as the hydrolysis of adenosine triphosphate (ATP), is termed primary active transport. A good example of this is the sodium-potassium ATPase pump that functions throughout most parts of the renal tubule. Transport that is coupled indirectly to an energy source, such as that due to an ion gradient, is referred to as secondary active transport. Reabsorption of glucose by renal tubule is an example of secondary active transport. But in passive transport there is no requirement of energy.
Within peritubular capillary network, molecules and ions are reabsorbed back into blood. Sodium chloride reabsorbed into the system increases the osmolarity of blood in comparison to glomerular filtrate. This reabsorption process allows water (H2O) to pass from glomerular filtrate back into the circulatory system.
Glucose and various amino acids also are reabsorbed into circulatory system. These nutrients have carrier molecules that claim the glomerular molecule and release it back into circulatory system. If all carrier molecules are used up, excess glucose or amino acids are set free into the urine. Other substances reabsorbed by active transport include sodium, calcium, potassium, phosphate and chloride. A complication of diabetes is inability of the body to reabsorb glucose. If too much glucose appears in glomerular filtrate it increases osmolarity of filtrate, causing water to be released into urine rather than reabsorbed by circulatory system. Frequent urination and unexplained thirst are warning signs of diabetes, due to water not being reabsorbed.
Major portion of water is reabsorbed by osmosis. Only 60–70% of filtrate reaches Henle loop. Much of this, especially water, sodium and chloride, is reabsorbed in the loop, so that only 15–20% of the original filtrate reaches distal convoluted tubule. More electrolytes are reabsorbed here, especially sodium, so the filtrate entering collecting ducts is actually quite dilute. The main function of collecting ducts is to reabsorb as much water as body needs.
Glomerular filtrate has now been separated into two forms: Reabsorbed Filtrate and Non-reabsorbed Filtrate. Non-reabsorbed filtrate is now known as tubular fluid as it passes through the collecting duct to be processed into urine.
Reabsorption of nitrogenous waste products, such as urea, uric acid and creatinine is very limited. maximum capacity of kidney for reabsorption of a substance is transport maximum, or renal threshold. For example, normal blood glucose level is 3.5–8 mmol/L (63 to 144 mg/100 mL) and if this rises above transport maximum of about 9 mmol/L(160 mg/100 mL), glucose appears in urine. This occurs because all carrier sites are occupied and mechanism for active transport out of the tubules is overloaded.
Parathyroid hormone, Antidiuretic hormones regulates selective reabsorption in urine formation process.