Urology is the study of the urinary system and how it works. There are four processes of the renal processes. Filtration, reabsorption, secretion, and excretion.
Filtration is the process of fluid being filtered through three layers that serve as selective filters. The fluid that enters to be filtrated is the filtrate. The filters are capillary fenestrae, glomerular basement membrane, and the slit diaphragm. The fluid first enters the glomerular capsule. glomerular capillaries are extremely permeable and have an extensive surface area. The filtration pressure produces a large volume of filtrate. The GFR (glomerular filtration rate) is the volume of filtrate that he kidneys produce per minute. 115 ml per minute is average for women and 125 ml per minute is average for men. This is equal to 180 L per day. Total bood volume averages 5.5 L in the human body. Total blood volume is filtered into the urinary tubules every 40 minutes. The majority of the filtered water must be returned immediately back to the vascular system. If this didn't happen, a person would urinate to death within a few minutes!
The ability of kidneys to maintain a constant GFR with fluctuating blood pressures is renal autoregulation. Tubuloglomerular feedback is a necessity in autoregulation of the GFR. A group of specialized cells called the macula densa is the sensor in tubuloglomerular feedback. NaCl and H2O passage and GFR homeostasis is maintained by the feedback system.


Reabsorption is the return of filtered molecules from the tubules back to the blood. Salt and water filtered from the blood is thus reabsorbed back through the proximal tubule wall. Water is reabsorbed by osmosis. Water follows transport of NaCl from the tubule into the surrounding capillaries. The majority of the water remaining in the filtrate is reabsorbed through the wall in the renal medulla called the collecting duct. About 99% of the filtrate must be returned to the blood. The other 1% is excreted in the urine.

http://www.britannica.com/EBchecked/topic/497969/renal-system/58630/Tubule-function (Source for picture)

http://www.britannica.com/EBchecked/topic/497969/renal-system/58630/Tubule-function (Source for picture)

The next process is secretion. This is the opposite of reabsorption. The molcules and ions move out of the peritubular capillaries and into the interstitial fluid. Then they get transported acoss the basolateral membrane of the tubular epithelial cells and go into the lumen of the nephron tubule. Molecules that have been filtered and secreted are excreted/eliminated to the urine faster then those molecules that have not been secreted. The rate of excretion in urine is equal to the rate of entering the filtrate minus rate of reabsorption from filtrate. This equation is used to measure the volume blood plasma filtered per minute by the kidneys. This is called glomerular filtration rate. This is very important in assessing kidney health. The video below shows the secretion or excretion of Na+, K+, glucose, Cl, bicarbonate, water, amino acids, and vitamins.

Kidney secretion

It is very important that these processes occur to maintain proper electrolyte balances in the body. Na+ plasma control is important in regulation of blood volume and blood pressure. K+ plasma control maintains proper function of cardiac and skeletal muscles. Electrolytes help in homeostastis of the body.

It is important as a nurse to recognize signs and symptoms of electrolyte imbalances. Hypernatremia, Hyponatremia, Hyperkalemia, and hypokalemia can be a result of an imbalance. Also, a nurse will take care of kidney dialysis patients or work in kidney dialysis. This is where blood is removed from the body via a surgical port and filtered through a "machine kidney" to remove wastes and clean the blood. These peope need dialysis because of kidney failure. Without this important procedure prognosis would be poor.


Fox, Stuart, Ira. Human Physiology, Tenth Edition