Formation of Urine

 The processes involved in the formation of urine are as follows:

 • ultra filtration,

 • selective reabsorption, and

 • tubular secretion.

     As blood circulates through the glomerulus, ultra filtration occurs. Small molecules such ad water, urea, mineral salts, sugar and plasma solutes pass through the one cell thick walls of the capillaries and the Bowman capsule into the capsular space. Bigger molecules like plasma proteins and the blood cells cannot pass through this barrier which thus act as a filter. a high pressure in the glomerulus is essential for the filtration process. This is brought about in the following ways:

 • The blood entering the kidney is already at high pressure because the renal artery branches off from the dorsal aorta at only a short distance from the heart.

 • The blood vessel leaving each glomerulus is narrower than the one entering it, thereby increasing the pressure of the blood in the glomerulus further.

    The fluid that filters into the Bowman's capsule is known as glomerular filtrate. It flows down of the tubule. As it passes through the proximal part of the tubule and the Henle loop, selective reabsorption takes place. In this process, water and useful substances like sugar, amino acids and salts are reabsorbed into the surrounding blood capillaries. The filtrate then moves into the distal part of the tubule. Here, large waste molecules like creatinine are secreted into the tubules. If necessary, ions (hydrogen, potassium and hydrogen carbonate) are secreted into the tubules to keep the osmotic concentration of the blood constant. The fluid that eventually remains in the tubule is concentrated and is known as urine.

    An average of 1.5 litters of urine is produced daily. The amount of urea excreted will depend on the protein content of the daily diet. The filtered blood leaving the kidney by the renal vein contains

 • less oxygen and glucose, and mor carbon dioxide, as a result of cellular respiration; and less nitrogenous wastes, salts and water as a result of excretion.

    The urine formed trickles down the ureter and collects in the bladder. When the bladder is full, it contracts discharging the urine out of the body through the urethra.

Note How the kidney helps to maintain a constant internal environment is going to be discussed in my next topics.