Urine Formation Processes

1. Urine Formation Processes

Glomerular filtration is a special case of the capillary fluid exchange process in which water and some solutes in the blood plasma pass from the capillaries of the glomerulus into the capsular space of the nephron through the pedicels of the podocytes.
It is a nonselective passive process and constitutes the first step of urine formation

This term reflects the fact that the glomerular filtration barrier is an extremely fine molecular sieve that allows the filtration of small molecules but restricts the passage of macromolecules (e.g., the plasma proteins).
The glomerular filtrate normally takes an extracellular route, through holes in the endothelial cell layer, the basement membrane, and the pores between adjacent podocytes.
The glomerular capillaries are relatively impermeable to proteins, so that the filtered fluid (glomerular filtrate) is essentially protein-free and devoid of cellular elements, including red blood cells.
Everything else is filtered into the nephron
As a result, glomerular filtrate is a plasma like fluid (an ultrafiltrate of plasma) in terms of salts and organic molecules concentration.
Exceptions to this generalization include a few low-molecular-weight substances, such as calcium and fatty acids, that are not freely filtered because they are partially bound to the plasma proteins.
The fluid that enters the capsular space is called the glomerular filtrate.
- On average, the daily volume of glomerular filtrate in adults is 150 liters in females and 180 liters in males.
- More than 99% of the glomerular filtrate returns to the bloodstream via tubular reabsorption, so only 1–2 liters are excreted as urine.
The fraction of blood plasma in the afferent arterioles of the kidneys that becomes glomerular filtrate is the filtration fraction.