Ultrafiltration occurs at the barrier between the blood and the filtrate in the renal corpuscle or Bowman’s capsule in the kidneys.
The Bowman’s capsule contains a dense capillary network, the glomerulus, through which blood flows through the afferent arteriole and leaves through the efferent arteriole.
The blood pressure in the afferent arteriole is higher than the blood pressure in the efferent arteriole, due to its smaller diameter.
Ultrafiltration occurs as a result of high hydrostatic pressure small molecules such as water, glucose, amino acids, sodium chloride and urea are forced through the filter, from the blood in the glomerular capsule across the basement membrane of the Bowman’s capsule and into the nephron.
The ultra filtered fluid is called glomerular filtrate.
Glomerular pressure is about 75 millimeters of mercury.
The glomerular pressure is opposed by osmotic pressure of 30 mmHg, and hydrostatic pressure of 20 mmHg of solutes present in capsular space.
This difference in pressure is called effective pressure and is 25 mmHg.
The layers of the glomerulus determine their permeability-selectivity, so that small ions such as sodium and potassium pass freely, while larger plasma proteins, such as hemoglobin and albumin have practically no permeability.
Negatively charged molecules will pass through far less frequently than positively charged ones.
Slow continuous ultrafiltration (SCUF) is an artificial method which mimics the ultrafiltration function of the kidneys.
SCUF is a continuous renal replacement therapy (CRRT) used to remove fluid from fluid overloaded patients suffering acute renal failure.
During SCUF blood is removed from the body and is passed through an extracorporeal circuit through a hemofilter and a predetermined percentage of plasma water is removed.
Up to 2 liters an hour of fluid is removed, with the remaining blood returned to the patient.
No dialysate or replacement fluids are used in SCUF.