Ultrafiltration (renal)
Encyclopedia
In biological terms, ultrafiltration occurs at the barrier between the blood and the filtrate in the renal corpuscle
or Bowman's capsule
in the kidney
s. The Bowman's capsule contains a dense capillary network called the glomerulus. Blood flows into these capillaries
through a wide afferent arteriole
and leaves through a narrower efferent arteriole. The blood pressure inside these capillaries is high because:
The high pressure forces small molecules such as water
, glucose
, amino acids, sodium chloride
and urea
through the filter, from the blood in the glomerular capsule across the basement membrane of the Bowman's capsule and into the nephron
. This type of high pressure filtration is ultrafiltration. The fluid formed in this way is called glomerular filtrate.
Glomerular pressure is about 75 millimeters of mercury
(10 kPa). It is opposed by osmotic pressure(30 mmHg, 4.0 kPa) and hydrostatic pressure (20 mmHg, 2.7 kPa) of solutes present in capsular space. This difference in pressure is called effective pressure(25 mmHg)(3.3 kPa).
It is also used in hemodialysis
to clean whole blood
while keeping its composition intact.
determine their permeability
-selectivity (permselectivity). For instance, small ions such as sodium
and potassium
pass freely, while larger plasma proteins, such as hemoglobin
and albumin
have practically no permeability at all.
(CRRT) generally 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 based upon a prescription. Typically, no more than 2 liters an hour of fluid is removed. The remaining blood is returned to the patient. Unlike hemodialysis
, hemofiltration
and hemodiafiltration, no dialysate or replacement fluids are used in SCUF.
Renal corpuscle
In the kidney, a renal corpuscle is the initial blood-filtering component of a nephron. It consists of two structures: a glomerulus and a Bowman's capsule. The glomerulus is a small tuft of capillaries containing two cell types. Endothelial cells, which have large fenestrae, are not covered by...
or Bowman's capsule
Bowman's capsule
The Bowman's capsule is a cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form urine. A glomerulus is enclosed in the sac...
in the kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
s. The Bowman's capsule contains a dense capillary network called the glomerulus. Blood flows into these capillaries
Capillary
Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...
through a wide afferent arteriole
Arteriole
An arteriole is a small diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries.Arterioles have muscular walls and are the primary site of vascular resistance...
and leaves through a narrower efferent arteriole. The blood pressure inside these capillaries is high because:
- The renal arteryRenal arteryThe renal arteries normally arise off the side of the abdominal aorta, immediately below the superior mesenteric artery, and supply the kidneys with blood. Each is directed across the crus of the diaphragm, so as to form nearly a right angle with the aorta....
contains blood at very very high pressure which enters the glomerulus via the short afferent arteriole. - The efferent arteriole has a smaller diameter than the afferent arteriole.
The high pressure forces small molecules such as water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
, amino acids, sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
and urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
through the filter, from the blood in the glomerular capsule across the basement membrane of the Bowman's capsule and into the nephron
Nephron
The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system, which is not part of the nephron....
. This type of high pressure filtration is ultrafiltration. The fluid formed in this way is called glomerular filtrate.
Glomerular pressure is about 75 millimeters of mercury
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...
(10 kPa). It is opposed by osmotic pressure(30 mmHg, 4.0 kPa) and hydrostatic pressure (20 mmHg, 2.7 kPa) of solutes present in capsular space. This difference in pressure is called effective pressure(25 mmHg)(3.3 kPa).
It is also used in hemodialysis
Hemodialysis
In medicine, hemodialysis is a method for removing waste products such as creatinine and urea, as well as free water from the blood when the kidneys are in renal failure. Hemodialysis is one of three renal replacement therapies .Hemodialysis can be an outpatient or inpatient therapy...
to clean whole blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
while keeping its composition intact.
Selectivity
The structures of the layers of the glomerulusGlomerulus
A glomerulus is a capillary tuft that is involved in the first step of filtering blood to form urine.A glomerulus is surrounded by Bowman's capsule, the beginning component of nephrons in the vertebrate kidney. A glomerulus receives its blood supply from an afferent arteriole of the renal...
determine their permeability
Vascular permeability
Vascular permeability, often in the form of capillary permeability, characterizes the capacity of a blood vessel wall to allow for the flow of small molecules or even whole cells in and out of the vessel. Blood vessel walls are lined by a single layer of endothelial cells...
-selectivity (permselectivity). For instance, small ions such as sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
and potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
pass freely, while larger plasma proteins, such as hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
and albumin
Albumin
Albumin refers generally to any protein that is water soluble, which is moderately soluble in concentrated salt solutions, and experiences heat denaturation. They are commonly found in blood plasma, and are unique to other blood proteins in that they are not glycosylated...
have practically no permeability at all.
Slow Continuous Ultrafiltration
Slow Continuous Ultrafiltration (SCUF) is an artificial method which approximately mimics the ultrafiltration function of the kidneys. SCUF is a continuous renal replacement therapyRenal replacement therapy
Renal replacement therapy is a term used to encompass life-supporting treatments for renal failure.It includes:*hemodialysis,*peritoneal dialysis,*hemofiltration and*renal transplantation.These treatments will not cure chronic kidney disease...
(CRRT) generally used to remove fluid from fluid overloaded patients suffering acute renal failure
Renal failure
Renal failure or kidney failure describes a medical condition in which the kidneys fail to adequately filter toxins and waste products from the blood...
. 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 based upon a prescription. Typically, no more than 2 liters an hour of fluid is removed. The remaining blood is returned to the patient. Unlike hemodialysis
Hemodialysis
In medicine, hemodialysis is a method for removing waste products such as creatinine and urea, as well as free water from the blood when the kidneys are in renal failure. Hemodialysis is one of three renal replacement therapies .Hemodialysis can be an outpatient or inpatient therapy...
, hemofiltration
Hemofiltration
In medicine, hemofiltration, also haemofiltration, is a renal replacement therapy similar to hemodialysis which is used almost exclusively in the intensive care setting...
and hemodiafiltration, no dialysate or replacement fluids are used in SCUF.