Urine Formation
MECHANISM OF URINE FORMATION :
The mechanism of urine formation involves three steps or processes:
1. Ultrafiltration or Glomerular filtration
2. Selective tubular reabsorption
3. Tubular secretion
(I) ULTRAFILTRATION OR GLOMERULAR FILTRATION :
- The first step in urine formation is the filtration of blood, which is carried out by the glomerulus.
- This process occurs in the malpighian corpuscle of the nephron.
- The glomerular capsular membrane (Filtration membrane) through which filtration of blood occur consists of three layers.
a. The endothelium of glomerulus blood vessels.
b. The epithelium of Bowmans capsule.
c. A basement membrane between these two layers.
- The epithilial cells of Bowman capsule called podocytes are arranged in an intricate manner so as to leave some minute spaces called as filtration slits or slit pore.
- The blood is filtered so finely through these membranes that almost all the constituents of the plasma except the protiens pass on to the lumen of the Bowmans capsules. Therefore it is considered as a process of ultra filtration.
- The plasma fluid that filters out from glomerular capillaries is called glomerular filtration rate (GFR). GFR in a healthy individual is approximately 125 ml/min i.e. 180 lit per day.
- On an average 1100-1200 ml of blood is filtered by kidney per minutes (Renal blood flow) which constitute roughly 20-25% of the blood pumped by each ventricle of the heart in an minute (Cardiac output) and of this blood about 650ml is the blood plasma (55%). This 650 ml is called Renal Plasma flow (RPF). About 20% 0f the blood plasma filtered by all nephrons of both kidney in a minute. It is 125ml which called glomerular filtration rate (GFR).
- The effective filtration pressure that causes ultrafiltration is determined by three pressure :
1. Glomerular hydrostatic presuure
2. Colloid osmotic pressure of blood
3. Capsular hydrostatic pressure
The glomerular hydrostatic pressure is the blood pressure in glomerular capillaries. It is the chief determinant of effective filtration pressure, It is due to the difference in diameter of afferent and efferent arteriole ie. The main driving force to cause filtration. (it is 60-75mmHg).
The colloidal pressure is the osmotic pressure created in blood of glomerular capillaries due to plasma proteins. It resists the filtration of fluid from capillaries (it is 30-32mmHg)
The capsular hydrostatic pressure is the pressure caused by fluid (filtrate) that reaches into Bowman�s capsule and resist filtration. (it is about 10-18mmHg)
The mechanism of urine formation involves three steps or processes:
1. Ultrafiltration or Glomerular filtration
2. Selective tubular reabsorption
3. Tubular secretion
(I) ULTRAFILTRATION OR GLOMERULAR FILTRATION :
- The first step in urine formation is the filtration of blood, which is carried out by the glomerulus.
- This process occurs in the malpighian corpuscle of the nephron.
- The glomerular capsular membrane (Filtration membrane) through which filtration of blood occur consists of three layers.
a. The endothelium of glomerulus blood vessels.
b. The epithelium of Bowmans capsule.
c. A basement membrane between these two layers.
- The epithilial cells of Bowman capsule called podocytes are arranged in an intricate manner so as to leave some minute spaces called as filtration slits or slit pore.
- The blood is filtered so finely through these membranes that almost all the constituents of the plasma except the protiens pass on to the lumen of the Bowmans capsules. Therefore it is considered as a process of ultra filtration.
- The plasma fluid that filters out from glomerular capillaries is called glomerular filtration rate (GFR). GFR in a healthy individual is approximately 125 ml/min i.e. 180 lit per day.
- On an average 1100-1200 ml of blood is filtered by kidney per minutes (Renal blood flow) which constitute roughly 20-25% of the blood pumped by each ventricle of the heart in an minute (Cardiac output) and of this blood about 650ml is the blood plasma (55%). This 650 ml is called Renal Plasma flow (RPF). About 20% 0f the blood plasma filtered by all nephrons of both kidney in a minute. It is 125ml which called glomerular filtration rate (GFR).
- The effective filtration pressure that causes ultrafiltration is determined by three pressure :
1. Glomerular hydrostatic presuure
2. Colloid osmotic pressure of blood
3. Capsular hydrostatic pressure
The glomerular hydrostatic pressure is the blood pressure in glomerular capillaries. It is the chief determinant of effective filtration pressure, It is due to the difference in diameter of afferent and efferent arteriole ie. The main driving force to cause filtration. (it is 60-75mmHg).
The colloidal pressure is the osmotic pressure created in blood of glomerular capillaries due to plasma proteins. It resists the filtration of fluid from capillaries (it is 30-32mmHg)
The capsular hydrostatic pressure is the pressure caused by fluid (filtrate) that reaches into Bowman�s capsule and resist filtration. (it is about 10-18mmHg)
