Kidneys form urine, which passes to bladder for storage prior to excretion.
Steps involved in the process of urine formation:
- Glomerular filtration or ultra-filtration
- Selective reabsorption
- Tubular secretion
This takes place through the semipermeable walls of the glomerulus and glomerular capsule.
Urine formation begins with ﬁltration of large amounts of ﬂuid through capillaries of glomerulus into Bowman’s capsule. Blood enters afferent arteriole and flows into glomerulus. Blood in the glomerulus has both filterable blood components and non-filterable blood components. Filterable blood components move toward the inside of the glomerulus while non-filterable blood components bypass filtration process by exiting through efferent arteriole. Filterable Blood components then take on plasma like form called glomerular filtrate.
Composition of the Glomerular Filtrate
The filtrate in the glomerulus is very similar in composition to plasma with the important exceptions of plasma proteins and blood cells.
The concentrations of other constituents of glomerular ﬁltrate, including most salts and organic molecules, are similar to the concentrations in plasma except a few low-molecular-weight substances, such as calcium and fatty acids that are not freely ﬁltered because they are partially bound to plasma proteins.
Almost one half of plasma calcium and most of the plasma fatty acids are bound to proteins, and these bound portions are not ﬁltered through glomerular capillaries.
Blood constituents in glomerular filtrate includes the following:
- Mineral salts
- Amino acids
- Keto acids
- Creatinine, Urea, Uric acid
- Some hormones and Some drugs (small molecules)
Blood constituents remaining in glomerular capillaries after ultrafiltration includes Leukocytes, Erythrocytes, Platelets, Plasma proteins and some drugs (large molecules).
The diameter of efferent arterioles is narrower than afferent arterioles. Due to this difference in diameter of arteries, blood leaving the glomerulus creates pressure known as hydrostatic pressure. The glomerular hydrostatic pressure forces blood to leave glomerulus resulting in filtration of blood.
A capillary hydrostatic pressure of about 7.3 kPa (55 mmHg) builds up in glomerulus. However, this pressure is opposed by osmotic pressure of blood, provided mainly by plasma proteins, about 4 kPa (30 mmHg), and by filtrate hydrostatic pressure of about 2 kPa (15 mmHg in glomerular capsule).
Therefore the net filtration pressure is,
55-(30 +15) = 10mmHg.
By net filtration pressure of 10mmHg, blood is filtered in glomerular capsule.
The volume of filtrate formed by both kidneys each minute is called the glomerular filtration rate (GFR). In a healthy adult GFR is about 125 mL/min, i.e. 180 liters of filtrate are formed each day by two kidneys.
It is a mechanism that protects renal blood flow therefore, glomerular filtration. Renal blood flow is maintained at a constant pressure across a wide range of systolic blood pressures (from around 80–200 mmHg).
It operates independently of nervous control and is a property inherent in renal blood vessels; it may be stimulated by changes in blood pressure in the renal arteries or by fluctuating levels of certain metabolites, e.g. prostaglandins.
In severe shock, systolic blood pressure falls below 80 mmHg, autoregulation fails and renal blood flow and hydrostatic pressure decrease, impairing filtration within glomeruli.