The renal parenchyma of human kidneys consists of two major parts, i.e., the outer renal cortex and the inner renal medulla. These parts comprise eight to 18 cone-shaped renal lobes, also known as renal pyramids, each containing a renal cortex surrounding the inside area of the renal medulla.

These capillaries emerge from afferent arterioles and form a network of glomerular capillaries (glomerular capillary tufts). This network of capillaries is surrounded by a specialized type of endothelial cells called Podocytes. They are called podocytes because they have foot processes in them.

They are connected with a double layer of cells, which is visceral epithelial cells and parietal epithelial cells.

These clusters of tiny blood vessels are supported through the mesangium, which is composed of intraglomerular mesangial cells and connective tissues.

READ MORE: What is Kidney Atrophy?

Between the endothelial cells of capillaries and epithelial cells of Bowman’s capsule, there is a basement membrane of connective tissues.

The glomerular capillary tuft has specialized endothelial cells with numerous pores called fenestrae. These perforations have a size of 70 to 100 nm per fenestration. These perforations are responsible for the filtration of fluid, blood plasma solutes, and protein; at the same time, they also prevent high molecular size molecules like red blood cells, white blood cells, and platelets from passing through them.

The glomerular basement membrane has a central dense area called Lamina Densa, while both sides of Lamina densa have a less dense area called Lamina Rara. It mainly consists of lamina type IV collagen.

BLOOD SUPPLY ENTERS THE GLOMERULUS:

The blood supply enters each kidney through the renal artery. The Glomerulus receives its blood supply through the Afferent Arteriole. It drains it into the Efferent Arteriole other than the venule, which makes it a specialized high-pressure system that facilitates ultrafiltration.

HOW GLOMERULUS FILTER THE BLOOD? (ULTRAFILTRATION):

· As the blood enters the renal artery into the Glomerulus, it enters the Afferent arteriole. It reaches into the glomerular capillary network (the usual capillary network has both arteriole and venule systems, which causes a decline in hydrostatic pressure of the blood that passes through it). Still, as we discussed earlier, Glomerulus has a specialized capillary network in which both sides of the capillaries have an arteriole system.

· High hydrostatic pressure in the capillaries facilitates the movement of molecules from the blood into Bowman’s capsule.

· When these molecules start to drain from the capillaries into the glomerular capsule, they pass through three consecutive barriers to reach the final filtrate.

· From glomerular capillaries, various types of molecules, including essential plasma proteins like albumin and globulin, except red blood cells, start to move through the endothelial cells of capillaries and reach the basement membrane.

· It is the 2nd barrier that is being crossed by the molecules. This layer is highly -ve charged, and due to the presence of relatively small size perforations compared to the endothelium of capillaries, this layer is impermeable for all types of plasma protein due to their large molecular size and -ve charge. Other small molecules easily pass from it.

· The semi-filtrate now reaches the final layer, which is a layer of epithelium along with podocytes (foot processes).there are small diaphragms in between the gaps of the cell, which give more restricted entry of molecules into the final filtrate.

· After passing through all these above layers, the substance is said to glomerular filtrate, reach into the Bowman’s capsule and move further towards the renal tubule.

· An efferent arteriole is responsible for the drainage of blood present in the Glomerulus to the renal vein. Constriction in the Efferent arteriole, as blood leaves through the Glomerulus, increases resistance in blood flow, which counters a pressure drop that will not be gained if blood flows through venules.

· The Efferent arteriole is narrower in diameter than the Afferent arteriole, which is relatively more comprehensive in diameter; this change in diameter between both arterioles will help regulate the alteration of blood pressure in the Glomerulus.

· The renal blood flow rate is about 1 litre/min (about 20% of the cardiac output). Also, it contributes to maintaining the glomerular filtration process at its optimum capacity and the whole process of urine formation in the kidneys.

GLOMERULAR FILTRATION RATE:

It is defined as the flow of plasma from the Glomerulus to Bowman’s capsule in a unit of time. It is a diagnostic test that describes the efficiency of kidneys. It is represented through Starling’s Equation.

                         

· GFR is the glomerular filtration rate

· Kf is the filtration coefficient—a proportionality constant

· Pgc is the glomerular capillary hydrostatic pressure

· Pbc is the Bowman’s capsule hydrostatic pressure

· πgc is the glomerular capillary oncotic pressure

· πbc is the Bowman’s capsule oncotic pressure

GLOMERULAR FILTRATION RATE RANGE:

The usual range of glomerular filtration rate in healthy adults is about 90 to 120 ml/min.

FACTORS AFFECTING FILTRATION AT GLOMERULUS:

Charge of the molecule:

The filtration rate is highly dependent upon the molecule’s charge as negatively charged molecules are highly repellent by the filtration membranes, i.e. (endothelial, basement, epithelial membrane). These membranes have highly negatively charged glycoproteins, so filtration membranes become impermeable to -very charged molecules, for example, albumin, globulins, and fibrinogen. So there is a total absence of plasma proteins in the filtrate.

 Size of the molecule: