Question

Describe how filtrate is formed and what substances normally end up in filtrate (and what substances do not). This should include a description of the anatomy and physiology of the glomerus and the glomerular capsule and how they interact with each other.

Answer 1

When blood passes through glomerular capillaries, the plasma is filtered into the Bowman capsule. This process is called glomerular filtration.

GLOMERULAR FILTRATION INTRODUCTION
Glomerular filtration is the process by which the blood is filtered while passing through the glomerular capillaries by filtration membrane. It is the first process of urine formation. The structure of filtration membrane is well suited for filtration.
Filtration Membrane
Filtration membrane is formed by three layers: 1. Glomerular capillary membrane
2. Basement membrane
3. Visceral layer of Bowman capsule.
1. Glomerular capillary membrane
Glomerular capillary membrane is formed by single layer of endothelial cells, which are attached to the basement membrane. The capillary membrane has many pores called fenestrae or filtration pores with a diameter of 0.1 μ.
2. Basement membrane
Basement membrane of glomerular capillaries and the basement membrane of visceral layer of Bowman capsule fuse together. The fused basement membrane separates the endothelium of glomerular capillary and the epithelium of visceral layer of Bowman capsule.
3. Visceral layer of Bowman capsule
This layer is formed by a single layer of flattened epi­ thelial cells resting on a basement membrane. Each cell is connected with the basement membrane by cytoplasmic extensions called pedicles or feet. Epithelial cells with pedicles are called podocytes . Pedicles interdigitate leaving small cleft­like spaces in between. The cleft­like space is called slit pore or filtration slit. Filtration takes place through these slit pores.
Process of Glomerular Filtration
When blood passes through glomerular capillaries, the plasma is filtered into the Bowman capsule. All the substances of plasma are filtered except the plasma proteins. The filtered fluid is called glomerular filtrate.
Ultrafiltration
Glomerular filtration is called ultrafiltration because even the minute particles are filtered. But, the plasma proteins are not filtered due to their large molecular size. The protein molecules are larger than the slit pores present in the endothelium of capillaries. Thus, the glomerular filtrate contains all the substances present in plasma except the plasma proteins.
METHOD OF COLLECTION OF GLOMERULAR FILTRATE
Glomerular filtrate is collected in experimental animals by micropuncture technique. This technique involves insertion of a micropipette into the Bowman capsule and aspiration of filtrate.
GLOMERULAR FILTRATION RATE
Glomerular filtration rate (GFR) is defined as the total quantity of filtrate formed in all the nephrons of both the kidneys in the given unit of time.
Normal GFR is 125 mL/minute or about 180 L/day.
FILTRATION FRACTION
Filtration fraction is the fraction (portion) of the renal plasma, which becomes the filtrate. It is the ratio between renal plasma flow and glomerular filtration rate. It is expressed in percentage.