Question

Describe the processes of glomerular filtration and the factors that influence filtration.

Answer 1

urine formation occurs in 3 steps.

1. glomerular filtration
2. reabsorption
3. Secretion

the filtration is carried out by the glomerulus. which is known as to glomerular filtration.The glomerular capillary blood pressure causes the filtration of the blood. through 3 layers

1. endothelium of glomerular blood vessels
2. .epithelium of bowman's capsule
3. the basement membrane between these 2 layers

the epithelium of bowman's capsule called podocytes are arranged in s single layer manner so as to leave a space in between known as slit pores. when the blood flows through the capillary network of bowman's capsule, it gets filtered through the walls of the capillaries, through which the water soluble small molecular wt substances such as glucose and amino acids and nitrogenous waste are filtered from the blood. Blood cells and proteins are not filtered out from the blood. This filtration is called ultra filtration. On an average 1100- 2100 millilitre of blood is filtered through the kidneys per minute..

Blood enters the afferent arteriole through the bowman's capsule and passes through the tuft of capillaries called glomerulus. The diameter of efferent arteriole which coming out of the glomerulus is less than the afferent arteriole; thus creating a high pressure inside the glomerulus to ultra filtration to occur. Hydrostatic and osmotic pressure in the capillary makes the plasma and solutes to be filtered from the blood into the capsular space of nephron through the filtration membrane.The filtration membrane allows only small particles to pass through it, this include water glucose amino acids inorganic contents and various metabolic waste products like urea,Creatinin etc. This fluid is called glomerular filtrate.
The amount of filtrate produced per minute is called glomerular filtration rate.
The efferent arteriole after exiting from the glomerulus forms a network of capillaries around the renal tubule called peritubular capillaries.

Filterability of a substance depends upon the size and charge of the substance.
-Small substances have a higher filterability and large substances have a lower filterability.
-positively charged ions have a higher filterability and negatively charged ions have a lower filterability

The components of glomerular filtrate
-No formed elements are seen such as red blood cells white blood cells and platelets
-No plasma proteins.
-contains inorganic ions such as Na+,Cl-,Mg+,Ca+,HCO-,Phosphate,K+,H+
-contains organic compounds such as glucose amino acids fatty acids and glycerol
-vitamins
-excretory products such as urea uric acid creatinine and bilirubin

• GFR (Glomerular Filtration Rate)

-glomerular filtration rate is defined as the amount of plasma filtered by all glomerulus of both the kidneys. it is measured in the unit millilitre per minute.
-the normal renal blood flow is 1200 to 1300 millilitre per minute.
-the normal GFR is 125 ml per minute.

Factors affecting GFR
1. NET FILTRATION PRESSURE (NFP)
-it is the balance between colloid osmotic force and hydrostatic force in the glomerulus and bowman's capsule.
-glomerular capillary hydrostatic pressure is the force that facilitate the glomerular filtration which is about 55 mmHg.(PBC)
-Bowman's capsular hydrostatic pressure is the force that opposes the filtration which is about 15 mmHg.(XGC)
-glomerular capillary colloid osmotic pressure is the force which tends to retain the fluid in the glomerulus and opposes the filtration process is about 30 mmHg.(PBC)
-as the glomerular filtrate does not contain proteins XBC is 0 mmHg
NFP = PGC-PBC-XGC = 10 mmHg.
= 55 mmHg - 15 mmHg - 30 mmHg = 10 mmHg.

2. FILTRATION COEFFICIENT OR kf
kf depends on 2 factors:-

1. Surface area &
2. Permeability

kf is the product of surface area and permeability of the filtration membrane.
GFR = kf X NFP
kf = GFR÷NFP
= 125 ml/min ÷10 mmHg = 12.5 ml/min/mmHg.

* When the surface area increases kf increases and accordingly GFR increases.
* When the permeability increases kf increases and GFR increases

3. RENAL BLOOD FLOW
When renal blood flow increases; the pressure inside glomerulus increases, and accordingly GFR increases.

4. DIAMETER OF GLOMERULAR BLOOD VESSELS

5. SYMPATHETIC STIMULATION
Sympathetic stimulation causes constriction of afferent arteriole and renal blood flow decreases and GFR decreases.

6. DISEASE CONDITIONS
In Hydronephrosis, urine outflow is obstructed. So the bowman's capsular hydrostatic pressure increases, and GFR reduces.
In diabetes mellitus, glycated protein accumulates in the glomerulus and the surface area of the filtration membrane decreases. Accordingly GFR decreases.

7. HORMONAL EFFECTS
ADH, aldosterone and angiotensin can affect the glomerular filtration rate.