Starling's forces and physiological basis of edema

Starling's forces

1. Capillary hydrostatic pressure

2. Capillary oncotic pressure

3. Interstitial fluid hydrostatic pressure

4. Interstitial fluid oncotic pressure

Forces at arterial end of capillary

Starling's Forces promoting filtration: 

1. Hydrostatic capillary pressure: 32 mmHg 

2. Interstitial oncotic pressure: 8 mmHg (due to proteins)

3. Interstitial hydrostatic pressure: -3 mmHg (Some fluid is also present in interstitial space. However, due to pulling by the tissues, a negative hydrostatic pressure is generated in interstitial space. So, it promotes filtration)

Total =  43 mmHg (by adding all forces)

Forces opposing filtration

2. Oncotic capillary pressure: 28 mmHg

Net driving force at arterial end of capillaries = Forces promoting filtration - Forces oppsong filtration

                               =  43-28 = 15 mmHg

Since it is a positive force, it drives water movement out of the capillaries.

Starling's Forces at venous end of capillary: 

Starling's Forces promoting filtration: 

1. Hydrostatic capillary pressure: 10 mmHg   (hydrostatic pressure decreased due to the movement of water outside the capillaries at the arterial end

2. Interstitial oncotic pressure: 8 mmHg (due to proteins)

3. Interstitial hydrostatic pressure: -3 mmHg 

Total =  21 mmHg (by adding all forces)

Forces opposing filtration

1. Oncotic capillary pressure: 28 mmHg

Net driving force at arterial end of capillaries = Forces promoting filtration - Forces oppsong filtration

                               =  21-28 = -7 mmHg

Since it is a negative force that means it draws in water into the capillaries. So there is the inward pull of water at the venous end. 

That means water which is filtered at the arterial end is reabsorbed at the venous end. 

But at arterial end, the net outward force was 15 mmHg and at the venous end, the net inward force was 7 mmHg so some fluid remains in the interstitial space. Lymphatics carry that excess fluid from the interstitial space. 

Now when can fluid accumulate inside interstitial space? That extra fluid accumulation in interstitial space is known as EDEMA. 

 So when can oedema occur? 

1. Increased hydrostatic pressure occurs in the case of hypertension or excess fluid accumulation. ( heart failure or iatrogenic) 

2. Decreased oncotic pressure that is due to decreased protein concentration. 

It may happen due to:

Malnutrition: Decreased protein intake 

Malabsorption: Decreased protein absorption

Liver disease: Decreased synthesis of proteins

Kidney disease: Increased loss of proteins

3. Increased capillary permeability can occur in case of local injury.

4. Blocked lymphatics can occur in case of filariasis, metastasis of cancerous cells to lymph nodes 

Edema is of 2 types

1. Transudate: This means that it is the water that is being accumulated inside the interstitial space. 

2. Exudate: along with water, there is a movement of proteins also from the capillary.  So exudate is water plus proteins. It occurs when there is increased permeability of capillaries causing movement of proteins also out of the capillaries.