Frank Starling Laws

Question 1

Describe Frank Starling’s Law of the Heart

Answer 1

Frank Starling’s Law of the heart states that CO will match Venous return at the heart. I.e. reduced venous return to the heart results in reduced EDV, which results in reduced stroke volume. Thus, increased venous return leads to increased EDV and increased SV. Therefore, the conclusion is that lower EDVs result in lower contractility, and higher EDV result in higher contractility of the heart (more powerful contraction) because the heart must match in- and out-flow

This does not necessarily apply so simply during exercise. During exercise, because of how rapid the heartbeat is, there is not time for much blood to fill into the ventricle during diastole, resulting in a lower EDV. However, because of the SNS outflow during exercise, the contractility of the heart will still be higher than it would normally for that EDV.

Question 2

Describe Starling’s Capillary Equation

Answer 2

Net filtration across capillary membrane=

Pc - Pif) - (colloid osmotic pressure of C - colloid osmotic pressure of IF

Question 3

What does Frank Starling’s Equation have to do with oedema?

Answer 3

Things that favour oedema = things that favour increased net flow out of C into the IF.

Thus, if Capillary protein very low, fluid will flow out down it’s osmotic gradient. I.e. IF colloid osmotic pressure will exceed C colloid osmotic pressure)

etc.

Question 4

How much fluid does lymph drain? What is lymph?

Answer 4

Lymph = ultrafiltrate plasma + some waste products.

On average, normal lymphatics will return ~2-3 L of lymph back into venous circulation per day. = ~120mls per hour.

Lymph can increase this average by 4x, before it cannot cope with the amount of fluid. This is when oedema occurs. *