CVS 20 - Integration of CVS response in haemorrhage

Question 1

Why does a man on admission for haemorrhage have low BP?

Answer 1

Haemorrhage -> loss of blood volume -> less venous return to heart so reduced SV so reduced CO so reduced MABP
(MABP = CO x TPR)

Question 2

Why is the patient’s HR elevated?

Answer 2

Compensation mechanism to increase BP

Question 3

How is BP maintained in haemorrhage?

Answer 3

Increased sympathetic activity on heart -> increase HR + force of contractility
Increased sympathetic tone to vessels -> vasoconstriction -> increased TPR

Question 4

How does increased sympathetic activity increase heart’s force of contraction?

Answer 4

Beta1 Receptors Respond to Noradrenaline via G-protein coupled Mechanism
Increased cAMP then PKA which phosphorylates calcium channel proteins- they open for a longer period of time so more Ca flows in so increased force of contraction

Question 5

How does increased sympathetic activity to vessels cause vasoconstriction?

Answer 5

NA acts on alpha receptors in smooth muscle

Makes muscle contract

Question 6

How is blood flow affected in the skin? skeletal muscles? gut? kidneys?

Answer 6

All reduced

Question 7

How does the kidney respond to haemorrhage?

Answer 7

Reduced flow to the kidney is indicated by a low renal artery pressure due to blood redistribution. This increases Renin secretion which converts Angiotensinogen to Angiotensin 1. ACE converts Angiotensin 1 into Angiotensin 2. AT 2 vasoconstricts blood vessels, and increases thirst. AT2 also promotes Aldosterone secretion from adrenal cortex which leads to sodium and water reabsorption

Low venous return also reduces left atrial pressure –> ADH is secreted from the posterior pituitary. ADH increases the kidney tubular permeability to water. This increases Water reabsorption. Combined with Sodium retention this leads to a low urine output.

All these mechachisms preserve blood volume and are supplemented with volume replacement

Question 8

Why does urea increase 48hrs after haemorrhage?

Answer 8

This is due to the low urine output caused by the compensatory mechanisms to preserve blood volume (ADH secretion, kidneys Na retention.

Question 9

What types of solution can be used for volume replacement?

Answer 9

Crystalloid
Colloid
Blood products (if no response to 2L of crystalloid)