Factors Affecting CO

Question 1

Define cardiac output

Answer 1

volume of blood ejected by one ventricle in 1 min (L/min)

Question 2

Define cardiac index

Answer 2

the CO, taking into account the size of the animal (L/min/m2)

Question 3

What are the factors that influence stroke volume?

Answer 3

• E of contraction - stretching myocardium in diastole increases end diastolic pressure and increases the energy of contraction
• Arterial pressure - increasing arterial pressure causes a decrease in SV - contractile energy used in increasing VBP during isovolumetric contraction so there is less energy for ejection

Question 4

What is Starlings law of the heart?

Answer 4

The energy of contraction of a cardiac muscle fibre is proportional to the initial fibre length at rest

Question 5

Define central venous pressure

Answer 5

P at the entrance to the RA, determines RV end diastolic P and resting distension. It depends on the total vol. of blood in circulation and the distribution of blood between peripheral and central veins.

Question 6

What are the factors affecting CVP and SV?

Answer 6

1. Lower blood vol. - 2/3 of blood is in venous circulation, haemorrhage or dehydration will cause a decrease in CVP and SV
2. Symp. nerves regulating peripheral venous tone - innervate skin, kidneys and splanchnic system, blood can be diverted from these organs to central veins
3. Venous muscle pump - rhythmic exercise compresses deep veins in the limbs and displaces blood centrally (increases CVP and SV)
4. Increased CO and decreased filling pressure - heart pumps out of venous system into arterial system. Increased pressure and volume in arterial system means a decreased volume and pressure in veins.
5. Respiration - during inspiration, intra-thoracic pressure becomes -ve and intra-abdominal pressure becomes +ve. Blood moves into thorax and fills vena cava, increases RV filling and SV. But decreases blood returen to LV, SV falls transiently. Counter balanced by sinus arrythmia.
6. Increased extramural pressure - forced expiration increases intra-thoracic pressure. Decreases ventricular transmural pressure, ventricular filling and CO

Question 7

Describe what is happening in each of the pressure-volume loops

Answer 7

Question 8

Define intrinsic regulation

Answer 8

change in contractile energy caused by a change in resting fibre length

Question 9

Define extrinsic regulation

Answer 9

Change in contractile energy by external chemical factors. Can change in contractility or inotropism.