Factors Affecting Cardiac Output
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What is cardiac output?
The cardiac output (CO) is the volume of blood ejected by one ventricle in one minute (l/min)
What is the cardiac index?
The cardiac index is the cardiac volume taking into account the size of the animal
CO = ? x ?
CO = heart rate (HR) x stroke volume (SV)
What two opposing factors influence stroke volume?
Energy of contraction and arterial pressure
What effect does an a) high energy of contraction b) high arterial pressure have on SV?
- a) increases stroke volume
- b) decreases stroke volume
How can the energy of contraction be increased?
By stretching the myocardium in diastole, through a rise in end-diastolic pressure.
What influences end-diastolic pressure?
Preload (venous filling pressure)
How can the strength of contraction of the heart be increased?
Through sympathetic stimulation and circulating hormones e.g. adrenaline
Which of the following are intrinsic/extrinsic properties that affect stroke volume? Preload, circulating hormones, sympathetic innervation, after load?
- Intrinsic - preload, afterload
- Extrinsic - sympathetic stimulation, circulating hormones
A rise in CVP results in an increase/decreased in the output of both ventricles?
Summarise Starling's law of the heart
The energy of contraction of a cardiac muscle fibre, like that of a sarcomere, is proportional to the initial fibre length at rest
What is central venous pressure?
Central venous pressure (CVP) is the pressure at the entrance to the right atrium
List the six factors that affect CVP
- Lower blood volume
- Sympathetic nerves regulating venous tone
- Venous muscle pump
- Increased CO reducing filling pressure
- Respiration's effect on extramural cardiac pressure
- Increased extramural pressure impairs filling
Give examples of situations that would lower blood volume
Dehydration, haemorrhage, upright posture in humans
How does lower blood volume change CVP?
Lower blood volume decreases CVP, and consequently SV
How does an increase in sympathetic innervation to peripheral venous tone affect CVP?
This redirects blood from peripheral veins to central veins therefore increasing CVP
Describe how the venous muscle pump affects CVP
Rhythmic exercise repeatedly compresses the deep veins of the limbs and displaces their blood centrally due to venous valves. This raises CVP. The converse is true for standing for long periods of time in hot weather.
Why does increased CO decrease CVP?
Increasing CO increases the volume of blood in the arterial system (increases after load) and reduces the volume and pressure of blood in central veins (reduces preload)
Does inspiration make the intrathoracic pressure more positive or negative?
Making the intrathoracic pressure more negative promotes filling of what vessel? What effect does this have on SV?
It promotes filling of the thoracic vena cava. This promotes right ventricular filling and increases SV.
What counteracts this effect and results in a transient decreased left SV during inspiration?
Expansion of the lungs increasing the pulmonary blood pool, which reduces return to the left ventricle
What does the heart do during inspiration to counteract the transient decrease in SV?
It increases HR on inspiration - sinus arrhythmia
Give examples of things that cause the intrathoracic pressure to be raised to positive levels
Forced expiration or coughing
What is arterial pressure?
The pressure against which the heart has to eject blood
What is normal arterial pressure?
What will a chronic increase in arterial pressure result in?
Left ventricular hypertrophy.
What are the short term/long term effects of LV hypertrophy?
- Short term - will increase contractile force for a while helping the ventricle cope with the hypertension
- Long term - overload of the ventricle will often result in heart failure
What is the difference between intrinsic and extrinsic regulation of contractile energy of the heart?
Intrinsic regulation is the change in contractile energy caused by changes in resting fibre length. Whereas extrinsic regulation is the change in contractile energy caused by external chemical factors.
Is sympathetic or parasympathetic stimulation an example of extrinsic regulation?
Give examples of when sympathetic stimulation would be increased in the body
Exercise, standing up, stress and haemorrhage.
What two things does sympathetic stimulation increase?
Heart rate and force of contraction
Why is the rate of relaxation much faster in a sympathetically stimulated heart?
If the heart rate increased but the relaxation period stayed the same the diastolic period would decrease. It is during the diastolic period that the heart supplies itself with blood and oxygen. The heart therefore relaxes faster to preserve diastolic filling time.
Give examples of hormones that increase contractility of the heart
Adrenaline, nor-adrenaline (beta-agonists)
Give examples of things that decrease contractility of the heart
- Parasympathetic activity and cholinergic agonists
- Beta-blockers eg propanolol, oxprenolol and atenolol
- Calcium channel blockers e.g. verpamil
- Baribituates and many anaesthetics
On an ECG, which interval decreases after exercise?
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