Cardiac Output And Blood Pressure Flashcards
What is the role of the smooth muscle in arteries ?
It gives rise to compliance and resistance which in turn give rise to blood pressure
What is compliance ?
It is the change in volume with respect to the change in pressure in arteries
What does compliance do ?
Compliance alters the arteries to act as pressure reservoirs to maintain continuous flow during diastole when the heart is refilling
How much blood is in the left ventricle during systole ?
130 ml and this is known as end diastolic volume
What happens when the left ventricle contracts ?
The heart pumps 70 ml of blood into the aorta
From this 70 ml, 30 ml will get trapped in the pressure reservoirs due to the compliance of the vessel and so 40 ml will flow down the cardiovascular tree
What happens during diastole ?
There is elastic recoil of the aorta and the remaining blood is pushed into circulation providing pulsatile but continuous flow
What are the factors that the arterial pressure fluctuations in the cardiac cycle are dependant on ?
1) rate of ejection from the ventricles
2) distension of arterial walls
3) rate of distribution - varies based on the resistance in the vessels
Describe how distension of the arterial walls affects arterial pressure fluctuations in the cardiac cycle
When there is low compliance there are large pressure fluctuations due to stiff arterial walls
High compliance highlights greater elasticity leading to less pressure fluctuations
What is the systolic phase ?
It is the peak pressure during the cardiac cycle
Peak of ventricular ejection
80 - 120 mmHg
What is the diastolic phase ?
It is measured just before ventricular ejection
60-80 mmHg
What is pulse pressure ?
It is the difference between systolic and diastolic blood pressure
~ 40 mmHg
What is mean arterial pressure ?
- heart spends more time in diastole than systole
- cardiac cycle is 800 ms
- systole is 300 to 350 ms
- mean arterial pressure is a time weighted average
- approximately 93 mmHg
What is the dicrotic notch ?
It is associated with the closing of the aortic valve during the falling phase of arterial pressure
How does the lub-dub sound of the heart arise ?
- The closure of the valves at the end of ventricular systole creates a lub-dub sound
- The dub is the closure of the aortic valve
How is blood pressure measured ?
- blood pressure is measured using the Auscultatory method
- it requires the use of a sphygmomanometer cuff around the upper arm
- attached to the sphygmomanometer is an inflation bulb
- when the brachial artery is pushed down using the inflation bulb there will be a point when the brachial artery is completely blocked
- the stethoscope is placed on the antecubital fossa so the korotkoff sounds can be heard
What is the equation to work out blood pressure ?
Blood pressure = total peripheral resistance x cardiac output
What is resistance to blood flow dependent on ?
- vessel radius
- vessel length
- blood viscosity
Describe how vessel radius affects resistance
Vasodilation and vasoconstriction control resistance in arteries and arterioles
Vasodilation decreases resistance
Vasoconstriction increases resistance
Describe how vessel length affects resistance
The longer the vessel the greater the resistance
Describe how blood viscosity affects resistance
The greater the viscosity of the blood the greater the resistance
How does the diameter of the blood vessel change ?
- sympathetic nervous system
- metabolites e.g. ATP and lactate
- circulating hormones
How does the the sympathetic nervous system change the diameter of the blood vessel ?
Noradrenaline binds to adrenergic receptors causing vasoconstriction of the blood vessel
This increases the mean arterial pressure by increasing total peripheral resistance
Tonic vasoconstriction during rest maintains blood pressure by sympathetic innervation
How do metabolites change the diameter of the blood vessel ?
H+ , ATP and lactate usually cause vasodilation of the smooth muscle cells lining the arterioles
How do circulating hormones affect the diameter of blood vessels ?
Adrenaline activity could influence vasoconstriction and vasodilation
Alpha and beta adrenergic receptors are important in this process
What is cardiac output ?
It is the volume of blood ejected by the left ventricle per minute (5L/min)
- it varies depending on the body’s demand e.g. in exercise it can go up to 25L/min
- total peripheral resistance and cardiac output are important in determining blood pressure
What is the value of resting heart rate ?
60 - 70 bpm
What is stroke volume ?
Stroke volume = end diastolic volume - end systolic volume
~ 70 ml
What is venoconstriction ?
Increase in blood flow from the vena cava into the heart which increases the end diastolic volume therefore having an impact on cardiac output
What does Starlings law suggest ?
It states that the energy of contraction of the ventricles is a function of the initial lengths of the muscle fibres in the walls of the heart
Increased stretch gives rise to stronger contraction
What is the relationship between the length of the sarcomere and tension ?
As sarcomere length increases so does tension but this only happens until a certain point
Beyond this point actin myosin cross bridges can’t form and so there is a decrease in stroke volume and end diastolic volume
What are the factors that affect end diastolic volume ?
- internal factors : venous pressure - blood pressure in the vena cava is low and this is greatly influenced by gravity
- external factors : during inspiration, there is a fall in intrathoracic pressure which promotes filling of the right ventricle therefore impacting end diastolic volume
What is the relationship between end diastolic volume and stroke volume ?
A greater end diastolic volume = greater stroke volume and therefore increased contractility
