Week 3: Cardiac Output and Haemodynamics of blood flow Flashcards
What is the definition of cardiac output? how can we calculate this?
The volume of blood pumped by each ventricle per minute
CO = HR x SV
What is the cardiac reserve?
the difference between CO at rest and Maximum Co
What is the parasympathetic NS influence on the heart? how does it do this? what nerve does it travel through?
The parasympathetic nerves travel as the vagus nerve to the heart primarily supplying the atrium (SA and AV nodes) with a few fibers passing to the ventricles.
- Main influence is to decrease HR by having acetylcholine increase the cells permeability to pottasium causing a reduction in the rate that the cells in the heart depolerise.
- Result of this: Hyperpolerises the membrane causing it to be further from threshold, this results in a long “drift” time between action potentials.
What is the sympathetic NS influence on the heart? how does it do this? what nerve does it travel through?
Sympathetic nerves travel to the haeart as cardiac nerves and supply the SA and AV node in the atrium along with the ventricular muscle fibres.
- The overall effect is to speed up the heart by increasing the rate of depolerisation so that the AP threshold is reached more rapidly in the AR cells.
- This is done by having noradrenaline released to accelerate the “deactivation of potassium channels” which will lessen the extent of hyperpolerisation along with a decrease of calcium influx and therefor allow for a higher frequency of AP’s effectively increasing HR.
- In contractile cells primarilly in the ventricles, the sympathetic NS results in an increase in permeability to Calcium resulting in an increase in contractile strength therfore allowing the pump to be more efficient.
How are the Parasympathetic and Sympathetic Nervous systems regulated? Which division is active during resting conditions?
The cardiovascular control centres (CCC) in the brainstem are coordinated as they are antagonistic divisions and only one can be active at a given time.
During resting conditions the parasympathetic division dominates causing a decrease in HR.
What are some extrinsic factors that can influence HR?
- Adrenaline released from the adrenal medulla increases HR
- HR is also sensitive to changes in temperature, plasma concentrations of certain ion and other hormones.
What are the 3 variables that determine the amount of blood that is ejected? explain them.
(intrinsic) Preload: The extent of ventricular filling; increase to EDV and increase is SV = increase in Preload. (the more muscle tissue that is stretched = the harder it must contract)
(Extrinsic) contractility: the strength of ventricular contraction; SNS and adrenaline both enable the heart to contract more forcefully and to squeeze out a greater percentage of the blood it contains.
Afterload: The aortic or pulmonary pressure that the heart must work against to eject blood suring systole. The semilunar valves are held shut but the volume of blood (afterload) “sitting” in the cusps. This pressure is about 120mmHg. Therefore pressure generated by ventricles must be >120mmHg.
What is stroke volume and how do we calculate it?
SV = Volume of blood ejected by the ventricle during each heartbeat
Calculated by: SV = EDV -ESV
What is Frank-Starling’s law of the heart?
The more muscle tissue that is stretched, the harder it contracts;
- The greater the EDV of blood in the ventricle, the greater the force of contraction which is required to eject the blood.
- This relationship is necessary because if venous return is increased through action of the SNS, the EDV will increase and therefore the SV will need to increase correspondingly.
How does sympathetic activity increase SV in relation to Preload and contractility?
As HR increases -> time availible for diastolic filling decreases -> sympathetic NS causes; quicker contraction and relaxation, increase in venous return, increase in EDV, Increase in contractility (leads to more complete ejection of the end diastolic ventricular volume).
This results in an increase to SV
What is the velocity of blood flow? how can we calculate it?
Rate of displacement of blood per unit of time based on a cross-sectional area of a vessel.
Velovity (cm/sec) = Volume flow (ml/sec) / cross sectional area (cm2)
(make sure V for velocity and Q for volume flow have a dot on the top of them)
What is the relationship between velocity of blood flow and cross sectional area?
There is an inverse relationship between the velocity of blood flow and cross sectional area.
- As blood vessel diameter increases, the velocity of blood flow through the vessel decreases.
True or False: The blood flow through a vessel decreases as the lumen of the vessel decreases.
Explain
Fasle:
The circulatory system is a closed circut so the volume of blood flow through all vessels is the same. The only things that can change is the velocity, and cross sectional area.
What are the two types of blood flow that can occur through an individual vessel?
Laminar: smooth/regular
Turbulent: Irregular “noisy”
What can cause turbulent flow? What is its tendancy directly proportionate to? what can clinically be heard from this?
- When rate of flow becomes too great
- When it passes an obsutruction in a vessel
- When it makes a sharp turn
The tendancy for turbulent flow increases directly in proportion to the velocity of blood flow.
Turbulent blood flow causes audible virbrations called murmurs.
Explain what is happening here?
Total blood flow is 4L/min and one of the vessels has constricted causing the other vessels to dilate proportionately to compensate; resulting in an unchanged total blood flow overall.
