Control of Cardiac Output and System responses Flashcards
what determines…
The arterial pressure
The venous pressure
Arterial: CO X TPR
Venous: rate blood enters veins and leaves, blood volume
If Cardiac Output doesn’t change, and
a) TPR falls
b TPR rises
what happens to arterial and venous pressure?
a) arterial pressure falls, venous pressure rises
b) arterial pressure rises, venous pressure falls
If TPR doesn’t change, and
a) CO falls
b CO rises
what happens to arterial and venous pressure?
TPR and arterial pressure are proportional:
a) Arterial pressure falls, venous pressure
b) arterial pressure rises, venous pressure falls
When does Cardiac output increase?
- A rise in venous pressure: increases SV through starling’s law and HR
- A fall in arterial pressure: Causes an increase in sympathetic activity: HR and FOC increase which also, in turn, increases the SV
What defines the stroke volume
difference between the end of diastolic volume and end of the systolic volume (the upstroke)
What is the ventricular compliance curve?
Why is it important to have a long diastole? How is this related to venous pressure?
Relationship between ventricle volume and venous pressure
A longer diastole means there’s more time for perfusion to reach tissues and more venous pressure means the heart fills better in diastole (more ventricular volume)
Explain how starling’s law of the heart is related to venous pressure
Name one other factor that aids contractions
The more the heart fills (with venous blood) the harder it contracts, stronger contractions increase the stroke volume
Muscle fibre length is proportional to the energy in a contraction
Name 2 things that assist starling’s law
- Increasing central venous pressure: venoconstriction increases pressure and drives venous blood up to the heart
- Lowering BP: activates the sympathetic system
What two things determine the end-systolic volume
End-systolic volume: how much the ventricles emptied
1. The strength of the contraction: contractility (sympathetic activity) and end-diastolic volume (how much ventricles were filled)
- How difficult it is to eject blood: aortic impedance
Depends on the TPR, which is proportional to arterial pressure. A low TPR means low arterial pressure, therefore it is easier to eject blood and SV rises and end-systolic volume falls
Where are rises in venous pressure sensed?
Explain the Bainbridge reflex
Sensed in the R atria,
BB reflex: senses an increase in central venous pressure and signals a reduction in parasympathetic activity and an increase in HR
Explain how the CVS is restabilized after a meal
- Post-meal: gut releases vasodilatory metabolites
- This lowers the TPR and arterial pressure, venous pressure rises
- Baroreceptors in the carotid sinus and aortic arch signal to the medullary oblongata to increase the HR and CO
- a rise in venous pressure signals the Bainbridge reflect to increase HR
- Once HR has increased, arterial pressure is re-raised and venous pressure falls
Back to normal!
What happens in this scenario: HR is the only thing that increases, all other factors stay the same.
What can you conclude from this?
If only the HR increases, it makes no difference to the system
1. Initially CO rises but TPR stays the same
2. CO rising reduces the venous pressure
3. SV falls (due to starling’s law: heart filling up less)
4. CO falls back to its initial value
Therefore, HR is driven by circulation (not the other way around)
What does exercise do to arterial pressure?
Using muscles pushes extra blood back to the heart; decreasing arterial pressure. BUT TPR lowers to compensate, and keeps the arterial BP raised
What is a pulmonary consequence of having venous pressure that is too high?
The amount of blood flow going through the R ventricle is so high that it can back up in the lungs and cause a pulmonary edema
What does the brain do at the beginning of exercise to ensure that venous pressure doesn’t become too high?
Increases HR
Explain the difference between static and dynamic exercise
How does this affect the gym life of certain patients?
Static: Weights raise BP
Dynamic: Cardio increases the HR (which also increases CO) but the mean BP doesn’t rise as much, muscular vasculature eventually vasodilates so diastolic BP decreases over time
You should discourage static exercise in patients with ischemic heart disease; such as angina and hypertension, as a higher BP will increase vascular resistance and put more strain on the L ventricle
What happens to venous pressure in postural hypotension?
Gravity allows blood to ‘pool’ in the superficial veins so central venous pressure falls, this means the SV falls (starling’s) and CO falls. This decreases the arterial BP
Baroreceptors sense the fall in arterial BP and increase the HR
This lowers venous pressure further as:
- More blood is being pumped through circulation (spending less time in veins)
- And increasing HR will bring back the arterial BP and TPR which also lowers venous pressure
Explain how a hemorrhage effects venous pressure
Blood escapes from a ruptured blood vessel, lowering blood volume and thus venous pressure
- Starling’s: CO falls
- Arterial pressure and TPR falls
- Baroreceptors sense a low arterial pressure and increase HR: lowering venous pressure further
Name 2 things you can do to increase the venous pressure
- Veno constriction
2. Auto-transfusion: the person receives own blood for transfusion
What is the most important long term regulator of blood pressure, and which organ is responsible for controlling it?
Blood volume, controlled by the kidney
Name 4 roles of starling’s law
- Increases SV during upright exercise
- Lowers CO during: orthostasis (standing, and hemorrhage and shock
- Balances outputs of RV and LV
- restores CO in response to IV infusions
Name 3 things you can do to help a fall in central venous pressure (one is a type of drug)
- Bed rest (no gravity)
- Warmth (vasodilation)
- alpha-adrenoceptor blocker (inhibit vasoconstriction)
What is the function of the coronary sinus?
Collection of veins that collect blood from the myocardium and deliver less-oxygenated blood to the R atria
How does digoxin help with heart failure?
Slows the HR down to improve filling of the ventricles
