Week 6 - Responses of the Whole System Flashcards
What effect does reduced TPR have on cardiac output?
Reduced TPR = reduced arterial pressure = reduced afterload = higher CO
What effect does increased venous pressure have on cardiac output?
Increase venous pressure = increased venous return = increased cardiac output
What happens when there is a rapid decrease in arterial pressure?
Increased flow resistance to the skin and gut
Veno-constriction - pushes blood back to heart
Discuss the CVS responses to eating a meal
Increased metabolic activity, so local vasodilatation. This means TPR is reduced, and therefore arterial pressure is also reduced.
Therefore cardiac output rises, which raises arterial pressure again.
Does increasing heart rate alone change cardiac output?
Why?
No.
If heart rate rises but TPR stays the same, venous pressure will fall.
This means that diastolic filling is reduced, and hence stroke volume falls.
If stroke volume falls to the same degree that heart rate rose, cardiac output remains the same.
Discuss the effect of exercise on arterial and venous pressures.
Vasodilatation in the muscles, as huge increase in need for blood flow.
As TPR falls, arterial pressure falls.
Fall in TPR, along with increased muscle pumping, means venous pressure rises.
Why does exercise lead to a risk of pulmonary oedema?
Increased venous pressure means the left ventricle is at risk of overfilling - it reaches the top of the Starling curve, and therefore stroke volume falls.
As stroke volume in the L ventricle has fallen, and rate is always the same, the outputs of the R and L ventricles cannot be matched.
Therefore blood pools in lungs - risk of oedema.
How is pulmonary oedema prevented during exercise?
The sympathetic nervous system triggers an increase in heart rate immediately before the onset of exercise.
Therefore, when venous pressure begins to rise, heart rate is already high, allowing cardiac output to rise whilst keeping stroke volume low.
Why does cardiac output fall in response to rising from lying to standing?
Gravity increases the transmural pressure in the superficial veins of the legs.
This results in blood pooling, which in turn results in a transient fall in central venous pressure, and hence also cardiac output.
When rising from lying to standing, cardiac output falls.
How is this corrected?
Reduced cardiac output means arterial pressure falls.
The baroreceptor reflex ensures that heart rate rises.
In addition:
- vasoconstriction in the skin and gut - i.e. TPR up, so arterial pressure up
- venoconstriction - blood pushed towards heart –> CO up
Why does arterial pressure fall when blood is lost?
Blood is mainly lost from the veins, so venous pressure falls.
This means that there is less diastolic filling, and hence cardiac output falls, meaning arterial pressure will fall too.
What is the sequence of events following a fall in arterial pressure during haemorrhage?
Vasoconstriction - raises TPR and hence arterial pressure, but this lowers venous pressure more.
Heart rate also increases - lowers venous pressure further.
Heart rate rises and rises, making the situation progressively worse.
How does the body try to restore venous pressure during haemorrhage?
Venoconstriction
Autotransfusion - fluid enters the blood vessels from the ECF in order to raise blood volume.
Both only work up to around 1 litre of blood loss.
Describe postural hypotension
Postural hypotension = low arterial bp when standing.
Occurs when venous pressure is already low (e.g. due to old age), and thus baroreceptor reflex doesn’t work.
How does a sustained increase in blood volume lead to hypertension (long-term raised arterial pressure)?
Increased blood volume leads to increased diastolic filling. This increases cardiac output, and hence raises arterial pressure.
More blood is forced through the tissues, so tissue “autoregulate” - vasoconstriction to restrict flow, i.e. TPR increases.
This means arterial pressure increases further –> hypertension.
