Contraction of the smooth muscleWatch
I did a practice question the other day for homework which was describe how the contraction of the smooth muscle layer in the arteriole walls helps to assist in the circulation of blood.
I put about how the contraction of the smooth muscle layer causes the constriction of the diameter of the arteriole which increases resistance to flow and reduces the flow of blood to ensure blood pressure isn't too high when flowing through the capillaries. However, when we marked it - the teacher said that the lumen diameter is smaller when the smooth muscle contracts which increases pressure and flow rate, which allows blood to flow but surely you don't want the pressure to be too high? I'm just slightly confused as I thought pressure would decrease
Let me walk you through this difficulty (btw, I won't hold your hand, because even guys 3 times my size scream when I shake their hand, and I don't want to hurt you! ).
OK, I am trying to think of an analogy that might make this easier to understand - yes, got it!
Let us take an example of a rubber balloon that is inflated - if you fit its mouth on say, a water pipe [of about 2-3 cm diameter] and squeeze the balloon, it will be easier to squeeze THAN if you fit its mouth to say, a metal straw [probs does not exist, but imagine one = 0.5 cm diameter [similar to your plastic Mac straw you use to suck chocolate milkshake e.g. - yummy! ]] - would you agree that the resistance to flow of air in the second case (narrower straw) would be greater = greater pressure, yes, but air flow would be reduced, so it will take longer to empty the balloon in the latter case (narrower straw)? Yes?
It's like a bottleneck effect with a narrow tube compared to a wider one.
Ok so far so good. NOW DO NOT CONFUSE BLOOD PRESSURE AND BLOOD FLOW! When the arteriolar diameter is smaller (vasoconstriction), the pressure is higher (as in the balloon analogy above), BUT BLOOD FLOW IS REDUCED.
BLOOD PRESSURE IS A FUNCTION OF CARDIAC OUTPUT (volume of blood pumped by the heart in one minute [approx, 5 litres in a healthy average adult man]) TIMES TOTAL PERIPHERAL RESISTANCE. So the higher the resistance (when the vessels are constricted, as you correctly state), the HIGHER the blood pressure.
Your teacher's Q asks about the effect of vessel diameter on CIRCULATION of blood:
a) organs that require more blood e.g. brain [or muscle during exercise] will experience a reduction of tone [=relaxation] in the circular muscle of the arteriole causing vasodilatation to increase blood flow. In muscle, this is achieved mainly by the stimulation of beta-2 adrenoceptors (receptors in the sympathetic division of the autonomic nervous system) by noradrenaline (NA) and adrenaline.
b) an important point to remember is that, following from the Frank-Starling Law (which states that the force of contraction of the heart is proportional to the diastolic venous return to the atria [measured as the end diastolic volume][effected by stretch receptors in the myocardium]), if there is too much vasodilatation, particularly of the veins, then pooling of blood in the periphery means that venous return to the heart is reduced, so cardiac output falls and blood pressure drops [look at the first parameter in the above equation [in bold]](confusingly, this will reduce blood flow to all organs).
I hope this helps!