In speaking of blood-pressure, arterial blood-pressure is always meant, unless otherwise stated.

As the blood-pressure depends on the difference between the quantity pumped into the arterial system by the heart at one end, and the quantity flowing out through the arterioles into the veins at the other in a given time, it is evident that The blood-pressure will remain constant when these quantities remain equal to each other.

It will rise when (a) More blood is pumped in by the heart.

(b) When less flows out through the arterioles in a given time. It will fall (a) When less is pumped in by the heart; or,

(b) More flows out through the arterioles; or, to look at it another way: -

Heart

more active.

Blood-pressure rises.

less ,,

,, ,, falls.

Arterioles

contract

,, ,, rises.

dilate

,, ,, falls.

The heart may throw more blood into the arteries, either by pulsating more rapidly, or by pulsating more vigorously and more completely, so that at each contraction a larger amount of blood is expelled. But increased activity can only affect the blood-pressure so long as there is a free supply of blood entering the heart. If there exist any obstruction to its entrance the increased cardiac action will have no effect. Hence obstruction of the pulmonary circulation will also lower the blood-pressure.

The causes of alteration in the blood-pressure may be tabulated as follows: Blood-Pressure

May be raised 1. By the heart beating more quickly.

2. By the heart beating more vigorously and more completely, and sending more blood into the aorta at each beat.

3. By contraction of the arterioles, retaining the blood in the arterial system.

May be lowered 1. By the heart beating more slowly.

2. By the heart beating less vigorously and completely, and sending less blood into the aorta at each beat.

3. By dilatation of the arterioles, allowing the blood to flow more quickly into the veins.

4. By deficient supply of blood to the left ventricle, as from contraction of the pulmonary vessels, or obstruction to the passage of blood through them, or from stagnation of blood in the large veins, e.g., in shock.

The influences on the pressure exerted by (a) the number of beats, and (b) by the amount of blood sent out by the heart at each beat, to a certain extent, though by no means completely, counteract each other; for, when the heart is beating quickly, it has not time to fill completely, and so sends out little blood at each beat: but, when beating slowly, it becomes quite full during each diastole, and sends out a larger quantity of blood at each contraction.

It is evident that the amount of blood which the heart can send into the arteries at each beat will depend also upon the completeness with which the ventricle relaxes during diastole. If the relaxation be incomplete very little blood will enter the ventricle, and thus a drug which increases the contractile power of the heart may, by unnecessarily prolonging the systole, lower the blood-pressure as much as a drug which paralyses the heart and prevents the ventricle from expelling its contents.