The heart sends blood into the arteries not steadily but intermittently; each beat forces in some blood, and then comes a pause before the next beat. Accordingly the flow in the larger arteries is not even and continuous, but jerky, as indicated by the pulse.

But in the capillaries the flow is quite steady, and yet the capillaries are supplied by the smaller arteries. We have to inquire how this is brought about.

The disappearance of the pulse is due to two things, (1) the fact that in the tiny capillaries the blood meets with considerable resistance to its flow, dependent on friction, and (2) that the arteries are very clastic.

On account of friction in the capillaries the arteries have difficulty in passing on blood through them; blood therefore accumulates in the aorta and its large branches and stretches their elastic walls. The stretched arteries press all the time on the blood inside them, and constantly keep squeezing it on into the small arteries and the capillaries; both while the heart is contracting and between two heartbeats. The heart, in fact, keeps the big elastic arteries over-distended with blood; before they have had time to nearly empty, another systole occurs and fills them up tight again; so all the while the walls of the arteries are stretched and keep pressing on the blood inside them, and steadily forcing it on into the capillaries. The heart keeps the arteries over-full, and the stretched elastic arteries drive the blood through the capillaries. As the arteries are always stretched and always pressing on the blood the capillaries receive a steady supply, and the flow through them is uniform. This even capillary flow passes on a steady blood stream to the veins.*

How in number? Is the total blood channel greater in arteries or capillaries? In veins or capillaries?

Why is the blood-flow in the great arteries not steady? Name vessels in which it is steady.

To what is the loss of pulse in the capillaries due? What results from friction in the capillaries? What is done by the stretched arteries?

The object of having no pulse in the capillaries is to diminish the danger of their rupture. As we have seen, materials from the blood have to ooze through their walls to nourish the organs of the body, and wastes from the organs to soak back into the blood that they may be carried off. Their walls have therefore to be very thin; and if the blood were sent into them in sudden jets at each beat of the heart, they would run much risk of being torn.

When? What does the heart do? What happens before the arteries have had time to empty? What is the condition of the arterial walls all through life? What results from their stretched condition? What keeps the arteries tightly filled? What sends blood through the capillaries? How do the capillaries get a steady blood supply? Why do we find a uniform current in the veins?

What is gained by having no pulse in the capillaries? What must food materials in the blood do before they can nourish the body? What must the wastes of the organs do? Why must the capillary walls be very thin?

* " Every inch of the arterial system may, in fact, be considered as converting a small fraction of the heart's jerk into a steady pressure, and when all these fractions are summed up together in the total length of the arterial system no trace of the jerk is left. As the effect of each systole becomes diminished in the smaller vessels by the causes above mentioned, that of this constant pressure becomes more obvious, and gives rise to a steady passage of the fluid from the arteries towards the veins. In this way, in fact, the arteries perform the same functions as the air-reservoir of a fire-engine, which converts the jerking impulse given by the pumps into the steady flow of the delivery hose."— Huxley.