Fill in the blanks.
(a) In mammals, the ………… is divided into four parts.
(b) The heart ………… continuously.
(c) The vessels carrying blood from the heart to the different parts of the body are called ………… .
(d) The vessels carrying blood from the parts of the body to the heart are called ………… .
The upper chambers of the heart which pump blood into the ventricles are called ________ .