Blood Vessels and Circulation
The blood that is pumped out of the ventricles enters the two large blood vessels—the aorta and pulmonarytrunk (Figures 8.7 and 8.8). The blood vessels carryblood from the heart to the tissue and back to the heart. The blood flows through the vessels primarily because of the pumping of the heart. In the case of the blood returning from the body to the heart, the elastic recoil of the artery walls during diastole, compression of veins by skeletal muscles during contraction, and the negative pressure created in the thorax during inspira-tion help draw blood toward the heart (venous return).
The blood from the right ventricle flows to the lungs via the pulmonary trunk and this is known as pul-monary circulation. The blood from the left ventricleflows to the rest of the body via the aorta and this is known as systemic circulation. The systemic circula-tion is made up of numerous different circuits in par-allel (see Figure 8.16), which allows for wide variations in regional blood flow without changing the total sys-temic flow. For example, blood flow to the gastroin-testinal system alone can be increased at mealtime.