Contractions from the heart eject blood into the lungs and body. The vascular network must be vast in complexity and length to cover all of this volume. The combined length of all of the tubing would cover nearly 60,000 miles (100,000 kilometers) - more than TWICE around the earth - if plumbed in a straight line! The vascular system, with the help of neuroendocrine input, can contract and relax different arteries and veins to guide both the speed and destination of blood flow, depending on the needs of the body. This facilitates circulation in performing more specialized tasks.
During physical exertion, the vessels going to muscles can dilate to deliver more oxygen to those tissues. Furthermore, blood also serves as a method to distribute our body heat. By dilating blood vessels at our skin surface, the body allows conductive heat loss during exercise, while contracting their diameter conserves body heat when the environment becomes colder.
As mentioned before, the circulatory system is a closed loop. If there are any serious blockages or changes in vessel resistance along the line, it is like putting a bend or kink in a garden hose - flow through the bend is slowed and pressure to push through the reduced opening may increase. This resistance to flow is of concern in heart failure, not only because of the workload it imposes on the heart, but because it hampers delivery of blood's precious cargoes (food, oxygen, etc.). Placing the heart under such stressful conditions (i.e. increased heart rate and contraction force with a curtailed oxygen supply) will reduce its ability to compensate for the needs of the body.
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