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Introduction to the Circulatory System
What does the circulatory system do?

img_circ Our circulatory system can be thought of as a network of living pipes that allows our body to transport oxygen as well as nutrients to the tissues. Chemical wastes produced by our cells enter the blood to be filtered and eliminated by our body (largely by the kidney and liver). Our body circulates about 5 liters (5.3 quarts) of blood; at rest, the heart moves it at a rate of 5 liters per minute. In summary, even without activity, blood can run the entire circuit of the body in one minute. During heavy exercise, the whole loop can be completed in ten seconds. The main components of the circulation are pictured on the left:

  1. Arteries – the red vessels represent arteries. Hemoglobin, the oxygen carrying protein in red blood cells, takes on blood’s distinctive red color when it is combined with oxygen. Arteries carry blood away from the heart and branch into smaller vessels (first arterioles, then capillaries) in order to facilitate delivery of its cargo to the organs and other tissues.
  2. Veins – the blue vessels represent veins. This bluish tint is also due to hemoglobin. After releasing oxygen to living cells, hemoglobin undergoes a chemical change that gives it a distinctive reddish-blue tone. Veins also have a series of one-way valves built into their structure. This is crucial for proper circulation, as gravity could cause blood to pool below the heart otherwise!

The body’s circulation is a “closed loop”. This necessarily means that an irregularity or back-up in one part of the system will affect blood flow elsewhere in the loop. If the structures in the path seem unfamiliar, click here for a graphical review.


How does the circulatory system work?

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. For more information on high blood pressure, also known as hypertension, click here.