Humans have circulatory systems designed much like water delivery systems. A pump—the heart—circulates fluid through a series of pipes, or blood vessels. The purpose of this system is to maintain the cells. In a multicellular organism, not only is it important for each cell to be provided with the molecules it needs to sustain itself, cells must also be able to communicate with one another. The circulatory system provides for both these needs.
Function
The heart is at the center, functionally speaking, of the circulatory system. It’s responsible for sending oxygen-rich blood to cells so that they have the oxygen they need to utilize nutrients. Dr. Gary Thibodeau, in his book “Anatomy and Physiology,” explains that in addition to providing oxygen, circulating blood also picks up nutrients absorbed by the intestines and distributes them to cells. Finally, blood leaving the cells carries away the waste product carbon dioxide and sends it to the lungs to be exhaled.
Features
Because the circulatory system is responsible for carrying both oxygen-rich blood meant for the cells and oxygen-poor blood that is full of carbon dioxide waste, it’s important that the system keep these two types of blood separate. To this end, notes Thibodeau, the human heart has two sides. The left side sends blood to the cells, and as the blood returns, it enters the right side of the heart, which sends it to the lungs for fresh oxygen. Blood returning to the heart from the lungs enters the left side, and so blood sent out to the cells is always fully saturated with oxygen.
Considerations
Because blood returning from the cells is comingled, sent to the lungs, then returned to systemic circulation, human circulation allows for an excellent cellular communication opportunity. An organ needing to communicate with a distant organ can simply secrete a molecule recognized by the target organ into the bloodstream, and odds are that within a heartbeat or two, the target organ will begin receiving some of the communication molecules. Dr. Lauralee Sherwood notes in her text, “Human Physiology,” that one example of this is the mechanism through which the cells of the pancreas communicate with body cells. When blood sugar is too high, the pancreas releases the hormone insulin, which is distributed through the bloodstream to all body cells. Body cells respond to insulin by taking up glucose, which reduces blood sugar. This communication would not be possible without circulation.
Expert Insight
The circulatory system, notes Sherwood, is mechanical in nature, and as such is susceptible to blockages. Some components of your diet, including saturated fat and cholesterol, can form hard plaques in the arteries that receive blood from the heart, blocking the passage of blood and hardening the arteries. This can result in arterial bleeding and blood clots, which can cause heart attacks or strokes.
Misconceptions
One interesting feature of the circulatory system in humans is that while it's common to think that pregnant women share circulation with their fetuses, they in fact maintain separate systems, and each depends on her own heartbeats and blood vessels to circulate blood. In their book "What To Expect When You're Expecting," Heidi Murkoff and Sharon Mazel point out that fetuses receive oxygen and nutrients by absorbing these substances from their mothers' blood vessels into vessels of their own in close proximity, but that the blood of mother and fetus never mix.
References
- “Anatomy and Physiology”; Dr. Gary Thibodeau; 2007
- “Human Physiology”; Dr. Lauralee Sherwood; 2004
- “What to Expect When You’re Expecting”; Heidi Murkoff and Sharon Mazel; 2008
