Biology 42 – Human Biology (Spring 2007)
Roy the Blood Cell
[Students and professors, please read.]
Assignment: Describe the path of a Red Blood Cell through the human heart and circulatory system. Go into detail when describing the blood vessels.
The path of a Red Blood Cell (named Roy) through the human heart and circulatory system, with emphasis on the blood vessels.
Roy was born in bone marrow, eventually making his way into the blood stream and through to the heart — what he likes to call “home base”. He likes to pretend that he is a professional tourist, paying his fare by way of hauling oxygen to various locations in the body, and hauling carbon dioxide to the lungs for expiration. It is our pleasure to tag along with Roy while he makes his rounds.
First, the most interesting thing to note at the outset of this journey, is that Roy must go through the lungs to get from the right side of the heart to the left side of the heart, so the heart is rather like a train station. Roy always enters the right atrium from the superior vena cava (if coming from the head, chest, or arms) or the inferior vena cava (if returning from the lower body regions), which pump oxygen poor blood cells like Roy into the heart from a return trip from the body. He always knows when he’s getting close to the heart from that familiar “lub dup” that gets louder and louder as he approaches. He becomes part of what makes that noise as he is pumped through the heart. Roy always passes through the atrioventricular valve (the tricuspid valve), down to the right ventricle and on through the pulmonary semilunar valve into the pulmonary trunk, which divides into two pulmonary arteries, which go to the lungs. He loves going into the lungs, because by that time he can really use the oxygen, and is ready to dump that heavy carbon dioxide load. Once he’s shouldered his oxygen, he’s pumped back to the heart, by way of one of four pulmonary veins into the left atrium, through the atrioventricular (bicuspid, or mitral) valve down to the left ventricle, and in a great, roller-coaster rush is whooshed through the semilunar valve into the aorta (the vein with the largest diameter and thickest walls) to somewhere in the body. He has noted that never once in his life has he ever entered the right side of the heart without needing oxygen, and never once in his life has he ever entered the left side of the heart without having first acquired that much-needed oxygen from his side-trip to one lung or the other (that side-trip he refers to as “the pulmonary circuit”). He has also noted that in the pulmonary circuit, two pulmonary arteries carry oxygen-poor blood to the lungs, and four pulmonary veins carry oxygen-rich blood to the heart – whereas in the rest of the body, arteries carry oxygen-rich blood away from the heart, and veins (with valves preventing Roy from getting all turned around) carry oxygen poor blood to the heart. Though it may seem he has a repetitive job, Roy has yet to see the entire body, and here’s why…
Once he zooms out into the aorta through the left ventricle, fully loaded with oxygen for whoever may need it, he’s begun what he calls “the systemic circuit”. He has observed a pattern in his career as professional tourist: left ventricle—aorta—big artery—branch artery—capillaries—branch vein—big vein—inferior or superior vena cava—right atrium. When he goes to a lower leg, for example, from the aorta he goes to the common iliac artery, to the femoral artery, to the lower leg capillaries, to the femoral vein, to the common iliac vein, to the inferior vena cava, to the right atrium, ending that particular tour of the systemic circuit (he never does get to see the whole body on one tour – it’s like touring the Hearst Castle, only there are many more possible tours, and he doesn’t get to choose which one he takes) and beginning again the pulmonary circuit.