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Body Basics – Red Blood Cells, Hemoglobin, Oxygen and Carbon Dioxide

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What are red blood cells (RBCs)? 

Red blood cells have the responsibility of transporting oxygen throughout the body. About 33% of the weight of a red blood cell is attributed to a specialized protein called hemoglobin. Because of this, red blood cells are often referred to as “bags of hemoglobin.”

Hemoglobin is a large and complex protein that contains four atoms of iron and can bind to oxygen so that it can be transported in circulation. There are about 42 to 52 million red blood cells per cc of blood; and each red blood cell contains about 250 million hemoglobin molecules. Since each hemoglobin molecule can carry four oxygen molecules, the potential exists to transport one billion molecules of oxygen in each red blood cell.

There are two reasons for the need for such a large amount of hemoglobin in our blood. First, oxygen does not dissolve very well into our blood. Second, the demand for oxygen is extremely high in our body. Therefore, hemoglobin increases the ability of the blood to carry oxygen tremendously. Any situation that significantly decreases either the number of red blood cells or the level of hemoglobin they carry can compromise oxygen delivery to our tissues and potentially compromise function and health.


How do we bring oxygen into our body and get rid of carbon dioxide? 

When the heart pumps, blood is propelled from the right ventricle into the pulmonary arteries for transport to the lungs. Pulmonary means lungs. Upon reaching the lungs and the pulmonary capillaries, carbon dioxide exits the blood and enters into the airways of our lungs. It is then removed from our body when we exhale. At the same time, oxygen enters the blood from the airways of our lungs and binds with hemoglobin within red blood cells. The oxygen-containing blood leaves the lungs and travels back to the heart as part of circulation. Thus every breath you take serves to exchange gases, bringing needed oxygen into your body while removing carbon dioxide.


How does the heart supply blood throughout our body? 

As our heart contracts, blood is pumped from the left ventricle into the aorta. Blood moves from the aorta into the arteries, then arterioles, and finally tiny capillaries in our tissue. The blood leaving our left ventricle is rich with oxygen while the blood returning to our heart from tissue throughout our body has given up oxygen to working cells while acquiring carbon dioxide. This blood is then pumped by the right ventricle to the lungs to load the hemoglobin with oxygen and release carbon dioxide. 


What is cardiac output? 

If we were to measure the amount of blood pumped out of our heart during one heartbeat, whether it be from the left or right ventricle, we would know our stroke volume. Then, if we multiply the stroke volume by our heart rate (heartbeats per minute) we would know the cardiac output.


Cardiac Output = Stroke Volume (mL) × Heart Rate (beats/min) 


Cardiac output is the volume of blood pumped out of the heart, either to the lungs or toward body tissue, in one minute. It should not matter which of the two destinations we consider, as they occur simultaneously and will have a similar stroke volume of about 5 to 6 liters (or quarts) per minute. During exercise both heart rate and stroke volume increase, which consequently increases cardiac output. For some of us, cardiac output may increase as much as five to six times during heavy exercise. This allows for more oxygen-rich blood to be delivered to working skeletal muscle 

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