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The Cardiovascular System

The cardiovascular system delivers blood, nutrients, ions, gases, and heat throughout your body. That is its job, a transportation system. If you are running, it moves oxygen (O2) down to your legs, and moves lactic acid, that chemical that gives your muscles that burning sensation when worked hard, out of your legs. It moves heat from your body core to your toes, fingers, and head on a cold day. And it also distributes nutrients from your meals throughout your body, feeding your bones, nerves, organs, and tissues in your body.

The pump the drives the cardiovascular system is your heart. The hearts muscles, the among most powerful in your body, drives blood down your trunk, your legs, in and out of your arms, throughout every organ in your body. But your heart does not drive blood throughout your body in a circle, but in a more of a figure-8 pattern.

The heart pumps blood along two pathways in your body, the pulmonary circuit and the systemic circuit. In the pulmonary circuit, the heart pumps blood to the lungs, then back to the heart. From this pathway, blood picks up oxygen and drops off carbon dioxide in the lungs. Put simply, blood is oxygenated in the lungs. In the systemic circuit, the longer of the two, the heart pumps oxygenated blood through the rest of your body. As the blood passes through organ after organ, tissue after tissue, blood drops off oxygen, and picks up carbon dioxide.

How can the heart keep the oxygenated blood separated from the deoxygenated blood (blood without oxygen)? The heart has four chambers that work in pairs. The chambers are the right atrium, the right ventricle, the left atrium, and the left ventricle. One pair of chambers, the right atrium and right ventricle, is directly connected to the pulmonary circuit. In this circuit, deoxygenated blood from the body enters the heart through the right atrium. That chamber then pushes the deoxygenated blood on to the right ventricle, which then pumps that blood to the lungs. After the blood has passed through the lungs, it is oxygenated, full of oxygen.

Now the heart must pump the oxygenated blood to your organs, tissues, and bones. The blood from the lungs reenters the heart through the left atrium. The left atrium them pushes blood into the left ventricle, which in turn pumps the oxygenated blood throughout your body. Notice that blood always enters the heart through an atrium, and always leaves the heart through a ventricle. Figure 1 shows the heart's chambers.

Figure 1

A group of four values controls the flow of blood within the heart. These valves prevent blood moving in the wrong direction through the heart. A pair of valves called the tricuspid and bicuspid controls the blood flowing from the atrium into the ventricle. Blood in the left atrium passes through the tricuspid valve to enter the left ventricle, while blood in the right atrium passes through the bicuspid valve to enter the right ventricle. A pair of valves called semilunar valves controls the blood flow leaving the heart. Blood moving from the heart to the lungs passes through the pulmonary semilunar valve as the blood leaves the left ventricle. The word pulmonary refers to the lung. Oxygenated blood leaves the heart through the aortic semilunar valve, leaving the right ventricle and going to the body. Figure 2 shows the heart's valves.

Figure 2

Following figure 3, you will see that blood flows from the left atrium, to the left ventricle, then out of the heart to the lungs. From the lungs, blood flows back to the heart, entering the right atrium, then flows into the right ventricle, and out of the heart to the body.

Figure 3

Such is the flow of blood through the heart, but what is its path through our bodies? Blood always flows away from the heart through arteries. Also, blood always returns to the heart through veins. The path to and from the lungs is simple (Figure 4). Blood leaves the heart through the pulmonary trunk, which immediately splits into the left and right pulmonary arteries. The pulmonary trunk is one of two large arteries which leaves the heart. Afterwards, the now oxygenated blood returns to the heart through the left and right pulmonary veins, entering the right atrium. The right ventricle then gives a might squeeze, pushing the blood out of the heart, and through the aorta, the largest artery leaving the heart. Blood going through the aorta travels through many branches of arteries running throughout the body. As the blood flows through smaller and smaller areas, the arteries divide into arterioles, then eventually capillaries. It is through the small walls of the capillaries that heat, gases, and nutrients are exchanged between the cardiovascular system and the organs and tissues. Depleted of oxygen, the blood returns to the heart through many branches of veins, which eventually merge into the superior vena cava and inferior vena cava.

Figure 4

How can the heart move blood throughout our bodies by itself? The arteries are the largest vessels in our bodies. They have a muscular wall that helps the heart move blood through our bodies. Although slightly thiner, the veins also have a muscular wall that helps return blood to the heart. The muscles of the veins are smaller and weaker than the arteries, but the veins have small valves throughout the body, valves that prevents blood from backing up our cardiovascular system. As I said earlier, gases, nutrients and heat moves from the cardiovascular system to our organs and tissues, and back into the cardiovascular system through the capillaries. Capillaries are nothing more than a thin-walled tube, called endothelium, with no muscles (see pg 878).

When the heart muscles squeeze, it exerts a pressure on the blood, which pushes the blood out of the heart and through the body. That pressure, called blood pressure is measured with a sphygmomanometer, which gives a reading of two numbers: systolic and diastolic pressure. The systolic reading indicates the maximum pressure exerted by the blood on the arterial walls; this high point occurs when the left ventricle of the heart contracts, forcing blood through the arteries. Diastolic pressure is a measure of the lowest pressure on the blood vessel walls and happens when the left ventricle relaxes and refills with blood. Healthy blood pressure is considered to be around 120 millimeters of mercury systolic, 80 millimeters of mercury diastolic (usually presented as 120/80).

Many people can experience temporary increases in blood pressure, particularly under stressful conditions. When blood pressure is consistently above 140/90, however, physicians diagnose hypertension. The disorder can generally be managed with the help of special diets, exercise regimens and medication.

One of the main jobs of the cardiovascular system is to deliver oxygen (O2) to the body. As our bodies work, it uses up oxygen, replacing it with carbon dioxide (CO2). The exchange of oxygen and carbon dioxide occurs in our lungs. The blood that reaches our lungs has a lot more CO2 than O2. But the air in our lungs normally has a lot more O2 than CO2. Because of this large difference, blood naturally drops of CO2 in our lungs, and picks up O2. How does this happen? It happens because the pressure of O2 in our capillaries as it travels through our lungs is lower than the pressure of O2 in our lungs. Since high pressure gases want to travel into areas of low pressure, O2 naturally moves from our lungs into our blood. The reverese is true for CO2. The pressure of CO2 in our capillaries is higher than the pressure of CO2 in our lungs, so CO2 naturally travels from our blood and into our lungs.

This gas exchange occures in sacs called alveoli. The lung is made of two large sacks, which is divided and folded into much smaller pouches and sacs. Each sac is connected to a tube called bronchi. The bronchi are connected to our mouth through another tube called the trachea. As we breath, air enters our nose and mouth, travels down the trachea, and into the lungs through the bronchi. Just like our arteries, the bronchi further divide into smaller tubes called bronchioles. The bronchioles end in the alveoli. Alveoli are surrounded by cappilaries, so the exchange of gases occur through very thin wall vessels.

Air enters our lungs through pressure changes. When we inhale, the muscles on our ribs and our diaphram contract, expanding our chest. When our chest expands, the air pressure in our lungs drop, pulling air into our lungs. When we relax our muscles, elastic cartilage pulls our ribs in, pushing the air out of our lungs.

Figure 5