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Structure of Heart
The human heart is four-chambered consisting of two atria and two ventricles. The heart is pear shaped and about the size of a fist. The heart is located beneath the protective ribs on the left side. The heart is an involuntary muscle, meaning that unlike the leg or arm muscles, the cardiac muscle is not under voluntary control. Different animals may have different chambered hearts or none at all. Following is the general anatomy of the human heart.
Superior/Inferior Vena Cava
The Superior Vena Cava and the Inferior Vena Cava both bring deoxygenated blood to the heart. Generally, the Superior Vena Cava carries blood from the upper part of the body while the Inferior Vena Cava carries blood from the lower part of the body. Like all veins, the Superior and Inferior Vena Cava are made of smooth muscle. Unlike arteries which require their elasticity to withstand the pumping of the heart, veins are more inelastic. These two large major veins feed into the right atrium.
The blood collects here. As directed by the pacemaker (sinoatrial node), located in the upper wall of this atrium, the two atria (left and right) are simultaneously contracted. This allows the continuation of blood flow through the tricuspid valve between the right atrium and the right ventricle. This valve does nothing more than allow the blood to flow into the right ventricle without letting blood reenter the right atrium.