The Human Heart
The heart starts to beat practically at conception and continues it’s life-sustaining role for a hundred years or longer. It’s size and mass are proportional to each person, our hearts being roughly the size of our clenched fist. This durable muscle powers the circulatory system that pumps blood into the lungs, exchanging CO2 for oxygen.
The heart is positioned in front of the T5-T8 vertebrae of the spinal cord behind the sternum, and it’s monitored by the parts of the brain stem that control the involuntary workings of the respiratory, digestive, and lymphatic systems.
As the heart is pounding away and keeping us healthy its protected by the sternum and rib cage and encased within the pericardial sac. The outside of the double-layered pericardium is a tough membrane attached to the inner wall of the chest cavity to prevent the heart from shifting around. The inner pericardium is connected to the heart by a thin layer of connective tissue and fat. In between is the pericardial fluid that forms a cushion to lubricate the heart and prevent infection.
Despite it’s vital function, the structure of the heart isn’t complicated … a right and left atria, a right and left ventricle, four valves, and the surrounding arteries and veins of the circulatory system
A Single Heart Beat
De-oxygenated blood enters the right atrium in the upper right side of the heart through the vena cava
An electrical impulse is caused by an interchange of sodium and calcium ions on the cells of the sinoatrial node, causing the right atrium to contract
Blood flows through the tricuspid valve and down into the right ventricle
The electrical impulse travels to the atrioventricular node and along a pathway that branches off to the right and left ventricles, stimulating them to contract
As the right ventricle contracts, the pulmonary valve opens and the de-oxygenated blood is pushed into the lungs through the pulmonary artery
Carbon dioxide diffuses into the lungs to be exhaled during respiration. Oxygen is absorbed into the hemoglobin of red blood cells, and the oxygen-rich blood flows through the pulmonary veins into the left atrium in the upper left side of the heart
When the left atrium contracts, blood flows down through the mitral (or bicuspid) valve into the left ventricle
And finally, when the muscular left ventricle contracts, the aortic valve opens and blood leaves the heart through the aorta, completing one heartbeat