The human body requires many things to survive, and one major requirement is oxygen. Every moment of every day, the human body feeds itself oxygen by breathing air. Breathing involves external respiration and internal respiration. The former is the act of bringing air into the lungs for gas exchange to occur within the lungs, and the latter is the exchange of gases within the cells of all body tissues. Both processes are essential for delivering oxygen to the brain and thus preventing tissue damage or death.
Air enters the body through the nose or mouth and travels through the nasal cavity to the pharynx. The air reaches a crossroad at the end of the pharynx where it must choose between the trachea or the esophagus. Air can only travel through the trachea, so this is where it goes.
The larynx, also known as the voice box, is between the trachea and pharynx. During speech, the vocal cords within the larynx vibrate in response to air passing through this area.
At this point, the air needs to enter the lungs, which will extract oxygen for the use of the body. This is made possible by the action of the diaphragm. The diaphragm is a muscle that contracts during inhalation, creating a vacuum within the thoracic cavity which draws air through the pharynx, trachea, bronchi, and lungs. Each contraction and relaxation of the diaphragm is responsible for inhalation and exhalation.
After the diaphragm contracts, the air travels along the bronchi; the bronchi consist of two main branches with each branch going into each lung. Within the lungs, the bronchi divide into smaller branches called bronchioles. At the end of each bronchiole are alveoli, or air-sacs. The exchange of oxygen and carbon dioxide occur between the walls of the alveoli. Once the necessary oxygen has been extracted from the main air supply, it leaves the walls of the alveoli to enter the surrounding capillaries which lead to pulmonary veins. The oxygen is now part of the bloodstream and ready for transport to the heart.
Oxygen is transported to the cells of the body by erythrocytes, also known as red blood cells. Red blood cells are the most abundant of cells in the blood. The hemoglobin portion of the red blood cells holds oxygen. The erythrocytes travel to the body cells through blood vessels such as veins, arteries and capillaries.
The heart receives oxygen from the lungs via pulmonary veins. The heart is the control center of the blood vessel network which is the life source for all tissue within the body. In other words, it is responsible for delivering oxygen to all body tissues and helping to remove waste gases via blood vessels. The heart itself is located in the mediastinum, which is the cavity between the lungs. This location allows for efficient exchange of gases between heart and lungs. The oxygenated blood passes through the pulmonary veins into the left atrium chamber of the heart, then the left ventricle chamber, and finally exiting the heart through ithe aorta. The aorta is the largest artery in the body and from here, the oxygen-rich blood enters arterial circulation.
The oxygen-rich blood travels through various arteries, into arterioles, then into capillaries. Oxygen and waste exchange occurs through the walls of the capillaries between the tissues and cells of the body. The red blood cells deliver oxygen and pick up gases such as carbon dioxide to carry back through the capillaries. From the capillaries, the deoxygenated blood enters venous circulation and travels back to the heart and lungs.
Ever wonder why nurses and phlebotomists only take blood samples from your veins and not your arteries? It's because the blood in your veins has not been recently pumped from the heart. As a result, veins don't pulsate in the way that arteries do. When taking a blood sample from an artery, there is an increased risk of blood spurting uncontrollably from the artery once punctured by the needle. It also takes longer for artery walls to heal compared to the walls of veins. For this reason, arterial blood samples are rarely taken. When they are, it is usually to check for abnormal gas percentages in the blood.
The red blood cells re-enter the heart via the superior and inferior vena cava, emptying into the right atrium chamber, then the right ventricle chamber. It exits through the pulmonary arteries to enter the air spaces of the lungs. Here, carbon dioxide and other waste gases are extracted from the blood to be exhaled from the body.
The entire process is called inhalation and exhalation. Each beat of the heart is one cardiac cycle.