The human body needs oxygen to support an active metabolism. The cardiovascular system includes the heart, the lungs, and the pathway of veins and arteries that transport oxygen and other vital nutrients around the body.
Our hearts beat approximately 50-90 times per minute, pumping blood into the lungs and then outward through miles and miles of arteries. It’s size and mass are proportional to each person, our hearts being roughly the size of our clenched fist. The heart and lungs work together to re-oxygenate the bloodstream and to clear waste. Red blood cells travel through the arteries to deliver oxygen to the cells of the body in exchange for carbon dioxide that flows through the veins and back to the heart and lungs to start the cycle again.
De-oxygenated blood flows into the right atrium, down through the tricuspid valve, and into the right ventricle where it’s pumped into the lungs. Carbon dioxide diffuses into the lungs from the bloodstream and oxygen is absorbed and stored in red blood cells. The re-oxygenated blood flows into the left atrium of the heart. From there it travels through the mitral valve and into the powerful left ventricle where its pumped outward into the systemic circulation.
Though we often find ourselves taking a deep breath to relax during a busy day, we aren’t usually thinking about the workings of our heart and lungs since the process of respiration is controlled by structures within the brainstem. Receptors in the brainstem monitor levels of CO2 and oxygen in the bloodstream, sending impulses through the autonomic nervous system to the diaphragm, the abdominal muscle that controls the airflow into and out of the lungs.
When we breathe in through the nose or mouth, air first passes through the pharynx and into the larynx where it flows past the vocal cords. The trachea, or windpipe, is an 8-10 cm (5-7 in) tube lined with disks of cartilage. The trachea ends at the bronchus which divides into the right and left bronchial tubes that stretch deep into the lungs. The airways become smaller and narrower, ending at the alveoli, the tiny air sacs covered with capillaries where CO2 is exchanged for oxygen.
The lungs are positioned in front of the thoracic vertebrae and behind the sternum and ribs. The right lung has an upper, middle, and lower lobe. The smaller left lung has only two lobes and is shaped by the cardiac “notch”, the place where the heart fits into the lungs
The powerful “thump” of the left ventricle resonates across the walls of the arteries as oxygenated blood flows away from the heart. Arteries are the widest and strongest blood vessels. They’re constructed of an outer layer of elastic collagen covering a thin middle layer of muscle that compresses the blood as it flows along the innermost layer of smooth epithelial cells.
Stemming from the aorta is a series of large arteries that travel away from the heart in different directions, slowly decreasing in size. The diameter of the artery and the thickness of the arterial wall begin to narrow, eventually tapering off into capillaries. Nutrients and oxygen are delivered to the cells of the body as they diffuse across the capillary walls into the interstitial fluid that the cells of the body are “floating” in on a microscopic level. Carbon dioxide and cellular waste flow in the opposite direction.
The journey back to the heart starts at this same point. The capillaries containing deoxygenated blood flow into slightly wider venules and then into veins. Blood pressure in the veins is lower than that in the arteries so the walls of the veins are lined with tiny valves that prevent blood from flowing backward.
At the end of one complete loop around the systemic circulation, blood enters the right atrium of the heart through the vena cava and is pumped through the pulmonary circulation to start the process again.
Blood is red due to the prevalence of red blood cells, or erythrocytes. These large, flexible cells contain the hemoglobin that binds to oxygen in the lungs and transports it into the smallest capillaries of the body. A person’s blood type is an inherited genetic trait determined by antigens found on the surface of red blood cells.
White blood cells, or leukocytes, work closely with the lymphatic system to cleanse the blood of cellular debris and to fight infection. There are different types of white blood cells and each one specializes in a particular type of pathogen or immune response. WBC’s adapt to recognize and kill specific toxins either by engulfing the disease-causing microbe or secreting antibodies.
The third type of blood cell is the platelets, or the thrombocytes. Platelets gather at the site of a cut or wound and interact with coagulation factors in the bloodstream to form a blood clot.