Cardiovascular system
The cardiovascular system consists of the heart, which is an anatomical pump, with its intricate conduits (arteries, veins, and capillaries) that traverse the whole human body carrying blood. The blood contains oxygen, nutrients, wastes, and immune and other functional cells that help provide for homeostasis and basic functions of human cells and organs.
The pumping action of the heart usually maintains a balance between cardiac output and venous return. Cardiac output (CO) is the amount of blood pumped out by each ventricle in one minute. The normal adult blood volume is 5 liters (a little over 1 gallon) and it usually passes through the heart once a minute. Note that cardiac output varies with the demands of the body.
The cardiac cycle refers to events that occur during one heart beat and is split into ventricular systole (contraction/ejection phase) and diastole (relaxation/filling phase). A normal heart rate is approximately 72 beats/minute, and the cardiac cycle spreads over 0.8 seconds. The heart sounds transmitted are due to closing of heart valves, and abnormal heart sounds, called murmurs, usually represent valve incompetency or abnormalities.
Blood is transported through the whole body by a continuum of blood vessels. Arteries are blood vessels that transport blood away from the heart, and veins transport the blood back to the heart. Capillaries carry blood to tissue cells and are the exchange sites of nutrients, gases, wastes, etc.
Heart
The heart is a muscular organ weighing between 250-350 grams located obliquely in the mediastinum. It functions as a pump supplying blood to the body and accepting it in return for transmission to the pulmonary circuit for gas exchange. The heart contains 4 chambers that essentially make up 2 sides of 2 chamber (atrium and ventricle) circuits; the left side chambers supply the systemic circulation, and the right side chambers supply the pulmonary circulation. The chambers of each side are separated by an atrioventricular valve (A-V valve). The left-sided chambers are separated by the mitral (bicuspid) valve, and right-sided chambers are divided by the tricuspid valve. Blood flows through the heart in only one direction enforced by a valvular system that regulates opening and closure of valves based on pressure gradients
Unique properties of cardiac muscle
Cardiac muscle cells are branching striated, uninucleate (single nucleus) cells that contain myofibrils.
Adjacent cardiac cells are connected by intercalated discs containing desmosomes and gap junctions. The myocardium behaves as a functional syncytium because of electrical coupling action provided by gap junctions.
Cardiac muscle has abundant mitochondria that depend on aerobic respiration primarily to generate adenosine tri-phosphate (ATP), the molecule that provides energy for cellular function
Systemic Circulation
The systemic circuit originates in the left side of the heart and functions by receiving oxygen-laden blood into the left atrium from the lungs and flows one way down into the left ventricle via the mitral valve. From the left ventricle, oxygen rich blood is pumped to all organs of the human body through the aortic semilunar valve
Pulmonary Circulation
The pulmonary circuit is on the right side of the heart and serves the function of gas exchange. Oxygen-poor systemic blood reaches the right atrium via 3 major venous structures: the superior vena cava, inferior vena cava, and coronary sinus. This blood is pumped down to the right ventricle via the tricuspid valve and eventually through the pulmonic valve,leading to the pulmonary trunk that takes the oxygen deprived blood to the lungs for gas exchange. Once gas exchange occurs in the lung tissue, the oxygen-laden blood is carried to the left atrium via the pulmonary veins, hence completing the pulmonary circuit (see the image above).
Coronary Circulation
Coronary circulation is the circulation to the heart organ itself. The right and left coronary arteries branch from the ascending aorta and, through their branches (anterior and posterior interventricular, marginal and circumflex arteries), supply the heart muscle (myocardial) tissue. Venous blood collected by the cardiac veins (great, middle, small, and anterior) flows into the coronary sinus. Delivery of oxygen-rich blood to the myocardial tissue occurs during the heart relaxation phase
Vessel Anatomy
An artery is a blood vessel that carries blood away from the heart to peripheral organs (see the image below). They are subdivided into larger conducting arteries, smaller distributing arteries, and the smallest arteries, known as arterioles, that supply the capillary bed (the site of active tissue cells gas exchange).
Capillaries are vessels that are microscopic in size and provide a site of gas, ion, nutrient, and cellular exchange between blood and interstitial fluid. They have fenestrations that allow for and enhance permeability for exchange of gas, ion, nutrient, and cellular elements
A vein is a blood vessel that has a larger lumen, and sometimes veins serve as blood reservoirs or capacitance vessels, containing valves that prevent backflow. This system of vessels in general returns blood to the heart from the periphery
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