2. CIRCULATORY SYSTEM
The cardiovascular system is an organ system that delivers nutrients, hormones, and oxygen to all cells in the body, as well as removes metabolic byproducts, such as carbon dioxide.
It consists of three components:
· blood (a fluid consisting of plasma, red and white blood cells, and platelets)
· Blood vessels running through the entire body.
The heart is situated in the mid-center of the chest cavity and made of specialized cardiac muscle tissues (myocardium) that allow it to act as a pump within the circulatory system. It is divided into four chambers – one upper (atrium) and one lower (ventricle) chamber on each side of the heart.
The circulatory system consists of two main circuits – pulmonary and systemic.
The heart pumps deoxygenated blood to the lungs for aeration through pulmonary circulation and oxygenated blood into the thick-walled, muscular aorta for distribution throughout the body in the systemic circulation.
The circulatory system consists of three types of blood vessels
· Arteries – vessels that transport blood towards the tissues and away from the heart. Aorta is the main and the largest artery in the human body. Its split into the network of arteries and smaller arterial branches, called arterioles. All arteries have thick, but an elastic muscular wall, which can withstand high blood pressure, and can either constrict (vasoconstriction) or relax (vasodilation) and thereby change the diameter of the vessels. By doing so, arteries (especially arterioles) regulate which organ receives more blood.
· Veins – vessels that carry blood away from the tissues and towards the heart. After the capillary bed, blood passes through the smallest veins called venules, which then merged into bigger veins and eventually into superior and inferior vena cava. Their walls are thinner and less muscular compare to arteries (since blood pressure is lower).
· Capillaries – the smallest blood vessels, that make direct contact with body tissues. Their extremely thin walls allow molecules of O₂ moves from the blood to the tissue and molecules of CO₂ vice versa (gas exchange). Some capillaries are so narrow that only one blood cell at a time can squeeze through. Capillary bed combines 10-100 individual capillaries to provide tissues with adequate O2 and removal of waste products.
Since Oxygen is not very soluble, only a small percent (about 5%) can be dissolved directly in the blood. The majority requires the special carrier, hemoglobin (Hb), to transport in the blood. Hemoglobin binds up to 4 molecules of O₂ and normally is almost completely saturated (96-98%).
O₂ concentration in the blood depends on the Hb concentration in the red blood cells (RBC), the number of RBC (hematocrit), and on the adequacy of perfusion of the lungs.
Not all of the O2 bound to Hb is released in the tissues. At rest, only about 25% of the O2 in the blood is released. But during intense activity blood gives up virtually all its oxygen.
Carbon dioxide is a product of oxidative metabolism. Unlike Oxygen, CO2 is very soluble and does not need a carrier for transportation in the blood. Approximately 5-10% move into physical solution in the plasma as free carbon dioxide.
Most (60-80%) of the carbon dioxide in the blood is transported as bicarbonate (HCO₃⁻).
Some of the carbon dioxide in solution slowly combines with water to form carbonic acid. Much faster (actually 5000 faster!) this reaction happens with the help of special enzyme within the red blood cell.
CO2 + H₂O ↔ H₂CO₃ ↔ HCO₃⁻ + H⁺
Once formed, the HCO3⁻ is transported out of the RBC into the plasma in exchange for Cl⁻. H⁺ stay within RBC
The remaining CO2 (about 20%) combines with Hb to form carbamino-hemoglobin compounds.
Because CO2 diffuses 20X more rapidly than O2, a rise in blood CO2 can be compensated by an increase in the ventilatory rate. Hyperventilation increases the amount of CO2 removed from the body and increases the unloading of CO2 from the blood in the lung.
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