BLOOD VESSELS; Show
A blood vessel is a tube that carries blood. Oxygen rich blood leaves the left side of the heart and enters the aorta. The aorta branches into arteries, which eventually branch into smaller arterioles. Arterioles carry blood and oxygen into the smallest blood vessels, the capillaries. Capillaries are so small they can only be seen under a microscope. The walls of the capillaries are permeable to oxygen and carbon dioxide. Oxygen moves from the capillary toward the cells of the tissues and organs. Carbon dioxide moves from the cells and into the capillaries. Blood leaves the capillary and enters the small venules. These venules become progressively larger vessels called veins. The vena cava are the two largest veins that carry blood into the right upper chamber of the heart (the right atrium). The superior vena cava carries blood from the brain and arms into the top of the right atrium. The inferior vena cava carries blood from the legs and abdominal cavity into the bottom of the right atrium. The vena cava are also called the "central veins". Central venous catheters are inserted with the tip in or close to the superior of inferior vena cava. Blood is pumped from the right side of the heart into the blood vessels of the lung. When blood enters the small capillaries of the lung (called the pulmonary capillaries), fresh oxygen enters the blood and carbon dioxide is removed. This is called "gas exchange" or "respiration". Because this is the exchange of gases between the atmosphere and the bloodstream, it is also called "external respiration". When freshly oxygenated blood reaches the capillaries of the tissues, oxygen moves from the blood toward the tissues, and carbon dioxide moves from the tissues toward the blood. This gas exchange that occur between the blood and the cells of the tissues and organs is called "internal respiration". Blood vessels have a muscle layer that is able to relax or contract. When we need to increase our blood pressure, the muscle layer contracts and makes the blood vessel diameter smaller. This is called "vasoconstriction". When the muscle layer of a blood vessel relaxes, the blood vessel diameter becomes larger. This is called "vasodilation". Vasodilation lowers the blood pressure. Drugs that cause the diameter of the blood vessels to change are called vasoactive drugs. Drugs that make blood vessels constrict are used to treat low blood pressure and are called vasocontrictors. Drugs that relax the blood vessels and make them relax are used to treat high blood pressure. They are called vasodilators.
Image 1: Blood Vessels. Arteries (in red) carry oxygen rich blood from the left side of the heart to the tissues and organs. After oxygen leaves the blood and moves into the tissues, the level of oxygen in the blood becomes low. The veins (in blue) carry blood that has a low level of oxygen back to the right side of the heart. Blood from the veins is pumped from the right side of the heart through the blood vessels of the lungs, where new oxygen is picked up. This oxygen rich blood flows from the lungs to the left side of the heart. NamingCoronary ArteriesThere are two main coronary arteries which branch to supply the entire heart. They are named the left and right coronary arteries, and arise from the left and right aortic sinuses within the aorta. The aortic sinuses are small openings found within the aorta behind the left and right flaps of the aortic valve. When the heart is relaxed, the back-flow of blood fills these valve pockets, therefore allowing blood to enter the coronary arteries. The left coronary artery (LCA) initially branches to yield the left anterior descending (LAD), also called the anterior interventricular artery. The LCA also gives off the left marginal artery (LMA) and the left circumflex artery (Cx). In ~20-25% of individuals, the left circumflex artery contributes to the posterior interventricular artery (PIv). The right coronary artery (RCA) branches to form the right marginal artery (RMA) anteriorly. In 80-85% of individuals, it also branches into the posterior interventricular artery (PIv) posteriorly.  Fig 1 – Anterior view of the arterial supply to the heart. Fig 2 – Overview of the branching structure of the coronary arteries. Cardiac VeinsThe venous drainage of the heart is mostly through the coronary sinus – a large venous structure located on the posterior aspect of the heart. The cardiac veins drain into the coronary sinus, which in turn, empties into the right atrium. There are also smaller cardiac veins which pass directly into the right atrium. The main tributaries of the coronary sinus are:
Fig 3 – Anterior view of the venous drainage of the heart. Supplied by the great and small cardiac veins Fig 4 – Posterior view of the heart, showing the venous drainage. Distribution of the Coronary ArteriesIn general, the area of the heart which an artery passes over will be the area that it perfuses. The following describes the anatomical course of the coronary arteries. See Appendix A for a tabular overview of the arterial distribution. The RCA passes to the right of the pulmonary trunk and runs along the coronary sulcus before branching. The right marginal artery arises from the RCA and moves along the right and inferior border of the heart towards the apex. The RCA continues to the posterior surface of the heart, still running along the coronary sulcus. The posterior interventricular artery then arises from the RCA and follows the posterior interventricular groove towards the apex of the heart. The LCA passes between the left side of the pulmonary trunk and the left auricle. The LCA divides into the anterior interventricular branch and the circumflex branch. The anterior interventricular branch (LAD) follows the anterior interventricular groove towards the apex of the heart where it continues on the posterior surface to anastomose with the posterior interventricular branch. The circumflex branch follows the coronary sulcus to the left border and onto the posterior surface of the heart. This gives rise to the left marginal branch which follows the left border of the heart. Fig 4 – Anterior view of territorial arterial supply to the heart. Fig 5 – Posterior view of territorial arterial supply to the heart. Clinical Relevance: Coronary Artery DiseaseCoronary artery disease or coronary heart disease (CHD) is a leading cause of death, both in the UK and worldwide. It describes a reduction in blood flow to the myocardium and has several causes and consequences. CHD can result in reduced blood flow to the heart as a result of narrowing or blockage of the coronary arteries. This may be due to atherosclerosis, thrombosis, high blood pressure, diabetes or smoking. All these factors lead to a reduced flow of blood to the heart through physical obstruction or changes in the vessel wall. Angina pectoris is one consequence of CHD. Angina pectoris describes the transient pain a person may feel on exercise as a result of lack of oxygen supplied to the heart. This pain is felt across the chest but is quickly resolved upon rest. Exercise is a trigger for angina as the coronary arteries fill during the diastolic period of the cardiac cycle. On exercising, the diastolic period is shortened meaning that there is less time for blood flow to overcome a blockage in one of the coronary vessels in order to supply the heart. If left untreated, angina can soon progress to more severe consequences, such as a myocardial infarction. The sudden occlusion of an artery results in infarction and necrosis of the myocardium. This means a section of the heart is unable to beat (which part of the heart depends on which artery has become occluded). The ECG leads on which an MI change appears can be used to locate the artery that had been occluded as shown in the table.
Diagnosis and Treatment of Coronary Artery DiseaseBy Maria A Pantaleo et al [CC-BY-2.0] via Wikimedia Commons Fig 1.6 – A coronary angiogram. Two critical narrowings have been labelled. A blockage in a coronary artery can be rapidly identified by performing a coronary angiogram. The imaging modality involves the insertion of a catheter into the aorta via the femoral artery. A contrast dye is injected into the coronary arteries and x-ray based imaging is then used to visualise the coronary arteries and any blockage that may be present. Immediate treatment of a blockage can be performed by way of a coronary angioplasty, which involves the inflation of a balloon within the affected artery. The balloon pushes aside the atherosclerotic plaque and restores the blood flow to the myocardium. The artery may then be supported by the addition of an intravascular stent to maintain its volume. Appendix A – Tabular Overview of the Vasculature of the Heart
What is it called when arteries get smaller?Clogging or narrowing of the arteries that supply blood to the heart can occur not only in the heart's largest arteries (the coronary arteries) but also in the heart's smaller blood vessels.
What are arteries divided into?When the arteries reach tissues or cells they divide into smaller blood vessels called capillaries. The capillaries are the site of exchange of gases and nutrients between blood and the body cells. Q. Artery gets divided into smaller vessels and the smallest vessels have walls which are one-cell thick.
What are the smallest branches of arteries called?Eventually, the smallest branch of the artery is called arterioles, which further divide into tiny vessels to form the capillary bed. Nutrients and wastes exchange between the blood and body tissues occurs at the capillary bed.
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