c1-CARDIOVASCULAR ANATOMY AND PHYSIOLOGY.docx

CARDIOVASCULAR ANATOMY AND PHYSIOLOGY ===================================== Heart anatomy ------------- Tapping into the cardiovascular system requires an understanding of the system structures and how they work together to maintain the body's equilibrium. It begins with the heart, which is the pu...

CARDIOVASCULAR ANATOMY AND PHYSIOLOGY ===================================== Heart anatomy ------------- Tapping into the cardiovascular system requires an understanding of the system structures and how they work together to maintain the body's equilibrium. It begins with the heart, which is the pump at the center of the cardiovascular system. It is a hollow, conical, muscular organ that is roughly the size of the patient\'s fist. It sits in the center of the chest, vertically between the second and sixth ribs, with a little more of it just to the left of the sternum. A membrane-the pericardium­ surrounds the heart and attached blood vessels. The wall-or body-of the heart has three layers: the outermost epicardium, myocardium layer, and innermost endocardium. Inside the heart are four chambers; the upper two are the right and the left atria, and the lower two are the right and left ventricles. Four valves keep the blood flowing in the right direction. They are the tricuspid valve between the right atrium and the right ventricle; the bicuspid valve (also known as the mitral valve) between the left atrium and left ventricle; the pulmonary semilunar valve between the right ventricle and the trunk of the pulmonary arteries; and the aortic semilunar valve between the left ventricle and the aorta. The heart pumps quickly. Just one beat corresponds with an entire cardiac cycle, which includes atrial contraction, ventricular contraction, and a recovery period. Most adult hearts beat 60 to So times per minute. Both atria begin in a state of relaxation, with blood flowing from the superior and inferior venae cavae into the right atrium and from the pulmonary veins into the left atrium. Then the right atrium contracts, the tricuspid valve opens, and blood flows into the right ventricle. Atrial contraction occurs simultaneously, forcing blood into both ventricles. As the atria relax, both ventricles contract and force blood out of the heart. The right ventricle forces deoxygenated blood to the lungs, and the left ventricle forces oxygenated blood to the body. What this cardiac cycle accomplishes is pumping blood that has been used for oxygenation of body tissues and organs from the veins to the pulmonary arteries, where it is reoxygenated, exchanging carbon dioxide for oxygen. Then it returns to the heart through the pulmonary veins, and the heart pumps that fresh blood to the arteries to supply the body\'s tissues and organs with oxygen again. The medulla of the brain is the command center of the cardiovascular system. It sends messages to the electrical pacemaker of the heart, called the sinoatrial (SA) node. This process plays a part in regulating cardiovascular function along with other systems and mechanisms in the body, such as the sympathetic and parasympathetic nervous systems. The cardiac cycle also requires a conduction system that sends electrical impulses to the heart, triggering contractions. The conduction system consists of two nodes: the SA node and the atrioventricular (AV) node, which transmits the signal to the bundle of His following atrial contraction. This collection of nerves located between the atria and ventricles then transmit the signal to the finger-like projections located throughout the ventricular muscle-called Purkinje fibers-to trigger ventricular contraction. These contractions move the blood through the heart\'s chambers to the pulmonary circulation and to the aorta for systemic circulation. The SA node triggers impulses at a rate of 60 to 100 per minute from the back wall of the right atrium. The impulse goes through both atria to the AV node, which is at the junction of the atria and ventricles. The AV node pauses the impulse just long enough to let the blood empty from the atria, then it releases it to the bundle of His, which sits between the right and left ventricles. This bundle of fibers has two branches, right and left. The impulse goes through both branches to the Purkinje fibers, where it then triggers ventricular contraction. This entire electrical force is what an EKG tracing demonstrates for the provider\'s evaluation. All these structures and processes work together to coordinate the functions of the cardiovascular system. To distribute the blood to and from the body organs and tissues, the heart requires a tubular network of arteries and veins, which are the blood vessels that comprise the vascular or circulatory part of the cardiovascular system. Vascular anatomy ---------------- Vascular structures include the arteries (which branch into arterioles and capillaries) and veins (which branch into venules and capillaries). Phlebotomists collect most blood specimens from the veins, but occasionally access arterial or capillary blood based on the type of testing necessary. Arteries carry freshly oxygenated blood to the body, with the exception of the pulmonary arteries, which carry deoxygenate \' blood to the lungs. The coronary arteries supply o: gen- and nutrient-rich blood to the heart muscle; femoral arteries supply blood to the lower extremities. Veins carry deoxygenated blood from the body back to the heart. The exception is the pulmonary veins, which carry reoxygenated blood back to the heart from the lungs. The jugular veins return deoxygenated blood from the head and neck to the heart; saphenous veins return deoxygenated blood from the lower extremities to the heart. Capillaries are permeable and function as exchange vessels. This is where oxygen and nutrients move into a body cell from a capillary, while carbon dioxide and other body wastes move into a capillary from body cells. Processes of osmosis, diffusion, and filtration facilitate these exchanges. Blood components ---------------- Blood is a living tissue containing various cells and substances. It circulates through the heart, arteries, veins, and capillaries, carrying nourishment, vitamins, electrolytes, hormones, antibodies, warmth, and oxygen to the body\'s tissues. Blood also transports wastes and carbon dioxide to organs that can excrete them from the body. Adults have 4 to 6 liters of whole blood in circulation, depending on sex and body size. About 55% to 60% of that blood volume is plasma, which is about 92% water and 8% a mixture of proteins, glucose, electrolytes, fibrinogen (which aids with blood clotting), and other substances. The other 40% to 45% is blood cells: red blood cells (erythrocytes), white blood cells (leukocytes), and platelets. Red blood cells contain hemoglobin, a complex iron-containing protein that carries oxygen throughout the body and gives blood its red color. They are flexible, so they can pass easily through the circulatory system. Red blood cells circulate in the bloodstream for about 120 days, after which they disintegrate and bone marrow replaces them with new red blood cells. White blood cells come in many varieties: monocytes, lymphocytes, and granulocytes (which include neutrophils, eosinophils, and basophils). Each has a specific function, but in general these cells defend the body against infection. They destroy pathogens and produce antibodies. Platelets are cells that are necessary for clotting. They function by sticking to the lining of blood vessels. Blood group systems ------------------- Each person has one of four blood types in the ABO system-A, B, AB, or 0-and one of two designations in the Rhesus (Rh) system-Rh-positive or Rh-negative. The presence or absence of the A and B antigens determines the type. - Type A blood has the A antigen. - Type B has the B antigen. - Type AB has both antigens. - Type O has neither antigen. Furthermore, type A plasma contains anti-B antibodies, type B plasma contains anti-A antibodies, type AB plasma contains no antibodies, and type O contains both antibodies. People who are Rh-positive have the antigen for the Rh factor; Rh-negative people do not. Rh-negative individuals may donate to Rh-positive recipients but should only receive Rh-negative blood. In an emergency, anyone can receive type O blood, and type AB individuals can receive blood of any type. Therefore, people who have type 0 blood are universal donors, and those who have type AB blood are universal recipients. In addition, AB donors can give plasma for all blood types. Type O is the most common ABO type, with type A a little less common, type B a lot less common, and type AB the rarest. Hemostasis and coagulation -------------------------- Hemostasis is the steady state (state of equilibrium) of blood. This includes components of the clotting process necessary for managing bleeding while blood is still circulating in its usual liquid form. In other words, hemostasis maintains blood throughout the body regardless of circumstances. When there is an injury to a small blood vessel, the hemostatic-or coagulation-process plugs the leak. Bleeding from larger vessels can require surgical repair. Coagulation has five distinct phases that occur rapidly. - First is the vascular phase. - The injured vessel narrows rapidly (constricts) to reduce the blood flow. - The second stage is the platelet phase. These blood cells clump and attach themselves to the injured portion of the blood vessel to plug the leak. - The third stage is the coagulation phase. This is a complicated process that involves substances such as fibrinogen, calcium, and clotting factors working - together to form a blood clot. A blood clot is a fibrin meshwork that seals off the injured portion of the blood vessel. - The fourth stage involves clot retraction. The blood - clot shrinks to bring the edges of the tear closer together - to heal. - Finally, fibrinolysis breaks up and dissolves the clot as other cells complete the repair. When phlebotomists access a vein to obtain a blood sample, the coagulation process repairs the tiny opening the needle made in the wall of the vein. Direct pressure assists in this process by slowing the flow of blood and constricting the vessel. This facilitates clot formation.