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last updated: 08/01

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The Heart
The heart is a powerful muscle that functions like a pump.  With each beat, it pushes blood through the elaborate circulation of the body.  The heart is composed of four chambers: two atria and two ventricles.  The right and left heart are separated by a septum.  The left side of the heart is the stronger pump, because it is responsible for propelling blood throughout the entire body. 
 

Blood Flow
Blood carrying carbon dioxide and cellular waste products from the whole body flows into the right atrium, primarily via the vena cava.  From the right atrium blood flows through the tricuspid valve into the right ventricle.  Next, blood flows through the pulmonic valve into the pulmonary arteries which carry it to the lungs.  Here the blood is oxygenated in exchange for carbon dioxide which is released into the lungs for expiration.  With each breath, carbon dioxide that originated from cells throughout the body is exhaled and freshly inhaled atmospheric oxygen is absorbed and begins the long trip through the circulation.  The newly oxygenated blood flows through the pulmonary veins into the left atrium and through the mitral valve into the left ventricle.  Finally, the blood is pumped through the aortic valve into the aorta where it branches off into arteries to arterioles to capillaries, delivering oxygen to every tissue in the body.   As the blood brings oxygen and nutrients to body tissues, it also removes carbon dioxide and waste products.  The oxygen-depleted blood then flows into venules to larger veins and finally back to the right atrium, where the cycle continues.
 

Heart Disease Pathophysiology
The cardiovascular system works continuously and, in most cases, efficiently.  But problems can arise when the blood flow is reduced or blocked.  If a vessel to the heart is completely blocked, the heart does not get enough oxygen and a heart attack results.  If the vessel that is blocked feeds the brain, the result is a stroke.  Both can be fatal, and, in fact, account for millions of deaths per year, making cardiovascular disease the leading cause of death in the US.  Heart disease can be a vicious cycle, because the narrowed vessels can not only damage the heart, but also make it harder for the heart to pump blood through the circulatory system.  Furthermore, as the heart is damaged it becomes less efficient and must work even harder to continue to deliver sufficient oxygen throughout the body.  Over time, heart disease leads to major problems involving the heart, the lungs, the kidneys, and eventually every body system, since every organ in the body relies on the heart for oxygen and nutrients.  Typically, the blockage which causes the problem is created by a sticky buildup known as an atherosclerotic plaque. 
 
 

Atherosclerosis
Atherosclerosis is a disease which preferentially affects the large and medium-sized arteries.  An atherosclerotic plaque is composed of cholesterol, fats, and other cellular debris.  As these are deposited along the artery walls, atherosclerosis develops and leads to decreased blood flow beyond the blockage because of increasing constriction of the passageway.  Atherosclerosis is a problem that tends to compound itself.  As the artery narrows, tiny clots capable of passing through a healthy artery become caught in the plaque and further obstruct the blood flow.  Furthermore, the plaque itself can initiate the formation of a clot. 

The exact pathogenesis of atherosclerosis is controversial and complex, but a simplified explanation is as follows: 
***Formation of the fatty streak: Dyslipidemia (high LDL or low HDL), 
     hypertension, cigarette smoking, or various other conditions cause an initial
     injury to the arterial endothelium.  Endothelial dysfunction allows 
     accumulation of lipoproteins in the subendothelial space, where chemical 
     modification of LDL can occur.  Modified LDL recruits monocytes into the 
     vessel wall, where these cells are converted to macrophages that engulf the 
     modified lipoproteins and become foam cells.  Fatty streaks do not cause 
     symptoms and are present in the aorta and coronary arteries of most 
     individuals by age 20. 
***Formation of the fibrous plaque: The endothelial injury also causes
     platelet aggregation and the platelets and macrophages release growth 
     factors which stimulate the proliferation of smooth muscle cells.  The 
     resultant fibrous plaque contains a necrotic core of cellular debris, 
     degenerating foam cells, and cholesterol crystals and is separated from the 
     arterial lumen by a fibrous cap.  Fibrous plaques are more advanced lesions 
     and lead to the clinical manifestations of atherosclerosis.
***Complications of fibrous plaques: Blood flow through the artery becomes
     restricted when one of the following complications ensues:
            1) The plaque calcifies, stiffening the vessel and increasing
                the fragility.
            2) The plaque ruptures or ulcerates, exposing thrombogenic material 
                and leading to the formation of a blood clot, or thrombus.  The 
                thrombus may block the artery, causing a heart attack or stroke, 
                or it may become incorporated into the plaque, enlarging its size.
            3) Pieces of the plaque, called atheroma may, break off and become 
                lodged in distal sites, a process called embolization.
***A bit more on thrombosis formation: It is critical to understand that 
     thrombosis on the surface of the plaque leads to vascular occlusion and 
     clinical impairment of blood flow and is the most common precipitant of a 
     myocardial infarction, or heart attack.  A thrombus forms when certain 
     connective tissue elements come into contact with blood.  Platelets are 
     stimulated to interact with collagen or fibrin on the exposed connective 
     tissue leading to platelet activation.  Activated platelets initiate the 
     intrinsic clotting pathway wherein fibrin is generated and cross-linked 
     and further platelet aggregation is stimulated.
***Hypertension and atherosclerosis: Hypertension is a recognized risk 
     factor for the development of atherosclerosis. It is unclear exactly how 
     this occurs, but studies suggest that high blood pressure enhances the 
     lipid accumulation in the vessel wall and increases the endothelial 
     permeability to lipoproteins such as LDL.  Continued vessel wall stress 
     exerted from the lumen probably plays a role in this.  Hypertension also 
     may increase platelet aggregation, thereby exacerbating the 
     atherosclerotic process.
 
 

Measuring Blood Pressure
One way to gauge the blood flow through the cardiovascular system is by measuring blood pressure.  Blood pressure is reported as two numbers.  The top or systolic blood pressure is the maximum force or pressure in the arteries when the heart contracts.  The lower or diastolic blood pressure is the measure of lowest pressure which occurs in the arteries when the heart relaxes between beats.  These together give an idea of the blood flow throughout the system.  Consistently high blood pressure may mean that constricted arteries are making the heart work harder than it normally would.