Coronary arteries supply the heart with a constant supply of blood. These arteries are essential, providing oxygen and nutrients that keep the heart constantly pumping. Arteries have three layers including the external layer, tunica external, middle layer, tunica media and the inner layer, called the tunica intima. The outer layer is made up of connective tissue with elastic and collagenous fibers. Tunica media is the thickest layer composed of smooth muscle and can change the diameter of the arteries to control blood flow and pressure. The tunica intima is composed of simple squamous epithelium (Patton & Thibodeau, 2013, 643).
There is a left and right coronary artery that “branch from the aorta just superior to the aortic valve. They lie on the epicardium of the hearts surface, encircling the heart in an atrioventricular groove” (Clancy & Andrew, 2009, 310). The right coronary artery breaks off into the “right marginal artery and the posterior descending artery” which pumps blood to the right atrium and ventricle. The left coronary artery diverges into the anterior descending branch which sends blood to the interventricular septum and anterior portion of the ventricles and the circumflex branch.
The circumflex branch sends blood to the left atrium as well as works alongside the right posterior artery providing blood to the back end of the ventricular wall. The coronary arteries are most active in sending blood during the heart’s relaxation period. So, when oxygen demand increases during times of exercise, “blood flow increases, but the metabolic stimulus to provide the additional blood has yet to be identified” (Clancy & McVicar, 2009, 311). However, the heart itself accommodates for exercise which, can be determines through a cardiac stress test.
The stress test is performed to check the heart’s adaptations to cardiac performance. The test looks at the results of the cardiac output, heart rate, stroke volume, Frank-Sterling mechanism, and other dictations of cardiac performance. These measurements will determine if a person’s heart is working properly or not during activity. The heart accommodates for exercise by adapting the different components of the cardiovascular system. One of the first accommodations the heart makes is through cardiac output.
Cardiac output is the amount of blood pumped out of the heart per minute. There are four determinants of cardiac output: heart rate, preload, afterload, and stroke volume. The equation used to calculate cardiac output is (CO = SV (per beat) X HR (per min)). Having a clear understanding between the four components will help with understanding cardiac output (Vincent, 2008, 174). Increased heart rate is one of the simplest measures of increased activity. A way to measure a subject’s heart rate is by counting the beats per minute from the radial or carotid artery.
Normal heart rate is about 60 to 80 beats per minute. However, as soon as exercise begins, the heart rate responds in proportion to the exercise intensity until reaching maximal heart rate. Maximal heart rate varies from person to person, therefore to calculate each subject’s specific maximum heart rate, use the formula: 220 minus the age of the person. Another response looked for from the stress test is the stroke volume which “is a major determinant of cardiorespiratory endurance capacity” (Wilmore, Costill & Kenney, 2015, 199).
Stroke volume is the amount of blood pumped out of the left ventricle per heartbeat. As the heart rate reaches near its max, stroke volume accommodates to these two factors: the heart’s capacity to fill the left ventricle during diastolic pressure and the ventricle stretching to pump out blood more efficiently during systolic pressure. During the heart’s diastolic filling time, the rate at which filling occurs lowers “from 500 to 700 ms at rest to about 150 ms” (Wilmore, Costill & Kenney, 2015, 201). This is due to the increased rate of blood caused by the lowering of fill time.
The Frank-Sterling mechanism (systolic pressure) must increase its contractility rate per beat, in other words, the amount of blood thats fills the heart increases while there is a greater stretch placed on the heart. The wider the stretch in the ventricle, there is a greater force of contraction (preload). Afterload is when the heart must overcome the pressure to create an ejection rate which, is impacted by arterial blood pressure and vascular stretch (preload). Another factor looked at through the stress test is a person’s blood pressure change during increased activity.
As the heart begins to pump harder, the diastolic pressure which, is the hearts relaxed filling time and the systolic pressure (pressure exerted during contraction). During exercise, diastolic pressure either stays the same or even decreases. While, systolic pressure in the arteriole increases. This is due from the increase in cardiac output. Like heart rate and stroke volume, as blood pressure reaches a steady state. Because of the heart’s systolic increase, a need for more oxygen intake is required. This is known as Myocardial oxygen intake (Wilmore, Costill & Kenney, 2015, 205).
Myocardial oxygen intake is the amount of oxygen supplied and used. With increasing exercise oxygen intake increases by increasing coronary blood flow. Uptake of oxygen and myocardial blood flow is related to the heart rate and systolic blood pressure. RPP is also a determent of myocardial oxygen intake, as RPP increases, demand for oxygen also increases (Wilmore, Costill & Kenney, 2015, 205). RPP measures the work and oxygen demand of the heart in a given state. Rate pressure product “is defined as the product of resting heart rate (RHR) and systolic blood pressure (SBP).
It is expressed as RPP = SBP ? HR/1000” (Sembuligam, K, Ilango 2015, 8). RPP is measured at different levels of exercise, to help determine safe levels of exercise by calculating max heart rate as well as any indicators of hemodynamic stress. RPP goes up significantly when a person’s stroke volume and heart rate increase. The heart is a muscle the size of a person’s fist. Yet, as the paper shows, it’s the most astounding and intricate muscle in the body. It truly is the workhorse that can accommodate almost anything.
