The heart is a very important organ in the human body and also one of the organs that are most commonly diseased. The risk of diseases incerases with age and with rising life expectancies, the number of people afflicted with heart diseases is also rising at an alarming rate. In these circumstances, it is important for people to be aware of what affects the heart functions and what could go wrong.
The most important function of the heart is to pump blood into the rest of the body. The two determinants of effective pumping of blood is the volume and the pressure determined by cardiac output and total peripheral resistance respectively. These two factors will determine whether the body tissues are receiving adeqaute amounts of nutrients and oxygen to sustain their activity.
The pumping of the heart can be affected by a few factors. One is the instrinsic health of the cardiac muscles. Coronary arteries supply blood to the cardiac muscles. These arteries are functional end artereis meaning that they do not anastomose enough to sustain blood supply to a region if any of the arteries are blocked. the modern diet rich in saturated fats leads to the build up of atherosclerotic plaques on the coronary arteries and as a person ages, it increases the risk of thrombus formation. A deep vein thrombosis may also lodge onto the coronary arteries. The region of the cardiac muscles that do nto receive the cardiac output will die in a process known as myocardial infarction. Heart muscles like brain cells do not generally have great replenishing capacity and the death of cardiac muscle cells will weaken the heart. the heart no longer can be an efficient pump and some of the blood remains in the heart at the end of systole. As such the body receives reduced amounts of cardiac output. If this amount is less than that required to sustain body fucntions, the heart is said to be in cardiac failure.
The rate at which the heart pumps determines the heart function too. Normally the heart oumps at a rate of 70 beatss per minute. Since cardiac output is the total volume of blood pumped by the heart in one minute. a low heart rate knwon as bradycardia (below 60 beats per minute) will reduce the cardiac output. heart block will cause bradycardia. The SA node is the natural pacemakerof the heart discharging at a rate of 60 to 100 beats per minute. However if there is a blockage at any point between the SA node, atrial muscle fibers, AV node, bundle of His or Purkinje fibers, there will be the presence of an ectopic pacemaker which beats at a lower rate since it does not have the high dischange frequency of the SA node. If the AV bundle is blocked as commonly happens, the Purkinje fibers will discharge at a ratse of 15 to 20 beats per minute on its own. Furthermore there is no coordination between the atrial contraction and the ventricular contraction such that the atria is unable to act as a primer pump and the ventricles do not efficienty get filled during diastole, further complicating the situation. the instrinsic heart rate is controlled by the parasympathetic system and can be incresaed by the sympathetic system.
The preload also determines heart functions. According to the starling mechanism, the stregth of of the cardiac muscle fiber determined by the end diastolic volume will determine the strength of the pumping by the heart and thus the cardiac output. . The more the stretch, the more the strength of the pumping action. The preload can be decresed due to blood loss by hemorrhage, diminished kidney functions rendering the water reabsorbing function of the kidney void, dilated veins and artereioles decreasing the total peripheral resistance and increasing the venous reservoir of blood and decreasing the amount of blood pumped into the heart and pooling of blood at the ankles (edema).
The afterload determines the fucntion of the heart. Afterload increases when hypertension increases and it increases the effort required by the heart to pump a certain amount of blood into the circulation. As a result, the amount of blod pumped into the circulation will decrease. An increase in total peripheral resistance, longer length of the arteries due to obesity or the decrease in compliance of teh arteries due to build up of athresclerotic plaque all leads to hypertension.
