In a rhythmic cycle, the heart contracts and relaxes. It pumps blood as it contracts and fills its chambers with blood when it relaxes. The cardiac cycle refers to a whole series of pumping and filling. Systole refers to the contraction phase of the cycle, whereas diastole refers to the relaxation phase .
The cardiac output is the amount of blood that each ventricle pumps every minute. The rate of contraction or heart rate (number of beats per minute) and the stroke volume or the amount of blood pumped by a ventricle in a single contraction, are the two parameters that influence cardiac output.
In humans, the average stroke volume is around 70 mL. Cardiac output of 5 L/min is obtained by multiplying this stroke volume by a typical resting heart rate of 72 beats per minute, which is roughly equivalent to the entire volume of blood in the human body. During strenuous activity, the increased demand for oxygen is supplied by a fivefold increase in cardiac output.
Heart and its valves
The heart has four valves that keep blood flowing and prevent backflow. The valves, which are made of connective tissue, flaps open when pushed from one side and close when pushed from the opposite.
Each atrium and ventricle has an atrioventricular (AV) valve. Strong fibres attach the AV valves, preventing them from turning inside out during ventricular systole. The AV valves are closed by the pressure generated by the ventricles’ strong contractions, preventing blood from flowing back into the atria. Semilunar valves are found at the heart’s two exits: the pulmonary artery leaving the right ventricle and the aorta leaving the left ventricle.
The pressure created by the contraction of the ventricles pushes these valves open. Blood pressure built up in the pulmonary artery and aorta seals the semilunar valves and prevents considerable backflow when the ventricles relax.
With a stethoscope or by pressing your ear against the chest of a buddy, you may listen to the two sets of heart valves close (or a friendly dog). “lub-dup, lub-dup, lub-dup” is the sound pattern. The rebound of blood against the closed AV valves produces the first heart sound (“lub”). The second sound (“dup”) is created by vibrations caused by the semilunar valves shutting.
The heart has four valves that prevent backflow and maintain blood flowing in the right direction.
The valves, which are made of connective tissue flaps, open when pushed from one side and close when pushed from the opposite. Between each atrium is an atrioventricular (AV) valve.
When blood squirts backward through a faulty valve, it can cause a heart murmur, which is an unnatural sound.
Heart murmurs are present in some persons from birth. Infection (for example, from rheumatic fever, an inflammation induced by infection with certain bacteria) might damage the valves in some people. Surgeons may implant a mechanical replacement valve when a valve problem is severe enough to put one’s health at risk. However, not all heart murmurs are caused by a valve problem, and most valve defects do not diminish blood flow efficiency to the point where surgery is necessary.
Contraction and Relaxation
When the heart contracts during ventricular systole, arterial blood pressure is at its greatest,Systolic pressure is the pressure at this point. Each ventricular contraction results in an increase in blood pressure, stretching the artery walls. Place the tips of your fingers on the inside of the opposite wrist to feel the pulse—the rhythmic bulging of the artery walls with each heartbeat.
The increase in pressure is attributed in part to the narrowing of arteriole apertures, which prevent blood from leaving the arteries. When the heart contracts, blood rushes into the arteries quicker than it can exit, stretching the vessels to a larger diameter as a result of the increased pressure.
The elastic walls of the arteries snap back during diastole. As a result, when the ventricles are relaxed, there is a decreased but still significant blood pressure (diastolic pressure). The heart beats again before enough blood has gone into the arterioles to completely release pressure in the arteries. Blood flows continuously into arterioles and capillaries because the arteries remain pressured during the cardiac cycle.
Controlling Blood Pressure
The diameter of arterioles is altered by homeostatic mechanisms, which regulate arterial blood pressure. Vasoconstriction occurs when the smooth muscles in arteriole walls contract, narrowing the arterioles. Vasoconstriction raises blood pressure in the arteries upstream. When the smooth muscles relax, the arterioles dilate, causing the blood pressure in the arteries to drop.
Blood, like all other fluids, flows from high-pressure places to low-pressure ones. Blood pressure is created when a heart ventricle contracts, exerting a force in all directions. Blood flows away from the heart, which has the maximum pressure when some of the force is directed lengthwise in an artery. The part of the force that is applied sideways strains the arterial wall.
The elastic artery walls’ recoil after ventricular contraction is crucial for sustaining blood pressure and thus blood flow throughout the cardiac cycle. The short diameter of these veins creates significant resistance to flow after blood enters the millions of tiny arterioles and capillaries. By the time blood enters the veins, much of the pressure generated by the pounding heart has dissipated due to this resistance.
Conclusion
Nitric oxide (NO), a gas, has been found as a key inducer of vasodilation, while endothelin, a peptide, has been identified as the most potent inducer of vasoconstriction. The neurological and endocrine systems control the generation of NO and endothelin in blood arteries, where their opposing activities maintain blood pressure homeostasis.
Changes in cardiac output, which impact blood pressure, are frequently associated with vasoconstriction and vasodilation. As the body’s demands on the circulatory system fluctuate, this coordination of regulatory processes ensures proper blood flow. The arterioles in working muscles, for example, dilate during intense activity, allowing more oxygen-rich blood to travel to the muscles. This increased blood flow to the muscles would produce a drop in blood pressure (and thus blood flow) throughout the body on its own. At the same time, cardiac output rises, keeping blood pressure stable and allowing for the necessary increase in blood flow.