The mammalian heart has two atria and two ventricles. The right atrium and ventricle serve the pulmonary circuit, and the left atrium and ventricle serve the systemic circuit. Valves that are open and shut by pressure differences prevent the backflow of blood. Modified cardiac muscle tissue in the right atrium functions to spontaneously generate pacemaker action potentials. Other modified cardiac muscle tissue between the atria and ventricles and throughout the ventricles conducts those signals and coordinates the heart contraction. Broad action potentials in ventricular muscle reflect Ca2+ cycling in the ventricular muscle cells and make sustained contractions possible.
learning outcomes
You should be able to:
Trace the path of blood through both sides of the heart, naming the major blood vessels and heart valves.
Describe how heart valves function.
Describe the cardiac cycle in terms of left ventricular volume and pressure, aortic pressure, and heart sounds.
Explain how calcium ion movement into and out of the sarcoplasm of cardiac muscle cells influences the rate and strength of contraction.
Valves of the heart can become stiff and calcified, limiting the flow of blood through them in a condition called stenosis. What would be the consequences of aortic valve stenosis and of pulmonary valve stenosis?
Aortic valve stenosis would decrease the flow of blood into the aorta. Consequences would include decreased pressure in the aorta and major arteries and decreased blood flow to tissues, causing fatigue and shortness of breath. Pressure in the left ventricle would increase, as would its work load. Blood could back up in the pulmonary circuit, causing pulmonary hypertension. Pulmonary valve stenosis would decrease blood flow to the lungs and therefore decrease delivery of oxygenated blood to the left heart, causing fatigue and shortness of breath. The pressure in the right ventricle would increase, as would pressure in the venous return vessels.
Digitalis is a drug that causes an increase in Ca2+ concentration in the cardiac muscle cells. Why is it used to treat patients with a weakened heart?
Contraction of the myocardium depends on the presence of Ca2+ ions in the sarcoplasm. Increasing Ca2+ in the sarcoplasm prolongs the duration of the Ca2+ pulse resulting from the cardiac action potential. Thus the heart beats more strongly, contractions last longer, and cardiac output increases.
Heart valve stenosis impedes blood flow, whereas heart valve prolapse allows blood to flow backward (regurgitation). Both of these conditions can be detected as heart murmurs when a stethoscope is used to listen to heart sounds. When in the cardiac cycle could a heart murmur indicate aortic valve stenosis, and when could it indicate aortic valve prolapse? Explain your answer.
Stenosis of the aortic valve should produce a heart murmur during systole, as that is when pressure in the left ventricle is pushing blood through the aortic valve. Prolapse of the aortic valve should create a heart murmur during diastole, as that is when greater pressure in the aorta is pushing blood back into the ventricle through the prolapsed aortic valve.
Next we will consider the composition of the blood and the characteristics of the vessels through which blood circulates around the body, illustrating once again how structure serves function. We will also consider the role of the lymphatic vessels that return interstitial fluid to the blood.