The vagus nerve is a major nerve of the parasympathetic nervous system, responsible for mediating numerous responses in the body. In relation to the heart, the vagus nerve provides constant inhibition to the sinoatrial node, slowing the heart rate. The sinoatrial node naturally fires at about 80 to 100 beats per minute, while a healthy resting heart rate is closer to 60 due to innervation by the vagus nerve.

The radial nerve is located in the forearm; the femoral nerve is located in the thigh; the subcostal nerve is located along the lower ribs.

The heart beats automatically through stimulation from the sinoatrial node. The group of neurons found in this node depolarize in a coordinated, spontaneous manner to allow for the contraction of the atria and ventricles in the heart. Without the sinoatrial node, heart contraction may be possible, but it would not be regular and would require additional regulation.

Depolarization of the sinoatrial node specifically leads to atrial contraction. The signal is then transmitted through the atrioventricular node and bundle of His to the purkinje fibers, which coordinate the contraction of the ventricles.

In a fetus, there are three fetal shunts: the ductus arteriosus, the ductus venosus, and the foramen ovale. Failure to fully close the foramen ovale during birth will allow blood to shunt directly from the right atrium to the left atrium, diverting the blood from the lungs. The ductus arteriosus also shunts blood away from the fetal non-functional lungs, allowing it to pass directly from the pulmonary artery to the aorta. Additionally, after birth, the ductus arteriosus becomes the ligamentum arteriosum.

Conversely, the ductus venosus is a structure in the fetus that diverts blood away from the fetal liver.

The left atrium has a higher pressure than the right atrium. As a result of the hole between the atria, there will be a left-to-right shunt of blood in the heart. Blood would normally go from the left atrium into the left ventricle. The shunt will allow blood from the left atrium to return to the right atrium, instead of proceeding to the left ventricle. Because of the shunt, less blood will go into the left ventricle.

There are four principle heart valves, each classified as either an atrioventricular (AV) valve or a semilunar valve. The atrioventricular valves separate the atria from the ventricles, while the semilunar valves separate the ventricles from arteries.

The tricuspid valve prevents backflow between the right atrium and right ventricle. The bicuspid, or mitral, valve prevents backflow between the left atrium and left ventricle. The pulmonary semilunar valve separates the right ventricle from the pulmonary artery, and the aortic semilunar valve separates the left ventricle from the aorta.

The lungs remain non-functional in the developing fetus. Instead, the fetus receives oxygen via gas exchange between maternal and fetal circulation in the placenta. To prevent inefficient blood flow to the lungs, blood is shunted in two ways. The ductus arteriosus shunts blood directly from the pulmonary artery to the aorta, allowing it to bypass the pulmonary circuit. After birth, this duct quickly collapses to become the adult structure known as the ligamentum arteriosum. The second shunt is the foramen ovale, which allows blood to pass directly from the right atrium to the left atrium. In adults, this structure becomes the fossa ovalis.

The ductus venosus is not found in the heart, and is used to direct blood from the umbilical vein to the inferior vena cava. This allows the oxygenated blood (returning from the placenta through the umbilical vein) to bypass systemic circulation and enter directly into the heart. The heart can then direct the blood through circulation.

Systolic blood pressure measures the force of blood exiting the heart into the arteries during contraction. This can measure how strong the heart tissue is. Diastolic pressure indicates the pressure when the heart is relaxed. Healthy systolic blood pressue is 120mmHg, and diastolic is 80mmHg.

Cardiac output is defined as the volume of blood pumped per minute. The two factors that will directly affect the cardiac output will be stroke volume (volume pumped per beat) and heart rate (beats per minute).

Systole can apply to either te atria or the ventricles and refers to the period during which the chambers contract. Diastole, in contrast, described the period of relaxation. During diastole, blood fills the relaxed chambers. During systole, blood is forced out of the chambers as they contract. Systole and diastole are coordinated between chambers such that both atria contract together (atrial systole) and both ventricles contract together (ventricular systole) in a rhythm that allows coordinated filling and emptying of the chambers without backflow or disruption.

Note that systolic blood pressure refers to blood pressure during ventricular systole, and diastolic blood pressure to pressure during ventricular diastole.

The correct answer is "it is striated." Similiar to skeletal muscle, cardiac muscle is striated. It has only one nucleus per cell, and its action is involuntary. Futhermore, it is composed of sarcomeres, which give it its striated appearance.