Answering this question requires you to know which organ assists in neutralizing acidic chyme once it enters the small intestine. This is the responsibility of the pancreas, which secretes bicarbonate ions. This helps raise the pH of the chyme in the small intestine, and prevents damage.

The heart has four valves, used to prevent backflow of blood during contraction of each chamber. The two atrioventricular valves separate the atria from the ventricles, and the two semilunar valves separate the ventricles from the exiting arteries. The right atrium and ventricle are separated by the tricuspid valve, while the left atrium and ventricle are separated by the bicuspid, or mitral, valve. The aortic semilunar valve separates the aorta from the left ventricle and the pulmonary semilunar valve separates the pulmonary arteries from the right ventricle.

Veins are responsible for returning blood to the heart, while arteries take blood away from the heart. The two veins listed are the pulmonary vein and the inferior vena cava. The pulmonary vein returns oxygenated blood to the heart and dumps it into the left atrium. The inferior vena cava returns deoxygenated blood to the heart and dumps it into the right atrium.

The heart has two ventricles which pump blood out of the heart and to either the lungs or the rest of the body. The right ventricle pumps blood into the pulmonary arteries which go to the lungs, and the left ventricle pumps blood into the aorta, which sends blood to the tissues of the body.

Many people have the misconception that arteries only carry oxygenated blood that has been pumped out fo the heart. The truth is that arteries are responsible for carrying all blood away from the heart, whether it be oxygenated or deoxygenated. For example, the aorta is an artery that carries oxygenated blood from the heart to the tissues, however, the pulmonary arteries carry deoxygenated blood from the heart to the lungs. Any vessel that travels away from the heart is classified as either an artery or an arteriole, regardless of the blood it contains.

Action potentials are spontaneously conducted so that the heart can pump automatically, without necessary stimulation from the central nervous system. These spontaneous action potentials are created by a group of cardiac cells called the sinoatrial node. Because it determines the heart rate, the sinoatrial node is considered the pacemaker of the heart.

After generation by the sinoatrial node, action potentials will cause the atria to contract and travel to the atrioventricular node. The atrioventricular node introduces a delay, which prevents the ventricles from contracting during atrial systole, which could push blood backward from the ventricle to the atrium. The atria relax and the signal is passed from the atrioventricular node to the bundle of His in the atrioventricular septum before spreading to the ventricles and causing ventricular systole.

Blood vessels can be constricted or dilated in order to adjust blood pressure and reroute blood to areas in need of nutrients and oxygen. This constriction is done by smooth muscle, which is primarily found wrapped around arterioles.

Arteries also have a thick lining of smooth muscle, but are generally too large in diameter to be useful in directing blood flow. Capillaries have no smooth muscle and cannot be used to direct blood. Venules may have small layers of smooth muscle, but are not nearly as effective as arterioles.

Before muscle contraction can take place, tropomyosin must be removed from the active site on actin, so that myosin heads can attach. Calcium ions are responsible for attaching to troponin, which will then pull tropomyosin away from the active sites. These calcium ions are stored in the sarcoplasmic reticulum until an action potential stimulates their release.

T-tubules serve to conduct the action potential to the interior of the muscle fiber, allowing for coordinated contraction of sarcomeres throughout the fiber. The sarcolemma is simply the cell membrane of the muscle fiber. Myosin is the filament responsible for binding actin, but does not directly interact with calcium.

Bone tissue is formed by four bone cell types. Osteogenic cells are the progenitor stem cells that differentiate into osteoblasts. Osteoblasts are responsible for creating bone matrix by depositing hydroxyapatite crystal. They will eventually become encapsulated by the bone matrix, and differentiate into osteocytes. Osteocytes are primarily involved in communication and nutrient transfer within the bond matrix. Osteoclasts perform the opposite action osteoblasts and resorb the bone matrix. This process increases mineral concentrations in the blood.

Muscle cells are considered quiescent, and are incapable of mitosis. Instead, muscle mass will increase by hypertrophy (cell growth without division). In the event of damage, muscle satellite cells will differentiate into new myocytes, but the mature myocytes will not divide.

Only skeletal muscle and cardiac muscle contain organized sarcomeres, leading to their striated appearance. Smooth muscle does not contain sarcomeres and does not appear striated. Instead, actin and myosin align in multiple directions, allowing non-linear contraction in smooth muscle. Smooth muscle and cardiac muscle are generally uninucleated, but skeletal muscle cells contain numerous nuclei.