The respitory system transports oxygen to the blood, removes CO₂ from the blood, regulates blood acidity, and helps to regulate body temperature. It is the endocrine system that secretes hormones into the blood.

The respiratory system is responsible for moving air into and out of the lungs, where gas exchange occurs between the alveoli and the pulmonary capillaries. The lungs exchange oxygen and carbon dioxide via alveoli and networks of small blood vessels known as capillaries. Production of gametes is a function of the reproductive system. Movement of blood and nutrients throughout the body is the primary function of the circulatory system. Movement and coordination is controlled by the nervous system and requires the structures of the muscular and skeletal systems.

Tidal volume is nominally and conceptually similar to an ocean tide. Under calm conditions, a tide is not particularly extreme at either its low or high points. During normal, relaxed respiration, an animal breathes in and out moderate volumes of air, creating its tidal volume. 

Blood enters the heart through the vena cavae into the right atrium. It flows through the right side of the heart, to the lungs, and back to the left side of the heart. When it arrives in the left ventricle, it is pumped into the aorta to be delivered to the body. The mitral valve separates the left atrium from the left ventricle, and is also known as the bicuspid valve. The sinoatrial node is the natural pacemaker of the heart. It is located in the right atrium and generated cardiac action potentials.

The cardiac sphincter divides the esophagus from the stomach, and is actually part of the digestive system.

In order for the entire heart to contract in unison, there needs to be a conduction pathway that sends an action potential throughout the entire heart muscle at once. There is a specialized group of cardiac cells responsible for initiating this action potential throughout the heart. This pacemaker structure is called the sinoatrial node.

The atrioventricular node and bundle of His are involved in coordinating and mediating the contraction of the heart, once it is initiated by the sinoatrial node. The atrioventricular septum is the muscular wall that divides the right and left sides of the heart.

When tracing blood flow through the heart, it is usually easiest to start at the vena cavae. Deoxygenated blood enters the right atrium through the superior and inferior vena cavae. From the right atrium it is pumped to the right ventricle and then to the pulmonary arteries, which carry it to the lungs for reoxygenation. After loading oxygen in the lungs, the blood returns to the heart through the pulmonary veins and enters the left atrium. From the left atrium it is pumped into the left ventricle, and then out of the heart into the aorta for systemic circulation. The blood travels through the body, and then returns to the vena cavae.

The tricuspid valve separates the right atrium from the right ventricle. After passing through this valve, blood will be in the right ventricle. The bicuspid, or mitral, valve separates the left atrium and ventricle.

The P wave of the electrocardiogram corresponds with atrial contraction (atrial systole). The pacemaker of the heart is the sinoatrial (SA) node. When the SA node stimulates the heart, it initiated atrial systole. The signal causes the P wave before traveling to the other regions of the conducting system of the heart. There is only one atrial pacemaker region, which ensures coordinated contraction.

The atrioventricular (AV) node, bundle of His, and Purkinje fibers are progressively lower in the conduction system and are not associated with P wave generation. They are involved in signal mediation and ventricular systole, which corresponds with the QRS complex.

The ejection fraction of a healthy heart is about 70% of its 100ml volume, or 70ml per stroke.

At a heart rate of 70 beats per minute, then approximately 5 liters is pumped by EACH side of the heart each minute.

Both the right and left sides of the heart must pump the same volumes since all blood from the right side returns to the left side after passing through the lungs.

The heart contains four chambers: two upper chambers, called atria, and two lower chambers, called ventricles. Valves are present between each of the chambers and prevent the backflow of blood into the previous chamber. The tricuspid valve prevents backflow from the right ventricle into the right atrium. The bicuspid valve prevents backflow from the left ventricle into the left atrium. The semilunar valves prevent backflow into the ventricles from the aorta and pulmonary arteries. The aortic valve prevents flow from the aorta to the left ventricle and the pulmonary valve prevents flow from the pulmonary artery to the right ventricle.

The pulmonary circuit is reponsible for carrying blood to and from the lungs. Blood enters the right atrium from the systemic circuit, it is then pumped into the right ventricle. From there it leaves the heart via the pulmonary arteries, and enters the pulmonary capillaries. Gas exchange occurs between the alveoli and pulmonary capillaries. Then, the blood returns to the heart via the pulmonary veins and enters the left atrium of the heart. From there, it is pumped into the left ventricle and out to the body through the aorta. Recall that the right side of the heart deals with the oxygen-poor blood returned from the systemic circulation; this same blood is then pumped to the lungs to become oxygen-rich. The oxygen-rich blood from the lungs comes to the left side of the heart, where it will be pumped to the body tissues.