Circulatory System - Biology

Deoxygenated blood returning from the body is carried in the vena cava, which empties into the right atrium. Blood passes through the tricuspid valve and enters the right ventricle. Then, the deoxygenated blood is carried to the lungs by the pulmonary arteries. Once in the lungs, the blood becomes oxygenated.

The oxygenated blood is carried back to the heart by the pulmonary veins. The blood enters the left atrium and passes through the bicuspid (mitral) valve to enter the left ventricle. From the left ventricle it is pumped out of the heart and into the aorta to be circulated through the body and deliver oxygen to the tissues

Always remember that the pulmonary arteries carry DEOXYGENATED blood and the pulmonary veins carry OXYGENATED blood. This is the exception to the rule that states that veins carry deoxygenated blood and arteries carry oxygenated blood.

Arteries are responsible for carrying blood to organs, traveling away from the heart, while veins are responsible for transporting blood back toward the heart. Arteries generally carry oxygenated blood to deliver oxygen to organs, and veins generally carry deoxygenated blood back to the heart for re-oxygenation. The singular exceptions are the pulmonary arteries and pulmonary veins. The pulmonary arteries still travel away from the heart, but carry deoxygenated blood from the right atrium to the lungs. The pulmonary veins still travel toward the heart, but carry oxygenated blood from the lungs to the left atrium.

A common misconception is that veins always carry deoxygenated blood. Veins always return blood to the heart, but the pulmonary veins are bringing oxygenated blood back to the heart so that it may be pumped to the body's tissues. Deoxygenated blood travels through the right ventricle to the pulmonary arteries. In the lungs it is reoxygenated, then returned to the heart in the pulmonary veins.

Blood is able to flow through the blood vessels due to pressure caused by the pumping of the heart. The pressure is the greatest in vessels just leaving the heart (arteries), and decreases as the blood vessels get further away from the heart. Veins have the lowest pressures out of all the blood vessels.

Arterial walls have three distinct layers: an inner smooth endothelium, a heavy coat of smooth muscle cells and elastic fibers, and a looser outer layer of connective tissue. Arteries are uniformly recognizable as robust, thick-walled structures compared with nearby veins.

Although some veins have muscular fibers in their walls (for example, those in the legs), this layer of cells is thin and often incomplete. Instead of muscle to counteract gravity, veins usually use valves to prevent blood from flowing backward. Arteries, veins, capillaries, and the heart chambers all have an endothelial layer.

In fetal mammals, oxygenated blood returns to the fetus via the umbilical veins. It then drains into the right side of the heart. There would be no use in sending oxygenated blood into non-aerated lungs, so it is diverted through a channel, the ductus arteriosus, into the aorta of the fetus.

At birth, this structure closes rapidly, allowing blood to pass the lungs of the newborn. A residual band of connective tissue, the ligamentum arteriosum, marks the site of the original structure. The foramen ovale is a developmental aperture between the atria, and it usually closes spontaneously as the heart develops.

The circulatory system of the body is composed of blood vessels. There are three main types of blood vessels: the arteries, the capillaries, and the veins.

Arteries always carry blood away from the heart. Usually the blood is oxygenated; the exceptions are the pulmonary arteries, which carry blood away from the heart to the lungs to become oxygenated. Veins carry deoxygenated blood towards to heart from tissue, with the exception of the pulmonary veins, which carry oxygenated blood from the lungs to the heart. Capillaries allow for exchange between blood and tissue.

Begin by remembering that arteries carry blood away from the heart. Veins carry blood from the body back to the heart. Note that the distinction between the blood in arteries and veins does not lie in the oxygen-content. Rather, it lies in the direction of flow. Arteries always take blood away from the heart, whereas veins always take blood back to the heart.

Arteries carry blood away from the heart. In all but one case, arteries carry oxygen-rich blood. The exception is the pulmonary arteries. They carry oxygen-poor blood away from the heart, to the lungs, to pick up more oxygen. Veins carry blood back to the heart. In all but one case, veins carry oxygen-poor blood. The exception is the pulmonary veins. They carry oxygen-rich blood from the lungs back to the heart.

Veins are the only circulatory vessels that contain valves. These structures contain valves because much of the pumping pressure that originated in the heart has been reduced by the time blood reaches veins, which return deoxygenated blood to the heart. The reduction of force can cause the blood to pool or move very slowly. Since veins rely mostly on gravity, blood flow force (originating from the heart), and muscular contractions to squeeze blood through them, it makes sense for them to have valves to keep blood from flowing backwards in between muscular contractions.

The pulmonary arteries are the only arteries that contain oxygen-poor blood because it carries blood into the lungs to be oxygenated. By definition, arteries carry blood away from the heart and veins carry blood to the heart. However, the pulmonary vein is the only vein in the body that carries oxygen-rich blood (from the lungs back to the heart to be pumped into the rest of the body) and the pulmonary artery is the only artery in the body that carries oxygen-poor blood (to the lungs to be oxygenated before being sent to the heart to be pumped into the rest of the body. The carotid artery is in the neck and delivers oxygen-rich blood to head. The femoral artery is in the leg and delivers oxygen-rich blood to the leg. The aorta is the main artery that delivers blood from the left ventricle of the heart to the rest of the body.

Blood types are named according to the antigens present on the surface of a person's red blood cells, specifically the ABO and Rh antigens. AB+ blood has all three antigens and therefore none of the antibodies, meaning it can receive any blood type without having an immune reaction to the donor blood.

O- blood is unique from the other blood types due to its lack of ABO and Rh antigens. Unlike A and B, O is used to denote no antigens rather than an "O type" of antigen. As a result, it can be given to any other blood type since there are no antigens on the red blood cells to have an immune reaction the recipients own natural antibodies.

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 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 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 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.