The activity of the skeletal-muscle pump aids in the return of blood to the heart. Muscles contract, squeezing the veins that flow through them. Veins have a set of one-way valves that allow blood to flow through them when they are pinched. The valves then close to prevent backflow. When muscles relax, the valves open again and continue returning blood to the heart.
The two main veins that carry blood from the lower body back to the heart are called the vena cava and the azygos vein. They both begin in the chest area but differ in their paths forward. The vena cava passes behind the breastbone and continues down into the abdomen, where it meets the diaplastic vein (also called the inferior vena cava). This last vessel merges with the superior vena cava to form one large vessel that returns blood to the heart. So in answer to the question, "How is venous blood returned to the heart?" The skeletal muscle pump plus the azygos and diaplastic veins are responsible for this action.
The azygos vein begins in the chest area with the brachiocephalic vein. This initial vessel divides into two smaller ones just before they enter the thoracic cavity. These are the hemiazygos and accessory hemiazygos veins, which drain the upper half of the body.
It makes its way back to the heart and into the left atrium. Blood is then pumped via the mitral valve into the left ventricle. This is the muscle pump that distributes blood throughout the body. When the left ventricle contracts, blood flows into the aorta via the aortic semilunar valve. This is also called the systemic circulation because it supplies blood to all parts of the body through arteries branching off from the aorta.
The right ventricle pumps blood directly into the pulmonary artery which branches into two pulmonary veins that carry deoxygenated blood toward the lungs. Once the blood reaches the lungs, it becomes oxygenated so it can be returned to the heart for further distribution. The entire process begins again as the left ventricle contracts moving blood through the aorta and out to the rest of the body.
There are two types of cardiac valves: bicuspidal and tricuspidal. The four cusps of the bicuspidal valve meet at right angles when it opens, allowing flow into but not out of the ventricle. The three cusps of the tricuspidal valve meet at an angle instead; if you looked down on this valve from above, you would see a triangle with its base on the inside wall of the ventricle and its apex pointing outward.
Veins transport blood back to the heart. After releasing oxygen and other chemicals from the blood into bodily tissues, the capillaries return the blood to the veins. Veins are hollow tubes that carry blood away from the heart to these capillary beds where it will receive its final supply of oxygen and other nutrients before being pumped back to the heart.
Lymph vessels are a network of fluid-filled tubes that connect many of the organs in the body with lymph nodes. The lymphatic system is responsible for returning fluid and soluble proteins to the bloodstream through two distinct pathways: efferent vessels and afferent vessels. Efferent vessels carry lymph away from specific organs such as the gut or skin to lymph nodes. Afferent vessels bring lymph back to the main artery near their origin within the organ or tissue.
Blood vessels contain three different layers: intima, media, and adventitia. The intima is the thin layer just inside the blood vessel wall that contains small blood cells called endothelial cells that line the walls of the blood vessels. The media is the middle layer of the blood vessel which supplies muscle fibers to the blood vessel. The outermost layer of the blood vessel is the adventitia which provides support for the blood vessel. Large arteries such as those found in the neck and chest cavity have muscular walls made up of multiple layers of smooth muscle fiber.
The blood in the veins is kept from flowing backward by valves. When blood is not correctly returned to the heart, it accumulates, or "pools," and lingers in the veins. This is why you should never stop moving your body when lying down. Even if you are not active, your body still needs oxygen, so it stays awake even when you sleep.
The valves in the veins are made of strong, flexible rings of muscle and bone called leaflets. These leaflets function like gaskets: when they are closed, the vessel is sealed; when they are open, the vessel is open for blood to flow through.
There are three types of valves in the veins: muscular valves in large veins that allow blood to flow in only one direction; venous valves in smaller vessels that prevent blood from flowing back in reverse; and bicuspid venous valves that resemble a pair of lips with three tongues projecting into the vein. They function much like the two-leafed valve in an artery—when they are closed, the vessel is sealed off from both high pressure blood flow and low pressure blood flow; when they are open, only blood flowing in one direction will open up these valves.
Any condition that causes the muscles surrounding the veins to tighten will close the valves tightly, preventing blood from leaking out and building up inside the veins.