Location of the Heart
The center of the circulatory system is the heart, which is the main pumping mechanism. The heart is made of muscle. The heart is shaped something like a cone, with a pointed bottom and a round top. It is hollow so that it can fill up with blood. An adult’s heart is about the size of a large orange and weighs a little less than a pound.
The heart is in the middle of the chest. It fits snugly between the two lungs. It is held in place by the blood vessels that carry the blood to and from its chambers. The heart is tipped somewhat so that there is a little more of it on the left side than on the right. The pointed tip at the bottom of the heart touches the front wall of the chest.

Structure of the Heart

If you looked inside your heart, you would see that a wall of muscle divides it down the middle, into a left half and a right half. The muscular wall is called a septum. The septum is solid so that blood cannot flow back and forth between the left and right halves of the heart. Another wall separates the rounded top part of the heart from the cone-shaped bottom part. So there are actually four chambers (spaces) inside the heart. Each top chamber is called an atrium (plural: atria). The bottom chambers are called ventricles. The atria are often referred to as holding chambers, while the ventricles are called pumping chambers. Thus, each side of the heart forms its own separate system, a right heart and a left heart. Each half consists of an atrium and a ventricle, and blood can flow from the top chamber to the bottom chamber, or ventricle, but not between the two sides.

The Valves
Blood can flow from the atria down into the ventricles because there are openings in the walls that separate them. These openings are called valves because they open in one direction like trapdoors to let the blood pass through. Then they close, so the blood cannot flow backwards into the atria. With this system, blood always flows in only one direction inside the heart. There are also valves at the bottom of the large arteries that carry blood away from the heart: the aorta and the pulmonary artery. These valves keep the blood from flowing backward into the heart once it has been pumped out.

Branching Blood Vessels
The heart is a pump whose walls are made of thick muscle. They can squeeze (contract) to send blood rushing out. The blood does not spill all over the place when it leaves the heart. Instead, it flows smoothly in tubes called blood vessels.

Arteries:

First, the blood flows into tubes called arteriesThe arteries leaving the heart are thick tubes. But the arteries soon branch again and again to form smaller and smaller tubes.
They are not just tubes through which the blood flows. Both arteries and veins have layers of smooth muscle surrounding them. Arteries have a much thicker layer, and many more elastic fibers as well. The largest artery, the aorta leaving the heart, also has cardiac muscle fibers in its walls for the first few inches of its length immediately leaving the heart.
Arteries have to expand to accept the blood being forced into them from the heart, and then squeeze this blood on to the veins when the heart relaxes. Arteries have the property of elasticity, meaning that they can expand to accept a volume of blood, then contract and squeeze back to their original size after the pressure is released.
for Example....A good way to think of them is like a balloon. When you blow into the balloon, it inflates to hold the air. When you release the opening, the balloon squeezes the air back out. It is the elasticity of the arteries that maintains the pressure on the blood when the heart relaxes, and keeps it flowing forward. if the arteries did not have this property, your blood pressure would be more like 120/0, instead of the 120/80 that is more normal. Arteries branch into arterioles as they get smaller. Arterioles eventually become capillaries, which are very thin and branching.




Capillaries:
The smallest blood vessels, called capillaries, form a fine network of tiny vessels throughout the body. Capillaries are really more like a web than a branched tube, having extremely thin walls so that the blood that they carry can come into close contact with the body tissues. The tiny red blood cells can then pass easily through the walls of the capillaries to deliver the oxygen they carry to nearby cells. As the blood flows through the capillaries, it also collects carbon dioxide waste from the body cells. The capillaries containing carbon dioxide return this used blood to the heart through a different series of branching tubes:




Veins

The capillaries join together to form small veins. The veins, in turn, unite with each other to form larger veins until the blood from the body is finally collected into the large veins that empty into the heart.



So the blood vessels of the body carry blood in a circle: moving away from the heart in arteries, traveling to various parts of the body in capillaries, and going back to the heart in veins. The heart is the pump that makes this happen.Veins do not have as many elastic fibers as arteries. Veins do have valves, which keep the blood from pooling and flowing back to the legs under the influence of gravity. When these valves break down, as often happens in older or inactive people, the blood does flow back and pool in the legs. The result is varicose veins, which often appear as large purplish tubes in the lower legs.

The Pulmonary and Systemic Circuits and the Blood Supply to the Heart. (Circulatory system) ( a Repeated Question in Css Exams)

The heart is responsible for pumping the blood to every cell in the body. It is also responsible for pumping blood to the lungs, where the blood gives up carbon dioxide and takes on oxygen. The heart is able to pump blood to both regions efficiently because there are really two separate circulatory circuits with the heart as the common link. Some authors even refer to the heart as two separate hearts--a right heart in the pulmonary circuit and left heart in the systemic circuit.
In the pulmonary circuit, blood leaves the heart through the pulmonary arteries, goes to the lungs, and returns to the heart through the pulmonary veins.
There are four chambers in the heart - two atria and two ventricles. The atria (one is called an atrium) are responsible for receiving blood from the veins leading to the heart. When they contract, they pump blood into the ventricles. However, the atria do not really have to work that hard. Most of the blood in the atria will flow into the ventricles even if the atria fail to contract. It is the ventricles that are the real workhorses, for they must force the blood away from the heart with sufficient power to push the blood all the way back to the heart (this is where the property of contracting with more force when stretched comes into play).



The muscle in the walls of the ventricles is much thicker than the atria. The walls of the heart are really several spirally wrapped muscle layers. This spiral arrangement results in the blood being wrung from the ventricles during contraction.

Between the atria and the ventricles are valves, overlapping layers of tissue that allow blood to flow only in one direction. Valves are also present between the ventricles and the vessels leading from it.
In the systemic circuit, blood leaves the heart through the aorta, goes to all the organs of the body through the systemic arteries, and then returns to the heart through the systemic veins. Thus there are two circuits. Arteries always carry blood away from the heart and veins always carry blood toward the heart. Most of the time, arteries carry oxygenated blood and veins carry deoxygenated blood. There are exceptions. The pulmonary arteries leaving the right ventricle for the lungs carry deoxygenated blood and the pulmonary veins carry oxygenated blood. If you are confused as to which way the blood flows through the heart, try this saying
"When it leaves the right, it comes right back, but when it leaves the left, it's left."

Fig Below: shows the Blood Path......



The blood does not have to travel as far when going from the heart to the lungs as it does from the heart to the toes. It makes sense that the heart would be larger on one side than on the other. When you look at a heart, you see that the right side of the heart is distinctly smaller than the left side, and the left ventricle is the largest of the four chambers.

While you might think the heart would have no problem getting enough oxygen-rich blood, the heart is no different from any other organ. It must have its own source of oxygenated blood. The heart is supplied by its own set of blood vessels. These are the coronary arteries. There are two main ones with two major branches each. They arise from the aorta right after it leaves the heart. The coronary arteries eventually branch into capillary beds that course throughout the heart walls and supply the heart muscle with oxygenated blood. The coronary veins return blood from the heart muscle, but instead of emptying into another larger vein, they empty directly into the right atrium.

Circulation in General: