Structure of the Heart

Arteries

• Arteries are elastic vessels that transport blood away from the heart. The largest artery of the body is the aorta. The aorta originates from the heart and branches out into smaller arteries. The smallest arteries are called arterioles which branch into capillaries.

Veins• Veins are elastic vessels that transport blood to the heart. The

smallest veins in the body are called venules. They receive blood from the arteries via the arterioles and capillaries. The venules branch into larger veins which eventually carry the blood to the largest veins in the body, the vena cava. The blood is then transported from the vena cava to the right atrium of the heart.

Vein Size 1 millimeter to 1-1.5 centimeters in diameter.

• Most veins have one-way flaps called venous valves that prevent blood from flowing back and pooling in the lower extremities due to the effects of gravity. These are in-foldings of the tunica intima. The precise location of veins is much more variable from person to person than that of arteries.

Venous return

• During physical exercise, muscles contract and expand laterally.

• The intramuscular pressure exerted on the veins by the surrounding muscle pushes blood through the one-way valves of the veins, returning it to the heart. This pumping action keeps blood from pooling in the lower limbs, and individuals that stand still for extended periods of time can experience reduced venous return to the heart and low blood pressure (hypotension) leading to dizziness or fainting (syncope).

List of important named veinsJugular veinsPulmonary veinsPortal veinAzygos veinSuperior vena cavaInferior vena cavaIliac veinFemoral veinPopliteal veinGreat saphenous veinSmall saphenous vein

Capillaries• Capillaries are extremely small vessels located within the tissues

of the body that transport blood from the arteries to the veins. Capillary walls are thin and are composed of endothelium (a single layer of overlapping flat cells). Oxygen, carbon dioxide, nutrients and wastes are exchanged through the thin walls of the capillaries.

Capillary Exchange• The Starling equation is an

equation that illustrates the role of hydrostatic and oncotic forces (the so-called Starling forces) in the movement of fluid across capillary membranes.

• Capillary fluid movement may occur as a result of two processes:

• diffusion• filtration

Respiratory Diffusion

Sinusoids• The liver, spleen and bone marrow contain vessel structures called instead of

capillaries. Similar to capillaries sinusoids are composed of endothelium. The individual endothelial cells however do not overlap as in capillaries and are spread out. Oxygen, carbon dioxide, nutrients, proteins and wastes are exchanged through the thin walls of the sinusoids.

Where Is Your Heart?The heart lies behind the body of the sternum, extending from the 2nd rib to the fifth intercostal muscle. About two thirds of the heart lies to the left of the mid line with the remaining third to the right.

An average adult heart is about the shape and size of a closed fist. Like a valentine heart, yours is slightly pointed at the lower end. The pointed end is called the apex

Although your heart is hollow, it isn't empty. In an average adult, about 5 quarts (4.7 litres) of blood flow through the heart each minute.

Your heart's walls are made mostly of strong muscle, called the myocardium. The myocardium is the strongest, hardest-working muscle in your body. It continuously pumps your blood through 60,000 miles (96,560 kilometres) of blood vessels for a lifetime, without rest!

Heart Chambers

The hollow centre of your heart is divided into four sections, called chambers. Each chamber is like a separate room, with doors that let blood in and out.

Where Blood Flows In — The AtriaThe two upper chambers in your heart are called the atria . The atria

are the receiving chambers of your heart. When blood flows into your heart from the body or lungs, it always flows into either the right or left atrium—never anywhere else. (One upper chamber is

called an atrium. Both upper chambers together are called the atria.)

Where Blood Is Pumped Out — The VentriclesThe two lower chambers in your heart are called ventricles. The ventricles are

the pumping chambers of your heart. When blood leaves your heart, it is always pumped out from the ventricles—never from anywhere else. The

ventricles are very strong because they have to pump hard enough to push blood through your lungs and entire body.

Your Heart's Right and Left SidesSometimes the right and left sides of your heart are called your right heart and left heart. The right atrium and right ventricle are, of course, on the right side of

your heart (the same side as your right arm), and the left atrium and left ventricle are on the left side of your heart. However, when you look at a picture of the

heart, the right heart is on your left

A wall, called the septum, separates the left and right sides of your heart. Blood that hasn't yet been to the lungs (blood with no oxygen) stays on the right side of the septum. Blood returning from the lungs (blood with oxygen) stays on the left

side of the septum.

right left

right

• The superior vena cava is a large, yet short vein that carries de-oxygenated blood from the upper half of the body to the heart's right atrium.

• It is formed by the left and right brachiocephalic veins, (also referred to as the innominate veins) which receive blood from the upper limbs and the head and neck, behind the lower border of the first right costal cartilage.

• The azygous vein (which receives blood from the rib cage) joins it just before it enters the right atrium, at the upper right front portion of the heart.

• The inferior vena cava (or IVC) is the large vein that carries de-oxygenated blood from the lower half of the body into the heart.

• It is posterior to the abdominal cavity and runs along side of the vertebral column on its right side (i.e. it is a retroperitoneal structure). It enters the right atrium at the lower right, back side of the heart.

• The right atrium (in older texts termed the "right auricle") is one of four chambers (two atria and two ventricles) in the human heart. It receives de-oxygenated blood from the superior and inferior vena cavae and the coronary sinus, and pumps it into the right ventricle through the tricuspid valve.

• The coronary sinus is a collection of veins joined together to form a large vessel that collects blood from the myocardium of the heart

• The atria do not have valves at their inlets

Tricuspid Valve• The tricuspid valve prevents the blood from returning to

the right atrium when the right ventricle contracts • The tricuspid valve is composed of tough connective

tissue covered in endocardium (thin layers of cells lining the entire heart.)

• The tricuspid valve has three cusps and is Y’ shaped.• It is one of the ATROVENTRICUALR valves.

• The right ventricle is one of four chambers (two atria and two ventricles) in the human heart. It receives de-oxygenated blood from the right atrium via the tricuspid valve, and pumps it into the pulmonary artery via the pulmonary valve.

• It is triangular in form, and extends from the right atrium to near the apex of the heart.

• The pulmonary arteries carry blood from the heart to the lungs. They are the only arteries (other than umbilical arteries in the fetus) that carry deoxygenated blood.

• In the human heart, the pulmonary trunk (pulmonary artery or main pulmonary artery) begins at the base of the right ventricle. It is short and wide - approximately 5 cm (2 inches) in length and 3 cm (1.2 inches) in diameter. It then branches into two pulmonary arteries (left and right), which deliver deoxygenated blood to the corresponding lung.

• The pulmonary valve, also known as pulmonic valve, is the semilunar valve of the heart that lies between the right ventricle and the pulmonary artery and has three cusps

left

• The pulmonary veins carry oxygen-rich blood from the lungs to the left atrium of the heart. They are the only veins in the post-fetal human body that carry oxygenated (red) blood.

• The pulmonary veins return the oxygenated blood from the lungs to the left atrium of the heart.

• They are four in number, two from each lung, and are without valves. They are

• right inferior • right superior • left inferior • left superior

The left atrium is one of the four chambers in the human heart. It receives oxygenated blood from the pulmonary veins, and

pumps it into the left ventricle.

• Normal left atrium may be up to 5.5cm in maximum diameter; any larger than this is a sign of cardiac failure

• The mitral valve (also known as the bicuspid valve or left atrioventricular valve), is a dual flap (bi = 2) valve in the heart that lies between the left atrium (LA) and the left ventricle (LV).

• In Latin, the term mitral means shaped like a miter, or bishop's cap. The mitral valve and the tricuspid valve are known collectively as the atrioventricular valves because they lie between the atria and the ventricles of the heart and control flow.

• The left ventricle is one of four chambers (two atria and two ventricles) in the human heart. It receives oxygenated blood from the left atrium via the mitral valve, and pumps it into the aorta via the aortic valve.

• The aortic valve has three cusps. These cusps are half moon shaped hence also called aortic semilunar valve.

• Dilatation of the wall of the aorta behind these cusps is called aortic sinus. When the aortic valve is open, the normal size of the orifice is 3-4 cm² in adults.

The aorta is the largest artery in the human body, originating from the left ventricle of the heart and bringing oxygenated

blood to all parts of the body in the systemic circulation.

The aorta is an elastic artery, and as such is quite distensible. When the left ventricle contracts to force blood into the aorta, the aorta expands. This stretching gives the potential energy

that will help maintain blood pressure during diastole, as during

this time the aorta contracts passively.

• The course of the aorta

• The aorta is usually divided into five segments/sections

• Ascending aorta — the section between the heart and the arch of aorta

• Arch of aorta — the peak part that looks somewhat like an inverted "U"

• Descending aorta — the section from the arch of aorta to the point where it divides into the common iliac arteries

– Thoracic aorta — the half of the descending aorta above the diaphragm

– Abdominal aorta — the half of the descending aorta below the diaphragm

What Do Your Valves Do?Your heart valves keep blood flowing in one direction through your heart, just like the one-way valves in your home's plumbing. They open to let blood flow through, and then close to prevent blood from flowing back the way it came. When a valve closes, flaps of tissue on the valve close tightly together to create a seal. These flaps of tissue are called leaflets.

Where Are Your Heart Valves?Your heart has four valves. Blood flows through each valve one time on its way through your heart. The four valves can be grouped by their job.

Heart ValvesWhat Makes the Sound of Your Heartbeat?When you listen to your heartbeat through a stethoscope ("lubb-dubb lubb-dubb"), you hear the sound of your heart valves closing. Although your heart has four valves, the valves open and close two at a time. That's why you hear only two thumps (one "lubb-dubb") per heartbeat, rather than four.

• Systemic circulation is the portion of the cardiovascular system which carries oxygenated blood away from the heart, to the body, and returns deoxygenated blood back to the heart

• In the systemic circulation, arteries bring oxygenated blood to the tissues. As blood circulates through the body, oxygen diffuses from the blood into cells surrounding the capillaries, and carbon dioxide diffuses into the blood from the capillary cells. Veins bring deoxygenated blood back to the heart.

• Pulmonary circulation is the portion of the cardiovascular system which carries oxygen-depleted blood away from the heart, to the lungs, and returns oxygenated blood back to the heart. The term is contrasted with systemic circulation.