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Cardiovascular anatomy and physiology
The Circulatory System The circulatory system consists of the heart and blood vessels. Together, these provide a continuous flow of blood to the body, supplying the tissues with oxygen and nutrients.
Arteries
- Carry blood away from the heart and distribute it to all parts of the body. Oxygenated blood is pumped out of the heart through the body's main artery- the aorta. Arteries that branch off the aorta transport blood throughout the body, supplying tissues with oxygen and nutrients.
- Have thicker and stronger walls than those of Veins
Veins
- Carry blood back toward the heart. Tiny vessels called capillaries in organs and tissues of the body deliver deoxygenated blood into small veins called venules, which join to form veins. Blood flows through the veins to the body's 2 main veins (called the vena cavae), which deliver the blood back into the heart.
- Contain valves that prevent backflow.
Capillaries
- Are the smallest blood vessels.
- Are the sites of exchange.
- Are absent in the Cornea, Epidermis, and Hyaline Cartilage. May not be present in some areas, where arterioles and venules have direct connections (this arteriovenous anastomoses called AV shunts and are present in the Skin, of the Nose, Lips, Fingers, Ears, where they conserve body heat.
Circulatory loops
Pulmonary circulation
Pumps blood from the right Ventricle to the Lungs through the Pulmonary arteries and returns it to the left Atrium of the Heart through the Pulmonary veins.
Systemic circulation
Pumps blood from the left Ventricle through the Aorta to all parts of the body and returns it to the right Atrium through the Superior and Inferior Vena Cavae and the Cardiac Veins.
Blood Pressure
The pressure exerted on the walls of blood vessels by the flowing blood, also varies during different phases of the cardiac cycle. Blood pressure in the arteries is higher during systole, when the ventricles are contracting and lower during diastole, as the blood ejected during systole moves into the body's capillaries. Blood pressure is measured in millimeters (mm) of mercury using a sphygmomanometer, an instrument that consists of a pressure recording device and an inflatable cuff that is usually placed around the upper arm. Normal blood pressure in an adult is about 120 mm of mercury during systole, and about 80 mm of mercury during diastole. Blood pressure is usually noted as a ratio of systolic pressure to diastolic pressure-for example, 120/80. A person's blood pressure may increase for a short time during moments of stress or strong emotions. However, a prolonged or constant elevation of blood pressure, a condition known as hypertension, can increase a person's risk for heart attack, stroke, heart and kidney failure, and other health problems.
Heart
- Hollow muscular organ that pumps blood through the body.
- Weight about 1 pound and is shaped like an upside-down cone and is located inside the chest cavity (Thorax). About the size of a closed fist.
- Is the circulatory system's power supply.
Heart chambers
Heart has four chambers. The upper two chambers, the right and left atria, are receiving chambers for blood. They collect blood from veins. The heart's lower two chambers, the right and left ventricles, are the powerful pumping chambers. The ventricles pump blood into arteries. The atria have relatively thin walls compared to the ventricles.
Heart wall
Heart wall consists of three layers: outer - Epicardium, middle- Myocardium, and inner- Endocardium. Lining of endocardium helps blood flow smoothly and prevents blood clots from forming inside the circulatory system.
Double-layered fibrous sac Pericardium surrounds the heart. The outer layer of the pericardium is attached to the breastbone (Sternum) and other structures in the chest cavity and helps hold the heart in place. Between the two layers of the pericardium is a thin space filled with a watery fluid that helps prevent these layers from rubbing against each other when the heart beats.
The heart is separated by a wall of tissue (Septum) into right and left sides. Each side pumps blood through a different circuit of blood vessels.
Coronary blood supply
The Heart is nourished not by the blood passing through its chambers but by a specialized network of blood vessels (coronary arteries), these blood vessels encircle the heart like a crown. Three main coronary arteries-the right, the left circumflex, and the left anterior descending-nourish different regions of the heart muscle. Veins running through the heart muscle converge to form a large channel called the coronary sinus, which returns blood to the right atrium.
Heart Valves
The heart has four valves which help prevent blood from flowing backward in the heart. The valves open easily in the direction of blood flow. Two of the valves are located between the atria and ventricles, and are known as atrioventricular valves. The right atrioventricular valve is formed from three flaps of tissue and is called the tricuspid valve, while the left atrioventricular valve has two flaps and is called the bicuspid or mitral valve. The other two valves are located between the ventricles and arteries. They are called semilunar valves because they each consist of three half-moon-shaped flaps of tissue. The right semilunar valve, between the right ventricle and pulmonary artery, is also called the pulmonary valve. The left semilunar valve, between the left ventricle and aorta, is also called the aortic valve. Although the right and left halves of the heart are separate, they both contract in unison, producing a single heartbeat.
The Cardiac Cycle
The sequence of events from the beginning of one heartbeat to the beginning of the next is called the cardiac cycle. The cardiac cycle has two phases: diastole, when the heart's chambers are relaxed, and systole, when the chambers contract to move blood. During the systolic phase, the atria contract first, followed by contraction of the ventricles. During diastole, both atria and ventricles are relaxed, and the atrioventricular valves are open. Blood pours from the veins into the atria, and from there into the ventricles. Systole then begins as the atria contract to complete the filling of the ventricles. Next, the ventricles contract, forcing blood out through the semilunar valves and into the arteries, and the atrioventricular valves close to prevent blood from flowing back into the atria. As pressure rises in the arteries, the semilunar valves snap shut to prevent blood from flowing back into the ventricles. Diastole then begins again as the heart muscle relaxes-the atria first, followed by the ventricles-and blood begins to pour into the heart once more.
Stethoscope
An instrument known as a stethoscope is used to detect internal body sounds, including the sounds produced by the heart as it is beating. The characteristic heartbeat sounds are made by the valves in the heart-not by the contraction of the heart muscle itself. The sound comes from the leaflets of the valves slapping together. The closing of the atrioventricular valves, just before the ventricles contract, makes the first heart sound. The second heart sound is made when the semilunar valves snap closed. The first heart sound is generally longer and lower than the second, producing a heartbeat that sounds like "lub-dup, lub-dup, lub-dup."
Properties and characteristics of Cardiac Cells
- Myocardial Cells from hearts muscular layer contain contractile filaments and contract when stimulated electrically by responding to outside stimulus and ionic imbalance.
- Pacemaker Cells cannot contract and spontaneously generates and conducts electrical impulses.
- Excitability (irritability)
Electrical conduction system of the heart
Heart muscle can contract of its own accord. Certain heart muscle cells have the ability to contract spontaneously, and these cells generate electrical signals that spread to the rest of the heart and cause it to contract.
The heartbeat begins with a small group of specialized muscle cells (sinoatrial (SA) node) located in the upper right-hand corner of the right atrium. Cells in the SA node generate their electrical signals more frequently than cells elsewhere in the heart, so the electrical signals generated by the SA node synchronize the electrical signals traveling to the rest of the heart. For this reason, the SA node is also known as the heart's pacemaker. Impulses generated by the SA node spread rapidly throughout the atria, so that all the muscle cells of the atria contract virtually in unison. Electrical impulses cannot be conducted through the partition between the atria and ventricles, which is primarily made of fibrous connective tissue rather than muscle cells. The impulses from the SA node are carried across this connective tissue partition by a small bridge of muscle called the atrioventricular conduction system.
The first part of this system is a group of cells at the lower margin of the right atrium, known as the atrioventricular (AV) node. Cells in the AV node conduct impulses relatively slowly, introducing a delay of about two-tenths of a second before an impulse reaches the ventricles. This delay allows time for the blood in the atria to empty into the ventricles before the ventricles begin contracting.
After making its way through the AV node, an impulse passes along a group of muscle fibers called the bundle of His, which span the connective tissue wall separating the atria from the ventricles. Once on the other side of that wall, the impulse spreads rapidly among the muscle cells that make up the ventricles.
The impulse travels to all parts of the ventricles with the help of a network of fast-conducting fibers called Purkinje fibers. These fibers are necessary because the ventricular walls are so thick and massive. If the impulse had to spread directly from one muscle cell to another, different parts of the ventricles would not contract together, and the heart would not pump blood efficiently. Although this complicated circuit has many steps, an electrical impulse spreads from the SA node throughout the heart in less than one second. Transmission of the impulse from the SA node to the ventricles called depolarization, which is followed by repolarization or electric recovery, and after, heart is resting or in polarization phase of cardiac cycle. Then, the entire cycle starts again. This means, that cardiac cycle, representing heart beat consists from depolarization, repolarization, polarization (Atrial contraction, Ventricular contraction, recovery and rest). Normal rhythm is 70-80 BPM (beats per minute).
Ramaz Mitaishvili 09:48, 6 October 2007 (UTC)