Blood Circulation

BLOOD CIRCULATION | General| Pulmonary Circulation| Systematic Circulation| Hepatic Portal System Circulation| Coronary Circulation |

General. The blood system of vertebrates is a closed system, i.e. the blood vessels are not open -ended. In the organs the arteries divide to form arterioles (small arteries) which in turn divide to form numerous capillaries. The capillaries unite to form larger vessels, called the venules (small veins). The venules leave the tissues and join with other venules to form veins. Veins leave the organs and eventually join the venae cavae. It is thus clear that the arterial and venous blood link up by means of capillaries in the tissues. In this section we will discuss the main circulatory system. Pulmonary Circulation. The pulmonary artery arises from the right ventricle and tranports deoxygenated blood (oxygen-poor) to the lungs, where the blood becomes oxygenated again. The four pulmonary veins return the oxygenated blood (oxygen-rich) to the left atrium of the heart. The pulmonary circulation is also referred to as the lesser circulation. The summary of the pulmonary circulation is thus: Right Ventricle - pulmonary artery - lungs - pulmonary veins - left atrium - left ventricle.

Pulmonary and Systemic Circulation

Systemic Circulation.

Oxygenated blood is pumped from the left ventricle into the aorta. Branches of the aorta convey blood to all the tissues and organs of the body (except the lungs). The tissue cells are oxygenated and deoxygenated blood returned to the heart via the superior and inferior venae cavae. The blood then flows via the tricuspid valve into the right ventricle, from where it joins the pulmonary circulation. The systemic circulation is also referred to as the greater circulation. The summary of the systemic circulation is thus: Left ventricle - aorta - organs - venae cavae - right atrium - left ventricle. The Hepatic Portal System of Circulation. This system serves the intestines, spleen, pancreas and gall bladder. The liver receives it blood from two main sources. The main sources are the hepatic artery, which as a branch of the aorta, supplies oxygenated blood to the liver and the hepatic portal vein, which is formed by the union of veins from the spleen, the stomach, pancreas, duodenum and the colon. The hepatic portal vein transports, inter alia, the following blood to the liver: •

absorbed nutrients from the duodenum;

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white blood cells (added to the circulation) from the spleen;

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poisomous substances, such as alcohol which are absorbed in the intestines, and

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waste products, such as carbon dioxide from the spleen, pancreas, stomach and duodenum.

The hepatic artery and hepatic portal vein open into the liver sinuses where the blood is in direct contact with the liver cells. The deoxygenated blood, which still retains some dissolved nutrients, eventually flows into the inferior vena cava via the hepatic veins.Coronary Circulation. This circulation supplies the heart muscle itself with oxygen and nutrients and conveys carbon dioxide and other waste products away from the heart. Two coronary arteries lead from the aorta to the heart wall, where they branch off and enter the heart muscle. The blood is returned from the heart muscle to the right atrium through the coronary vein, which enters the right atrium through the coronary sinus. The heart and circulatory system (also called the cardiovascular system) make up the network that delivers blood to the body's tissues. With each heartbeat, blood is sent throughout our bodies, carrying oxygen and nutrients to all of our cells. Each day, 2,000 gallons (more than 7,570 liters) of blood travel many times through about 60,000 miles (96,560 kilometers) of blood vessels that branch and cross, linking the cells of our organs and body parts. From the hard-working heart, to our thickest arteries, to capillaries so thin that they can only be seen through a microscope, the cardiovascular system is our body's lifeline.

What Is the Cardiovascular System? The circulatory system is composed of the heart and blood vessels, including arteries, veins, and capillaries. Our bodies actually have two circulatory systems: The pulmonary (pronounced: pul-muhner-ee) circulation is a short loop from the heart to the lungs and back again, and the systemic

(pronounced: sis-teh-mik) circulation (the system we usually think of as our circulatory system) sends blood from the heart to all the other parts of our bodies and back again.

The heart is the key organ in the circulatory system. As a hollow, muscular pump, its main function is to propel blood throughout the body. It usually beats from 60 to 100 times per minute, but can go much faster when it needs to. It beats about 100,000 times a day, more than 30 million times per year, and about 2.5 billion times in a 70-year lifetime.

The heart gets messages from the body that tell it when to pump more or less blood depending on a person's needs. When we're sleeping, it pumps just enough to provide for the lower amounts of oxygen needed by our bodies at rest. When we're exercising or frightened, the heart pumps faster to get more oxygen to our bodies.

The heart has four chambers that are enclosed by thick, muscular walls. It lies between the lungs and just to the left of the middle of the chest cavity. The bottom part of the heart is divided into two chambers called the right and left ventricles (pronounced: ven-trih-kulz), which pump blood out of the heart. A wall called the interventricular septum (pronounced: in-tur-ven-trih-kyoo-lur septum) divides the ventricles.

The upper part of the heart is made up of the other two chambers of the heart, called the right and left atria (pronounced: ay-tree-uh). The right and left atria receive the blood entering the heart. A wall called the interatrial septum (pronounced: in-tur-ay-tree-ul sep-tum) divides the atria, and they're separated from the ventricles by the atrioventricular (pronounced: ay-tree-oh-ven-trihkyoo-lur) valve. The tricuspid valve separates the right atrium from the right ventricle, and the mitral (pronounced: my-trul) valve separates the left atrium and the left ventricle.

Two other heart valves separate the ventricles and the large blood vessels that carry blood leaving the heart. These valves are called the pulmonic valve, which separates the right ventricle from the pulmonary artery leading to the lungs, and the aortic (pronounced: a-or-tik) valve, which separates the left ventricle from the aorta, the body's largest blood vessel.

Blood vessels carrying blood away from the heart are called arteries (pronounced: ar-tuh-reez). They are the thickest blood vessels, with muscular walls that contract to keep the blood moving away from the heart and through the body. In the systemic circulation, oxygen-rich blood is pumped from the heart into the aorta. This huge artery curves up and back from the left ventricle, then heads down in front of the spinal column into the abdomen. Two coronary (pronounced: kor-uh-ner-ee) arteries branch off at the beginning of the aorta and divide into a network of smaller arteries that provide oxygen and nourishment to the muscles of the heart.

Unlike the aorta, the body's other main artery, the pulmonary artery, carries oxygen-poor blood. From the right ventricle, the pulmonary artery divides into right and left branches, on the way to the lungs where blood picks up oxygen.

Arterial walls have three layers:

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The endothelium (pronounced: en-doh-thee-lee-um) is on the inside and provides a smooth lining for blood to flow over as it moves through the artery.

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The media (pronounced: me-dee-uh) is the middle part of the artery, made up of a layer of muscle and elastic tissue.

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The adventitia (pronounced: ad-ven-tih-shuh) is the tough covering that protects the outside of the artery.

As they get farther from the heart, the arteries branch out into arterioles (pronounced: ar-teer-eeolz), which are smaller and less flexible.

Blood vessels that carry blood back to the heart are called veins (pronounced: vaynz). They are not as muscular as arteries, but they contain valves that prevent blood from flowing backward. Veins have the same three layers that arteries do, but they are thinner and less flexible. The two largest veins are the superior and inferior vena cavae (pronounced: vee-nuh kay-vee). The terms superior and inferior do not mean that one vein is better than the other, but that they are located above (superior) and below (inferior) the heart.

A network of tiny capillaries (pronounced: kah-puh-ler-eez) connects the arteries and veins. Even though they're tiny, the capillaries are one of the most important parts of the circulatory system because it is through them that nutrients and oxygen are delivered to the cells. In addition, waste products such as carbon dioxide are also removed by the capillaries.

What Do the Heart and Circulatory System Do? The circulatory system works closely with other systems in our bodies. It supplies oxygen and nutrients to our bodies by working with the respiratory system. At the same time, the circulatory system helps carry waste and carbon dioxide out of the body. Hormones - produced by the endocrine system - are also transported through the blood in our circulatory system. As the body's chemical messengers, hormones transfer information and instructions from one set of cells to another.

Did you ever wonder about the process behind your beating heart? A healthy heart makes a "lub-dub" sound with each beat. Here's what happens to make that sound: One complete heartbeat makes up a cardiac cycle, which consists of two phases. In the first phase, the ventricles contract (this is called systole, pronounced: sis-tuh-lee), sending blood into the pulmonary and systemic circulation. To prevent the flow of blood backwards into the atria during systole, the atrioventricular valves close, creating the first ("lub") sound.

When the ventricles finish contracting, the aortic and pulmonic valves close to prevent blood from flowing back into the ventricles. This is what creates the second sound (the "dub"). Then the ventricles relax (this is called diastole, pronounced: dye-as-tuh-lee) and fill with blood from the atria, which makes up the second phase of the cardiac cycle.

A unique electrical system in the heart causes it to beat in its regular rhythm. The sinoatrial (pronounced: sy-no-a-tree-ul) or SA node, a small area of tissue in the wall of the right atrium, sends out an electrical signal to start the contracting of the heart muscle. These electrical impulses cause the

atria to contract first; they then travel down to the atrioventricular or AV node, which acts as a kind of relay station. From here the electrical signal travels through the right and left ventricles, causing them to contract and force blood out into the major arteries.

In the systemic circulation, blood travels out of the left ventricle, to the aorta, to every organ and tissue in the body, and then back to the right atrium. The arteries, capillaries, and veins of the systemic circulatory system are the channels through which this long journey takes place. Once in the arteries, blood flows to smaller arterioles and then to capillaries. While in the capillaries, the bloodstream delivers oxygen and nutrients to the body's cells and picks up waste materials. Blood then goes back through the capillaries into venules, and then to larger veins until it reaches the vena cavae. Blood from the head and arms returns to the heart through the superior vena cava, and blood from the lower parts of the body returns through the inferior vena cava. Both vena cavae deliver this oxygen-depleted blood into the right atrium. From here the blood exits to fill the right ventricle, ready to be pumped into the pulmonary circulation for more oxygen.

In the pulmonary circulation, blood low in oxygen but high in carbon dioxide is pumped out the right ventricle into the pulmonary artery, which branches off in two directions. The right branch goes to the right lung, and vice versa. In the lungs, the branches divide further into capillaries. Blood flows more slowly through these tiny vessels, allowing time for gases to be exchanged between the capillary walls and the millions of alveoli (pronounced: al-vee-uh-lie), the tiny air sacs in the lung. During the process called oxygenation, oxygen is taken up by the bloodstream. Oxygen locks onto a molecule called hemoglobin in the red blood cells. The newly oxygenated blood leaves the lungs through the pulmonary veins and heads back to the heart. It enters the heart in the left atrium, then fills the left ventricle so it can be pumped into the systemic circulation.

Things That Can Go Wrong With the Heart and Circulatory System Problems with the cardiovascular system are common - more than 64 million Americans have some type of cardiac problem. But cardiovascular problems don't just affect older people - many heart and circulatory system problems affect teens, too.

Heart and circulatory problems are grouped into two categories: congenital, which means the problems were present at birth, and acquired, which means that the problems developed some time when a person was a kid or teen.

Congenital (pronounced: kun-jeh-nuh-tul) heart defects. Congenital heart defects are heart problems that babies have at birth. Congenital heart defects occur while a baby is developing in the mother's uterus. Doctors don't always know why congenital heart defects occur - some congenital heart defects are caused by genetic disorders, but most are not. A common sign of a congenital heart

defect is a heart murmur. A heart murmur is an abnormal sound (like a blowing or whooshing sound) that's heard when listening to the heart. Lots of kids and teens have heart murmurs, which can be caused by congenital heart defects or other heart conditions.

Arrhythmia. Cardiac arrhythmias (pronounced: a-rith-mee-uz), which are also called dysrhythmias or rhythm disorders, are problems in the heart's rhythm. Arrhythmias may be caused by a congenital heart defect or a person may develop this condition later. An arrhythmia may cause the heart's rhythm to be irregular, abnormally fast, or abnormally slow. Arrhythmias can happen at any age and may be discovered when a teen has a checkup.

Cardiomyopathy. Cardiomyopathy (pronounced: kar-dee-oh-my-ah-puh-thee) is a long-lasting disease that causes the heart muscle (the myocardium) to become weakened. Usually, the disease first affects the lower chambers of the heart, the ventricles, and then progresses and damages the muscle cells and even the tissues surrounding the heart. Some kids and teens with cardiomyopathy may receive heart transplants to treat their condition.

Coronary artery disease. Coronary artery disease is the most common heart disorder in adults, and it's caused by atherosclerosis (pronounced: ah-thuh-ro-skluh-ro-sis). Deposits of fat, calcium, and dead cells form on the inner walls and clog up the body's arteries (the blood vessels that supply the heart) and get in the way of the smooth flow of blood. A clot of blood may even form, which can lead to a heart attack. Heart attacks are very rare in children and teens.

Hyperlipidemia/hypercholesterolemia (high cholesterol). Cholesterol is a waxy substance that is found in the body's cells, in the blood, and in some of the foods we eat. Having too much cholesterol in the blood, also known as hypercholesterolemia (pronounced: high-pur-kuh-les-tuh-ruhlee-me-uh) or hyperlipidemia (pronounced: high-pur-lih-puh-dee-me-uh), is a major risk factor for heart disease and can lead to a heart attack. About one out of 10 teens between 12 and 19 years old have high cholesterol levels that put them at increased risk of cardiovascular disease.

Hypertension (high blood pressure). Hypertension (pronounced: high-pur-ten-shun) is when a person has blood pressure that's significantly higher than normal. Over time, it can cause damage to the heart and arteries and other body organs. Teens can have high blood pressure, which may be caused by genetic factors, excess body weight, diet, lack of exercise, and diseases such as heart disease or kidney disease.

Rheumatic heart disease. Teens who have had strep throat infection may develop rheumatic (pronounced: roo-ma-tik) fever. This type of infection can cause permanent heart problems, mostly in kids and teens between 5 and 15 years of age. People who've had strep throat and received antibiotics right away are unlikely to develop this problem.

So what can you do to halt heart and circulatory problems before they start? Getting plenty of exercise, eating a nutritious diet, maintaining a healthy weight, and seeing your doctor regularly for medical checkups are the best ways to help keep the heart healthy and avoid long-term problems like high blood pressure, high cholesterol, and heart disease.

Reviewed by: Steven Dowshen, MD Date reviewed: April 2007