The Circulatory System: The Heart and Circulation of Blood

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. Every time the heart beats it goes “thump” against the chest wall. We  can feel the thumps if we press there with our hands. We  can also listen to them with our ear.


If you look 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. First, the blood flows into tubes called arteries. The arteries leaving the heart are thick tubes. But the arteries soon branch again and again to form smaller and smaller tubes. The smallest blood vessels, called capillaries, form a fine network of tiny vessels throughout the body. The capillaries have 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: 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.

The Circulation of Blood
The human circulatory system is really a two-part system whose purpose is to bring oxygen-bearing blood to all the tissues of the body. When the heart contracts it pushes the blood out into two major loops or cycles. In the systemic loop, the blood circulates into the body’s systems, bringing oxygen to all its organs, structures and tissues and collecting carbon dioxide waste. In the pulmonary loop, the blood circulates to and from the lungs, to release the carbon dioxide and pick up new oxygen. The systemic cycle is controlled by the left side of the heart, the pulmonary cycle by the right side of the heart. Let’s look at what happens during each cycle:
The systemic loop begins when the oxygen-rich blood coming from the lungs enters the upper left chamber of the heart, the left atrium. As the chamber fills, it presses open the mitral valve and the blood flows down into the left ventricle. When the ventricles contract during a heartbeat, the blood on the left side is forced into the aorta. This largest artery of the body is an inch wide. The blood leaving the aorta brings oxygen to all the body’s cells through the network of ever smaller arteries and capillaries. The used blood from the body returns to the heart through the network of veins. All of the blood from the body is eventually collected into the two largest veins: the superior vena cava, which receives blood from the upper body, and the inferior vena cava, which receives blood from the lower body region. Both venae cavae empty the blood into the right atrium of the heart.
From here the blood begins its journey through the pulmonary cycle. From the right atrium the blood descends into the right ventricle through the tricuspid valve. When the ventricle contracts, the blood is pushed into the pulmonary artery that branches into two main parts: one going to the left lung, one to the right lung. The fresh, oxygen-rich blood returns to the left atrium of the heart through the pulmonary veins.
Although the circulatory system is made up of two cycles, both happen at the same time. The contraction of the heart muscle starts in the two atria, which push the blood into the ventricles. Then the walls of the ventricles squeeze together and force the blood out into the arteries: the aorta to the body and the pulmonary artery to the lungs. Afterwards, the heart muscle relaxes, allowing blood to flow in from the veins and fill the atria again. In healthy people the normal (resting) heart rate is about 72 beats per minute, but it can go much higher during strenuous exercise. Scientists have estimated that it takes about 30 seconds for a given portion of the blood to complete the entire cycle: from lungs to heart to body, back to the heart and out to the lungs.

                Functions of the Circulatory System

1. To carry digested food from the small intestine to all areas in the body which need it.
2. To carry oxygen from the lungs to the rest of the body.
3. To aid in the disposal of all wastes from the body.
4. To distribute heat.
5. To fight diseases by using white blood cells to fight off infection.

Arteries and Veins have many differences:

Arteries:

1.    Carry blood away from the heart always.

2.    Have thick muscular walls

3.    Have a pulse

4.    Deep under the skin

5.    Have no valves

6.    Arteries branch at their ends, into tiny arterioles those then join capillaries.

Veins:

1.    Carry blood to the heart.

2.    Have thin walls

3.    Do not have a pulse   

4.    Near surface of the skin

5.    Have valves to stop back-flow of blood.

6.    Veins branch at their beginnings into tiny venules which join capillaries.

                Veins have valves which stop the blood from flowing backward. Compared to the arteries, pressure in the veins is very low. The blood is being pushed through the arteries as the heart squeezes the blood out. This is not true for the veins. Blood is pushed through the veins when body muscles contract and squeeze the veins. In this way the blood moves.

The contraction/relaxation cycles of skeletal muscles squeeze the veins forcing the contained blood towards the heart.

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Artery-Vein Comparison

                                  Arteries                                                            Veins

Transport blood away from the heart;	Transport blood towards the heart;
Carry Oxygenated Blood  (except in the case of the Pulmonary Artery);	Carry De-oxygenated Blood (except in the case of the Pulmonary Vein);
Have relatively narrow lumens (see diagram above);	Have relatively wide lumens (see diagram above);
Have relatively more muscle/elastic tissue;	Have relatively less muscle/elastic tissue;
Transports blood under higherpressure (than veins);	Transports blood under lower pressure (than arteries);
Do not have valves (except for the semi-lunar valves of the pulmonary artery and the aorta).	Have valves throughout the main veins of the body. These are to prevent blood flowing in the wrong direction, as this could (in theory) return waste materials to the tissues.

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Capillaries

Capillaries are small blood vessels acting as veins and arteries. They surround the body cells at the ends of the arteries and at the beginning of the veins. The walls of capillaries are only one cell thick so substances (Food or cells) can easily get through them and into the blood or out. If all the capillaries of the body were set end to end they would be 100,000 km long!

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SUMMARY OF BLOOD VESSELS

Structure Functions Arteries The walls (outer structure) of arteries contain smooth muscle fibre that contract and relax under the instructions of the sympathetic nervous system. Transport blood away from the heart; Transport oxygenated blood only (except in the case of the pulmonary artery). Arterioles Arterioles are tiny branches of arteries that lead to capillaries. These are also under the control of the sympathetic nervous system, and constrict and dilate, to regulate blood flow. Transport blood from arteries to capillaries; Arterioles are the main regulators of blood flow and pressure. Capillaries Capillaries are tiny (extremely narrow) blood vessels, of approximately 5-20 micro-metres  (one micro-metre = 0.000001metre) diameter. There are networks of capillaries in most of the organs and tissues of the body. These capillaries are supplied with blood by arterioles and drained by venules. Capillary walls are only one cell thick (see diagram), which permits exchanges of material between the contents of the capillary and the surrounding tissue. Function is to supply tissues with components of, and carried by, the blood, and also to remove waste from the surrounding cells ... as opposed to simply moving the blood around the body (in the case of other blood vessels); Exchange of oxygen, carbon dioxide, water, salts, etc., between the blood and the surrounding body tissues. Venules Venules are minute vessels that drain blood from capillaries and into veins. Many venules unite to form a vein. Drains blood from capillaries into veins, for return to the heart Veins The walls (outer structure) of veins consist of three layers of tissues that are thinner and less elastic than the corresponding layers of aerteries. Veins include valves that aid the return of blood to the heart by preventing blood from flowing in the reverse direction. Transport blood towards the heart; Transport deoxygenated blood only (except in the case of the pulmonary vein).

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The Heart

The heart is located slightly to the left of the centre of your chest between the 2 lungs. It is made of cardiac muscle and is surrounded by a double membrane called the pericardium. There is fluid between these 2 membranes called pericardial fluid. This fluid helps to reduce friction when the heart beats. The heart is about the size of a clenched fist.

The human heart is primarily a shell. There are four cavities, or open spaces, inside the heart that fill with blood. Two of these cavities are called atria. The other two are called ventricles. The two atria form the curved top of the heart. The ventricles meet at the bottom of the heart to form a pointed base which points toward the left side of your chest. The left ventricle contracts most forcefully, so you can best feel your heart pumping on the left side of your chest.

The left side of the heart houses one atrium and one ventricle. The right side of the heart houses the others. A wall, called the septum, separates the right and left sides of the heart. A valve connects each atrium to the ventricle below it. The bicuspid valve connects the left atrium with the left ventricle. The tricuspid valve connects the right atrium with the right ventricle. The semilunar valves allow the blood to flow out of the heart into the 2 main arteries (aorta and pulmonary artery). They prevent blood from returning to the heart.

The top of the heart connects to a few large blood vessels. The largest of these is the aorta, or main artery, which carries nutrient-rich blood away from the heart. Another important vessel is the pulmonary artery which connects the heart with the lungs as part of the pulmonary circulation system. The two largest veins that carry blood into the heart are the superior vena cava and the inferior vena cava. They are called "vena cava" because they are the "heart's veins." The superior is located near the top of the heart. The inferior is located beneath the superior.

The heart's structure makes it an efficient, never-ceasing pump. From the moment of development through the moment of death, the heart pumps. The heart, therefore, has to be strong. The average heart's muscle, called cardiac muscle, contracts and relaxes about 70 to 80 times per minute without you ever having to think about it. As the cardiac muscle contracts it pushes blood through the chambers and into the vessels. Nerves connected to the heart regulate the speed with which the muscle contracts. When you run, your heart pumps more quickly. When you sleep, your heart pumps more slowly.

Considering how much work it has to do, the heart is surprisingly small. The average adult heart is about the size of a clenched fist and weighs about 11 ounces (310 grams). Located in the middle of the chest behind the breastbone, between the lungs, the heart rests in a moistened chamber called the pericardial cavity which is surrounded by the ribcage. The diaphragm, a tough layer of muscle, lies below. As a result, the heart is well protected.

THE FLOW OF BLOOD THROUGH THE HEART

    

Day and night, the muscles of your heart contract and relax to pump blood throughout your body. When blood returns to the heart, it follows a complicated pathway. If you were in the bloodstream, you would follow the steps below one by one.

Oxygen-poor blood (shown in blue) flows from the body into the right atrium. Blood flows through the right atrium into the right ventricle. The right ventricle pumps the blood to the lungs, where the blood releases waste gases and picks up oxygen. The newly oxygen-rich blood (shown in red) returns to the heart and enters the left atrium. Blood flows through the left atrium into the left ventricle. The left ventricle pumps the oxygen-rich blood to all parts of the body.

SUMMARY OF BLOOD FLOW:

            FROM BODY:

1.              Superior and inferior vena cava (deoxygenated)

2.            Right atrium

3.            Tricuspid valve

4.            Right ventricle

5.            Semilunar valve

6.            Pulmonary artery

7.            Lungs

8.            Pulmonary veins (oxygenated)

9.            Left atrium

10.                       Bicuspid valve

11.                       Left ventricle

12.                       Semilunar valve

13.                       Aorta

TO BODY and then back to the superior and inferior vena cava (Step #1)

Click here to see a general animation of blood flow

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THE DOUBLE CIRCULATION SYSTEM

The heart has a double circulation system. The pulmonary circuit pumps blood to the lungs and the systemic circuit pumps blood to the body systems (i.e. The head limbs, and trunk).

This system separates the oxygenated and the deoxygenated blood. (See diagram below)

The right ventricle pumps blood through the shorter pulmonary circuit while the left ventricle pumps blood through the longer systemic circuit. As a result, the left ventricle’s walls are thicker and stronger.

The Systemic System can be divided into THREE SUBSYSTEMS

A. CORONARY CIRCULATION - SUPPLIES BLOOD TO THE HEART.

B. RENAL CIRCULATION - SUPPLIES BLOOD TO THE KIDNEYS.  Nearly one-forth of the blood that is pump into the Aorta by the Left Ventricle flows to the Kidneys.  The Kidneys Filter Waste From the Blood.

C. HEPATIC PORTAL CIRCULATION - Nutrients are picked up by capillaries in the small intestines and are transported to the Liver.  Excess nutrients are stored in the Live for future needs.  The Liver receives oxygenated blood from a large Artery that branches of the Aorta.

Blood for the heart to use:

    The heart also needs a supply of blood for its nourishment. It does not use the blood it pumps for itself. The blood flowing through the heart does not directly serve the heart. Like all other organs the heart muscle has its own blood circuit. The heart gets blood from the coronary arteries that emerge directly from the aorta. Two coronary arteries arise from the aorta just beyond its semilunar valve. The right coronary artery mostly serves the right atrium and right ventricle. The left coronary artery is much larger and supplies the left atrium and left ventricle. The blood then goes through arterioles into capillaries and then to the heart’s cells. Venules then carry the blood to the coronary veins and back to the right atrium.

HEARTBEAT CONTROL

            The pumping action of the heart is controlled by the pacemaker (also known as the sino-atrial node (SA). It is in the wall of the right atrium. The pacemaker receives messages from the brain. Two nerves from the medulla oblongata connect to it influencing its rate of contraction. One nerve quickly accelerates the heart rate and the other can quickly reduces it back to resting rate.  These messages are transmitted as electrical impulses that cause the atria (both left and right) to contract. This contraction is called atrial systole.  Contraction of the atria sends blood to the ventricles.

The electrical impulse then enters the ventricles at the atrio-ventricular node and travels through the septum to the ventricles at the atrio-ventricular node (AV). The ventricles contract and force the blood towards the openings of the arteries, pulmonary artery and aorta. This is called ventricular systole.

            

        The rate of heartbeat is very important. The average healthy person will have a heart rate of about 70-80 beats per minute. Brain impulses and hormones can change this rate. Also, exercise, temperature, emotions, and shock will increase the heart rate. Relaxation and sleep decrease the rate.

STAGES OF THE HEARTBEAT

A heartbeat is a two-part pumping action that takes about a second. As blood collects in the upper chambers (the right and left atria), the heart's natural pacemaker (the SA node) sends out an electrical signal that causes the atria to contract. This contraction pushes blood through the tricuspid and bicuspid valves into the resting lower chambers (the right and left ventricles). This part of the two-part pumping phase (the longer of the two) is called atrial systole.

The second part of the pumping phase begins when the ventricles are full of blood. The electrical signals from the SA node travel along a pathway of cells to the ventricles, causing them to contract. This is called ventricular systole. As the tricuspid and bicuspid valves shut tight to prevent a back flow of blood, the semilunar valves are pushed open. While blood is pushed from the right ventricle into the lungs to pick up oxygen, oxygen-rich blood flows from the left ventricle to the heart and other parts of the body.

After blood moves into the pulmonary artery and the aorta, the ventricles relax, and the pulmonary and aortic valves close. The lower pressure in the ventricles causes the tricuspid and bicuspid valves to open, and the cycle begins again. This series of contractions is repeated over and over again, increasing during times of exertion and decreasing while you are at rest.

When the atria are contracting the ventricles are relaxing. This is called ventricular diastole. Likewise, when the ventricles are contracting the atria are relaxing. This is called atrial diastole.

THE SOUNDS OF THE HEART

The sounds of the heart are caused by the closing of the valves.

1.      The  “lub” sound- quieter, longer, lower pitched- caused by the bicuspid and tricuspid valves closing.

2.      The “dub” sound- louder, shorter, higher pitched- caused by the semi lunar valves closing.

3.       A heart murmur is an abnormal sound of the heart. It is usually an indication of damaged valves.

Blood Pressure

    Blood pressure is the blood pressing against the blood vessel walls. It is measured by a sphygmomanometer.

   

    The pressure varies along the circuit – decreasing from artery to arteriole to capillary to venule to vein.

    Pressure is highest at the start of the artery and lowest at the entrance to the atrium.

    Blood pressure is much higher in the aorta than in the pulmonary artery.

      The measures of a person’s blood pressure it taken at a large artery in the upper arm. It is the pressure need to stop blood flow in this artery and is measured at diastole and systole. Standard healthy readings are 80 mm Hg diastolic, 120 mm Hg systolic.

    People with a diastolic number at or above 95 have hypertension (high blood pressure). It is usually caused by blockages in the arteries.

Effect of Smoking

1. Hardening of the arteries.

2. Increased risk of heart disease.

3. Raised blood pressure.

4. Increased risk of stroke.

Effect of Diet

   

1. High Blood Pressure: excessive salt.
2. Clogged arteries from fatty foods.
3. Low Blood Pressure: lack of protein.

Effect of Exercise

1. Lower resting heart rate – more efficient heart.

2. Dilated arteries caused by aerobic exercise – improved blood flow to all the organs and less risk of heart disease.