CARDIOVASCULAR SYSTEM 

The heart is the most important part of our cardiovascular system. It is a hollow mass of muscle that is slightly larger than a man’s fist and weighs only about a half pound. 

The human heart starts beating when a fetus is 4 weeks of age and is fully developed at 8 weeks. It will continue to beat once per second about 3 BILLION times in an average lifetime. 

The heart uses ten times the food and oxygen than the other organs in the body and moves the body’s 6 quarts of blood completely through the body 3 ounces at a time.  

When a person is relaxed, about 3 quarts of blood per minute is circulated. When a person is active, as much as 24 quarts can be transported to all areas of the body.  

The total volume of the body’s blood is cycled every 50 beats, or basically every 50 seconds.

In a normal lifetime, the heart beats several billion times (roughly 100,00 times a day) and moves as much as 9-10 tons of blood in a single day!

The circulatory system is comprised of the heart, the lungs, arteries, veins, arterioles, venules, and capillaries. 

All these parts work together to circulate blood to every one of the trillions of cells in the human body. 

In the blood oxygen and food are circulated and waste products are removed. 

The blood is transported through a long series of blood vessels.  

If all the blood vessels in the body were placed end to end, they would measure more than 60,000 miles in length. If they were placed side by side, they would measure over 800” in diameter. 

The constant circulation of blood is required for cells to remain alive and healthy. Some cells, such as brain cells, would be severely damaged or even die if they were without oxygen for even a few minutes. Other cells are hardier, however. 

The human heart rate changes as a person ages. The older the person, the slower the heart rate.  

Below is a chart of a human’s heart rate at various ages:   

Animal heart rates vary with the size of the animal. As a general rule, the smaller the animal, the faster the heart rate.   

Several other factors besides size can affect the heart rate, though.  

Activity can play a major part in how fast a heart beats. The heart rate increases when a person is standing rather than lying down. It can also increase because a person is physically active, emotional stress occurs, the temperature of the body changes, the temperature of the air changes, or as a response to specific foods he has eaten. 

A person’s heart rate can be felt by placing light fingertip pressure over any artery in the body. In a person, the heart rate is usually counted by putting pressure over the main artery in either the wrist or the neck because they are both easily available. 

When a heart rate is measured, there are 2 different factors to consider: how fast is the heart beating and how hard is it beating. 

A measurement of how fast a heart beats is called the pulse. A measurement of how hard a heart beats is called the blood pressure. 

About 200 years ago an Englishman stuck a small tube into the artery of a horse in an attempt to measure the strength (or pressure) the heart exerted to circulate the blood. He found this pressure was enough to force blood 8 feet upward inside a glass tube.  This was the first recorded example of an accurate blood pressure measurement. 

The human heart squeezes hard enough to force blood up a tube to a height of 5 feet. 

It would be impractical and painful to have to insert small tubes into a person’s veins and hook them up to five foot long tubes to measure blood pressure. So, a simpler method was created.

Today’s blood pressure machines consist of a hollow glass tube about 8 inches long filled with mercury. Scientists discovered that mercury weighs about 14 times as much as blood.  

Therefore, it would take the same amount of blood pressure to raise the mercury 1 inch as it did to fill the empty glass tube 1 foot.   

Unlike the muscles in a person’s arms or legs that move when he wants them to, the muscles that make the heart beat are involuntary. That means that a person has no conscious control over moving them. 

The heart is located a little to the left of the middle of the chest. It has 4 separate chambers and basically consists of a left pump and a right pump working side by side. 

The right pump is responsible for sending blood to the lungs for oxygen. The left pump sends blood to the various parts of the body. 

These two pumps are divided into two separate sections: the atrium and the ventricle. The right side of the heart has a right atrium and a right ventricle. The left side of the heart has a left atrium and a left ventricle.  

The two thin-walled atriums are smaller than the ventricles and both relax at the same time. They are mainly areas where blood returning from the veins pools when the ventricles relax.

When the two thick-walled ventricles contract (at the same time), the atriums empty into them.  

The right ventricle then sends the blood to the lungs and the left ventricle send blood to the rest of the body.

Blood leaves the ventricles in arteries. 

All blood movement is controlled by the relaxing and contracting of the two ventricles. 

The signal for the ventricles to relax or contract, causing the heart to beat, is controlled by the brain. There is a small area of muscle called the sinoatrial bundle (SA) located in the upper right atrium that triggers the ventricle to relax.  

A similar bundle in the lower part of the right atrium is called the atrioventricular bundle (AV), and it signals the ventricles to contract.  

By constantly signaling these two areas to trigger the ventricles to relax or contract, the brain keeps the steady beat of the heart going. 

Once the blood leaves the heart it travels from the largest vessels in the body (arteries and veins) through smaller and smaller vessels (arterioles and venules) until it reaches the smallest vessels in the body, the capillaries.  

From there the blood continues its journey in a circular manner through larger and larger vessels until it finally reaches the heart again. 

Arteries are thick-walled blood vessels that are made of muscle and elastic tissue. 

Veins are thin-walled blood vessels.  They are made of muscle and are very elastic, allowing them to stretch and store blood needed for eating and exercise.  Veins, just like the heart, contain valves that allow the blood to only go one direction. 

Capillaries are only one cell thick, which allows for the easy exchange of oxygen, carbon dioxide, and cellular waste.   

Capillaries are so small, in fact, that it would take 10 of them to equal the thickness of 1 human hair. You have lots of them, however.  If all your capillaries were put end to end, they would circle the globe 2 and a half times! 

Capillaries  are the connectors between arteries and veins. 

A wall called the septum separates the right and left sides of the heart.  

The left side of the heart is larger than the right side because it is more muscular. It needs to pump blood to all parts of the body while the right side simply pumps blood to the lungs.  

Two sets of one-way valves separate the atriums from the ventricles. These valves are as thin as tissue paper but stronger than iron. They open and close with every heartbeat, but only open in one direction. This allows the blood to only flow one way in the body.   

The tricuspid valve separates the right atrium and right ventricle.  The bicuspid valve separates the left atrium and left ventricle. 

A murmur is a condition where a valve does not seal tightly together and some blood is allowed to flow back through it. 

After traveling through the right atrium and right ventricle, the blood flows from the heart to the two lungs through the pulmonary arteries.  

Once in the lungs, the blood collects oxygen inhaled by the person and disposes of carbon dioxide, a waste produced by the cells and collected by the blood as it travels throughout the body. 

The blood exits the lungs and reenters the heart through the pulmonary veins. After it travels through the left atrium and left ventricle, the blood is pumped into the largest vessel in the body, the aorta. 

The aorta in about 1” in diameter, making it the largest vessel in the body, and blood gushes through it at a speed of 8” per second. The aorta is about half the diameter of a garden hose. 

Like all cardiac vessels, the aorta has a thick, tough outer wall and a smooth interior that won’t interfere with the rapid, easy flow of blood.  

The aorta splits into 3 separate branches called the aortic arch, above the heart which sends blood to the entire upper part of the body.

Below the heart, a single section of aorta sends blood to the lower parts of the body. 

After traveling to every cell in the body delivering oxygen and collecting carbon dioxide, the blood returns to the heart through the vena cava veins.   

The one leaving the top of the heart is called the superior vena cava.  The one which returns blood from the bottom of the body is called the inferior vena cava.   

The heart is covered with a thick-walled sac called the pericardium. 

It is filled with a thin layer of fluid and a layer of fat that support, cushion, and lubricate the heart as it beats. Certain diseases can thicken the pericardium and cause fluid build-up that presses on the heart and slows the heart rhythm. 

Other diseases can also affect the heart. As a person ages the valves and arteries are less elastic causing the blood to be pumped less effectively.

Deposits can harden the artery walls and slow the flow of blood. Weakened arteries may burst. And some arteries may develop clots in them that block the flow of blood to or from the heart. 

These conditions and others like them can affect the rate of the heartbeat.  

If a heart beats too quickly, it is called tachycardia.  

If a heart beats too slowly, it is called bradycardia.  

Both these abnormal beats can easily be detected by a doctor during a routine exam. 

Some heart conditions can be congenital, meaning they don’t occur because of disease or injury, but are present at birth. 

One such heart problem is called patent ductus arteriosus or PDA.  

When a fetus is developing before birth, the lungs do not breathe air. The blood, therefore, did not need to go to the lungs, it simply went from the left atrium back to the aorta through a small hollow tube of muscle called the ductus arteriosus.  

After birth this tube shrivels and seals itself closed. Sometimes this doesn’t happen and doctors have to surgically close the tube. 

Although the heart is filled with blood, it does not use any of this blood to keep itself alive. It receives blood from 2 arteries that branch off the aorta. 

They cover the pericardium with small arterioles and capillaries and transport the blood needed to keep the heart healthy.  

These 2 arteries are called the right and left coronary arteries. 

The following are the vessels in order that the blood flows through. 

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