HUMAN BIOLOGY

CHAPTER 23 THE HEART

SECTION 1: YOUR HEART: THE BODY'S PUMP

Your heart is the size and shape of your fist. It is located in the middle and slightly toward the left side of your chest. It rests safely and snugly between your two lungs. It is protected by the lungs and your rib cage (see Figure 1). The heart is inside a slippery bag called the pericardium. This bag helps lubricate the movements of the heart and it helps to hold the heart in place. The jolts of the beating heart are absorbed by the sponginess of the lungs.

Your heart is a muscle. It is called the cardiac muscle. The heart is made up of thousands of muscle cells called myocardium. These cells all have to work together. They relax to let the heart fill with blood, then contract to pump the blood out of the heart. 

Your heart beats more than 100,000 times a day, which is over 42 million times a year. It pumps about 10,000 quarts (40,000 cups or 2,500 gallons) of blood each day. In fact, each blood cell travels from the heart to the body and back to the heart in less than a second. These blood cells carry the oxygen and nutrients that keep your cells alive. 

The heart normally beats about 70 to 80 times a minute. Your heart beats faster when you exercise and it beats slower when you sleep. Your brain sends messages to your heart through coronary nerves. Through these nerves, the brain is able to tell the heart how fast it needs to beat to meet the body's current demands for oxygen. Your heart works hard but, it does get to rest. It actually relaxes and rests between each heartbeat while it fills up with blood. Your heart gets to "rest" about five hours a day. When it's not resting, the heart is contracting and pumping blood to your body.

Here's an experiment that you can do at home to "see" your heart beating. You will need a small amount of clay or play-doh and a match or small stick. Stick the end of the match into a tiny ball of clay or play-doh, so the match head sticks up into the air. If you don't have clay or play-doh, make a very thick mixture of flour and water and use that. Flatten the bottom of the clay. Place your arm and hand, palm side up, on a table. Feel for your pulse. You should find it about 1/2 inch to the right of the center of your wrist and about one inch below your thumb. Put the clay on your wrist at this point. Watch as the match starts to vibrate back and forth. Count how many times it vibrates in one minute.

The match should vibrate back and forth with a regular beat about 60 to 80 times a minute. It should vibrate faster, 80 to 140 times a minute, for children. Why does the match move? As the heart beats or contracts, blood is forced through the blood vessels. The blood moves through the vessels at a steady rate, which causes them to pulse or throb. The match vibrates because of the pulsing movement of the blood through your blood vessel.

To see how your pulse rate changes, exercise for a few minutes. Jog around the room or do 20 jumping jacks. Now try the experiment again. The match should move more times a minute than it did the first time. When you exercise, the cells in your muscles work harder and need more oxygen. Your heart has to pump blood faster to the cells. Your heart beats faster, so your pulse gets faster.

SECTION 2: HOW THE PARTS OF THE HEART WORK TO PUMP BLOOD

Blood follows a fairly simple pathway through your body, and it all starts in your heart. Your heart is divided into two sides, a right side and a left side. A wall of tissue called the septum separates the two sides. Each side of your heart has a different job to do. The right side of your heart pumps blood into the lungs where the blood cells get fresh oxygen. The blood leaves the lungs and empties back into the left side of your heart.

The left side of the heart pumps the blood into arteries that carry it all over your body. The arteries turn into small capillaries, which carry the blood to your tissues and organs. The capillaries turn into veins which carry the blood back to the right side of your heart, so the process can start all over again. Oddly enough, the two sides of your heart are not the same size. The left side is larger and more muscular than the right side because it has to pump blood all through the body.

Each of the two sides of your heart is divided into two area or chambers. The upper chambers are called atriums. You have a right atrium and a left atrium. The lower chambers are called ventricles. You have a right ventricle and a left ventricle. This gives you a total of four different chambers in your heart (see Figure 2).

The atrium and ventricle on the right side of your heart work together to pump blood to the lungs. The atrium and ventricle on the left side of your heart work together to pump blood to the arteries that carry it all over your body. Each atrium and ventricle are separated by a valve. The valves only open one way, so blood can only flow from an atrium into a ventricle. 

Now, let's look at the pathway that blood follows in more detail. Blood returns from the body in veins that empty into the right atrium. When this atrium fills with blood, it squeezes together (contracts or beats) and pumps the blood through the tricuspid valve into the right ventricle.

When the right ventricle fills up, it pumps the blood through the pulmonary valve and into a large artery called the pulmonary artery. This artery carries the blood straight into the lungs. As the blood passes through the lungs, it gets fresh oxygen that you've just breathed in. After this happens, the blood leaves the lungs and gets carried back to the heart in the pulmonary veins.

The pulmonary veins empty the blood into the left atrium. When the left atrium fills with blood, it contracts (beats) and pumps the blood through the bicuspid (or mitral) valve into the left ventricle.

When the left ventricle fills with blood, it contracts and pumps the blood through the aortic valve and into a large artery called the aorta. The aorta is the biggest blood vessel in your body. It is one inch wide and blood pumps through it at the speed of eight inches per second.

The aorta divides into many different arteries and finally into small capillary arteries which carry the freshly oxygenated blood to all parts of your body, including your lungs, heart, brain and other organs. Oxygen is released to the cells and waste products from the cells are picked up. Then, the blood passes from the capillary arteries into capillary veins.

The blood flows through the capillary veins into larger and larger veins that finally empty into the largest veins called the vena cava. You have two vena cava veins. The superior vena cava vein collects the blood from the upper chest and head areas. The inferior vena cava collects the blood from the lower half of your body. The two vena cava veins carry the blood back to the right atrium of your heart. From there, the process starts all over again (see Figure 3).

Actually, these two processes occur at the same time. As blood from the veins fills up the right atrium, blood returning from the lungs fills up the left atrium. The two atriums contract at the same time and force blood down into the ventricles. This contraction is the first half of your heartbeat.

When the ventricles are full, they contract at the same time. The blood from the right ventricle goes to the lungs. The blood from the left ventricle goes to the arteries that carry it to your whole body. This contraction is the second half of your heartbeat. Then the heart rests briefly as the atriums fill with blood again. Believe it or not, this whole process only takes 4/5 of a second, or slightly less than one second. Remember that your heart beats 70 to 80 times in one minute.

You may have noticed that the work of the heart is divided into two phases, filling and contracting. When the atriums or ventricles are relaxed and filling up with blood, they are said to be in diastole. When they are contracting and pumping blood out, they are said to be in systole. 

In order for the heart to beat properly, the cardiac muscle cells have to contract and relax at the right time. For instance, the cells in the ventricle can't contract before the ventricle fills with blood or there won't be anything to pump out. The heart also has to pump slower or faster to keep up with your body's oxygen demands. If you're running to catch a bus, your heart better beat faster to help your legs run.

The heartbeat is controlled by a special clump of tissue called the sinoatrial node. This node is located in the right atrium. The brain sends signals through the coronary nerve to this node. The signals make the cells in the sinoatrial node contract. When this happens, the node quickly sends a signal to the rest of the muscle cells in the atriums telling them to contract. A split second later, the atriums contract. Their contractions stimulate another node, the atrioventricular node, also in the right atrium. This node sends signals that make the cells in the ventricles contract. It's all a matter of split-second timing, and it all relies on the sinoatrial node. Since the sinoatrial node is responsible for the heartbeat, it's called the pacemaker of the heart.

The number of times your heart beats each minute is called your heart rate. The heart rate is actually controlled by the waste products (including carbon dioxide) from your cells. When you exercise or work hard, your cells work harder. They make more waste products, which the blood picks up. As the blood flows through your brain, a special area of your brain senses the increased amounts of waste products.

The brain wants to get rid of the waste products because they can be poisonous. It sends a signal down the coronary nerve to make the sinoatrial node contract faster. This makes your heart beat faster, which means your heart rate goes up. When the heart beats faster, the blood gets pumped through your lungs faster. This helps your blood cells get rid of their waste products faster. They also pick up oxygen faster, which is exactly what your body cells need when they are working harder.

When you stop exercising or working, the amount of waste products your cells give off decreases. The level of waste products in your blood drops. The brain senses this and sends signals to decrease your heart rate.

SECTION 3: PROBLEMS WITH THE HEART

There are many fairly common problems with the heart, including problems with valves, arteries, veins and the sinoatrial node. In fact, one out of ten babies is born with some type of heart problem, making it one of the most common birth defects.

One problem many babies are born with is a heart murmur. If a person has a heart murmur, it means blood does not flow through the heart correctly. A doctor can hear a blowing or rasping sound if he listens to the heart beating. Some murmurs are caused when the heart valves don't close properly. Each time the heart contracts, some blood goes backward through the partly opened valve, making a whooshing sound. Some murmurs are caused when the aortic artery is clogged or partly closed. The heart has to pump harder to force the blood through the narrowed artery, which causes a rasping sound. Some murmurs aren't serious or dangerous. In some cases, the problem even fixes itself as the baby's heart grows a little bigger. Other heart murmurs are serious and must be surgically fixed.

A very common problem many adults have is "clogged" arteries and veins. Your blood has special cells that clean out cholesterol and other debris. However, if too much debris is in your blood, the cleaning cells can't keep up. The debris that isn't cleaned out begins to stick to the linings inside your arteries and veins. Over time, the debris forms a thick layer around the inside of your artery. This is called stenosis, which means narrowing of a blood vessel. Stenosis leaves a narrower area for the blood to flow through. When this happens, your heart has to pump the blood with more force to push it through the narrower arteries. This can cause high blood pressure.

High blood pressure is also called hypertension. If a person has high blood pressure, it means that their heart is working harder than normal to pump blood through their body. High blood pressure can be caused by several things including narrowing of blood vessels, kidney disease, some kinds of tumors, certain drugs, and stress.

High blood pressure can cause many problems. When a person has high blood pressure, the blood is forced through their vessels with more pressure. The extra pressure makes the vessels weak. Sometimes the blood vessels can't take the extra pressure, and they burst. When a blood vessel bursts, the part of the body it goes to doesn't get blood or oxygen anymore. When this happens, the cells in that area die from the lack of oxygen. If a blood vessel in a person's brain bursts, the person can have a stroke. This means a part of their brain died because it didn't get enough oxygen.

Sometime the blood vessels in the heart burst. This can cause a heart attack. This means a part of the person's heart died because it didn't get enough oxygen. When a part of the heart dies, it can't pump blood anymore. This creates a serious problem. The rest of the heart tries to pump faster to keep up with the body's demands. However, if a large enough part of the heart dies, the rest of the heart can't keep up. It can't pump enough blood, even to its own cells. As more and more of the cells in the heart die from lack of oxygen, the heart stops working altogether. Unless the person quickly gets oxygen or their heart starts to work again, they'll die. 

A heart attack can also occur when the blood vessels that deliver blood and oxygen to the heart cells become too blocked with debris. If these vessels get too narrow, the cells in the heart muscle don't get enough oxygen. Then, if the person gets excited, stressed, or exercises, their heart tries to pump more blood to their body. The heart is then working harder, but it's not getting the oxygen it needs. The heart cells start to die from lack of oxygen, and the person has a heart attack. If a heart attack is mild, only a small area of heart cells are killed and the person will probably recover. However, if too many heart cells are killed, the heart can't pump anymore, and the person will die.

Finally, heart attacks can also be caused by a problem with the sinoatrial node or pacemaker. If it doesn't send out signals properly, the different parts of the heart beat at the wrong times. This is called an arrhythmia, which means lacking the right rhythm. When this happens, the heart doesn't pump blood correctly and your body doesn't get enough oxygen. This can cause a heart attack.

It is possible to live after a heart attack caused by a problem with this pacemaker, if help is received in time. Sometimes a doctor can shock a person's heart. This briefly stops all the signals in the heart cells.  Then, hopefully, the pacemaker will start the correct signals and rhythms again. Sometimes the node just stops working and can't be fixed. When this happens, surgeons may put an artificial pacemaker in the heart to send out the correct signals so the different parts of the heart beat at the right time.

Symptoms of a heart attack include a feeling of pressure or squeezing in the chest. The pain may spread to the shoulder, neck, arm and hand (usually the left ones), back, teeth or jaw. The person may also be nauseous, vomit, sweat and be short of breath. Most people experience several episodes of minor symptoms before they have a major heart attack. Most people get these early warning signs; however, many people ignore them. If a person ever has any of these symptoms, they should go to the hospital immediately. If they wait until they have a heart attack, it may be too late. Half of the people who have heart attacks die before they can get to a hospital.

End of Chapter 23.

The homework assignment is shown below.  Students would complete the multiple-choice study questions, then use them as a study guide to prepare to take a test over this chapter’s material. 

Each ½ credit course consists of 16 chapters with homework assignments/exams, two quarter exams, and a semester final.

Each 1 credit course consists of 32 chapters with homework assignments/exams, four quarter exams, and a course final.

HUMAN BIOLOGY

CHAPTER 23  STUDY QUESTIONS

SECTION 1

HUMAN BIOLOGY

CHAPTER 23  STUDY QUESTIONS

SECTION 4, PARTS OF THE HEART

For your exam, be able to: label the parts of the heart on a diagram, explain the function of each part of the heart, and describe the path that blood takes through your body.

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