Textbook: Zelman, M. (2015). Human Disease – A Systematic Approach (8th Edition). Publisher: Pearson Inc.
Chapter 6: Diseases of the Cardiovascular System
By the end of this lesson you should be able to
- Identify major parts of the heart and their function including all chambers, valves, and muscles
- Track the flow of blood from the lungs to the heart and back to the lungs
- Differentiate between arteries, veins, arterioles and capillaries
- Briefly explain how the heart’s electrical system works
Anatomy of the Heart
The heart is a pump that circulates blood around the body. It connects to various tubes to get the blood where it needs to go. For example, arteries carry blood from the heart to the tissues, and veins carry blood back to the heart. Both veins and arteries are connected to smaller vessels called capillaries that interact with tissue cells. As blood flows throughout the body, nutrients, oxygen, hormones, and white blood cells are delivered to cells, and waste is removed.
Since the heart is the driving force in the circulatory system, let’s look at its structure and how it works. The heart is a muscle in the chest that is constantly expanding and contracting (or beating) as it drives blood around the body. It consists of three major layers:
- Endocardium: Thin inner layer of the heart. All internal surfaces of the heart, including the valves, are covered by the endocardium. This layer provides a smooth surface to the inside of the heart that prevents sticking of heart tissues as the heart contracts and prevents sticking of blood to the surface of the heart which would cause blockage and infection. It also strengthens the valves.
- Myocardium: The powerful contractile middle muscle of the heart, called cardiac muscle, is responsible for pumping blood
- Pericardium: The outside layers of the heart. It anchors the heart in place, lubricates the heart, and protects it from injury and infection.
The heart consists of four chambers i.e. the right and left atrium for blood receival and the right and left ventricles for blood removal. Notice that the left side is to your right and vice versa. Think of this heart as representing a person standing in front of you. His left will be your right.
The flow of blood in and out of the heart is in a one-way direction. Any backflow would be a malfunction. Backflow is prevented by the four valves in the heart i.e. the bicuspid (mitral) valve, aortic valve, tricuspid valve, and the pulmonary valve.
Based on figure 4, can you map the path of blood through the heart? Let’s look on the left side first. By the way I should mention this. Notice that the heart has a red side and a blue side. Red means that the blood is oxygenated (filled with oxygen). Blue means that it is deoxygenated (lacks oxygen). These colors are used because the pigment in blood is actually red when it absorbs oxygen but turns blue when it lacks oxygen.
OK, let’s now track the path on the left. Pulmonary veins taking blood from the lungs contains lots of oxygen. The blood is pumped into the left atrium and travels past the bicuspid valve into the left ventricle. It is then pumped through the aortic valve and into the aorta which connects with other vessels to take oxygen to tissues and cells in the body.
Now let’s look on the right side of the heart. Deoxygenated blood with lots of carbon dioxide (waste) travels via the inferior and superior vena cava to the right atrium, moves pass the tricuspid valve, to the right ventricle. It is then pumped out through the pulmonary valve into the pulmonary artery which takes blood to the lungs.
When this blood gets to the lungs, carbon dioxide is breathed out and oxygen picked up.
Whenever the heart contracts (gets smaller) it pumps blood out and whenever it relaxes, it allows blood to flow in. The contraction phase is called systole and the relaxation phase is called diastole. When you take your blood pressure, you will get two values. The top one is called the systolic pressure and the bottom one is the diastolic pressure. Normal blood pressure is 120/80. 120 is the pressure exerted on the arteries when the heart contracts, and 80 is the pressure on the arteries when the heart relaxes.
Here is a look at your blood pressure readings that you took in your lab last week.
You can keep your blood pressure normal by making lifestyle changes such as:
- Exercising regularly (minimum of 150 minutes per week of moderate aerobic exercise or 75 minutes of rigorous exercise)
- Reducing sodium in your diet. Sodium increases water retention in the blood.
- Losing excess weight
- Consuming adequate potassium. Potassium relaxes your blood pressure walls. Get more calcium from eating more fruits and vegetables)
- Limit alcohol consumption (A maximum of 1 drink a day for women and 2 drinks a day for men)
- Quit smoking
- Reducing cholesterol intake
- Reducing intake of “bad” fats such as trans fats and saturated fatty acids
- Reducing stress. Stress hormones increase blood pressure
Body Mass Index (BMI)
How much is your BMI? BMI above normal increases your risk of heart disease. Here is the BMI data we collected in class for each student. Notice on the graph to the left that body weight is a good predictor of BMI. While you cannot change your height, you do have a say on what your weight will be.
What Makes the Heart Contract?
In order for the heart to contract and relax continually, its muscles need energy. Therefore cardiac (heart) muscles are continually supplied with oxygen via coronary arteries.
Apart from the energy supplies and nutrients that the heart needs to run on, it also needs electricity. The electrical current is generated in a small area of tissue in the heart known as the sinoatrial (SA) node or the anatomical pacemaker. The electrical current causes the atrial muscles (both left and right) to contract. The signal travels to the atriaventricular node and then to the ventricles via the bundle branches and the Purkinje fibers, causing the ventricles to contract.
This flow of electrical current can be measured by an instrument called an electrocardiogram. Any change will indicate that the heart is not beating properly. If there is no signal at all, the electrocardiograph will flat-line indicating that the heart is no longer beating. This is an indication of death.
Figure 11: Electrocardiogram. Source: Wiki Commons.
Definitions You Must Know
- Pulmonary: Relating to the lungs
- Systole: Contraction phase of the heart
- Diastole: Relaxation phase of the heart
- Electrocardiogram: Chart displaying electrical signals from the heart
- Capillary: Tiny blood vessels that form a network between the arterioles and venules
- Coronary: Relating to the arteries that supply the heart muscles with oxygen, energy, and nutrients