The Heart
Mission 1: It's the Beating of That Hideous Heart, Redux.
Mission Objectives. You should be able to...
1. Explain how artificial pacemakers regulate heart rate.
2. Describe how a defibrillator is used to treat life-threatening cardiac conditions.
3. List the causes and consequences of hypertension and thrombosis.
The heart is composed of cardiac muscle. The actin and myosin proteins are arranged in contracting units called sarcomeres. The cells containing sarcomeres are joined together by interconnections called intercalated discs. Those discs contain contain openings called gap junctions where cytoplasm from one cell freely passes to the next cell. The sharing of cytoplasm is what allows the cardiac muscle cells to pass an electrical signal so quickly from cell to cell. Without the gap junctions, the impulse to begin a heartbeat would spread too slowly through the muscle tissue to result in a unified event (Pearson, 2014).
The cardiac cycle is a series of events that is known as the heartbeat. One cardiac cycle is all the heart events that occur from the beginning of one heartbeat to the beginning of the next one. The frequency of the cardiac cycle is your heart rate in beats per minute. If your RHR (resting heart rate) is 60 beats/min, then that means there are 60 cardiac cycles each minute (Pearson, 2014)
When a heart chamber contracts, it is because the cardiac muscle of the chamber has received an electrical signal that has caused the muscle fibers of said chamber to contract. This causes an increase in pressure on the blood within the chamber and the blood leaves the chamber through any available opening. This process is called the systole. When a heart chamber is not in systole, the cardiac muscle of the chamber is relaxed. This process is called diastole. Both atria contract at the same time, and so do both ventricles. However, the ventricles undergo systole just after the atria (Pearson, 2014). See below.
Mission Objectives. You should be able to...
1. Explain how artificial pacemakers regulate heart rate.
2. Describe how a defibrillator is used to treat life-threatening cardiac conditions.
3. List the causes and consequences of hypertension and thrombosis.
The heart is composed of cardiac muscle. The actin and myosin proteins are arranged in contracting units called sarcomeres. The cells containing sarcomeres are joined together by interconnections called intercalated discs. Those discs contain contain openings called gap junctions where cytoplasm from one cell freely passes to the next cell. The sharing of cytoplasm is what allows the cardiac muscle cells to pass an electrical signal so quickly from cell to cell. Without the gap junctions, the impulse to begin a heartbeat would spread too slowly through the muscle tissue to result in a unified event (Pearson, 2014).
The cardiac cycle is a series of events that is known as the heartbeat. One cardiac cycle is all the heart events that occur from the beginning of one heartbeat to the beginning of the next one. The frequency of the cardiac cycle is your heart rate in beats per minute. If your RHR (resting heart rate) is 60 beats/min, then that means there are 60 cardiac cycles each minute (Pearson, 2014)
When a heart chamber contracts, it is because the cardiac muscle of the chamber has received an electrical signal that has caused the muscle fibers of said chamber to contract. This causes an increase in pressure on the blood within the chamber and the blood leaves the chamber through any available opening. This process is called the systole. When a heart chamber is not in systole, the cardiac muscle of the chamber is relaxed. This process is called diastole. Both atria contract at the same time, and so do both ventricles. However, the ventricles undergo systole just after the atria (Pearson, 2014). See below.
Heart Valves. Valves keep blood moving in a single direction. Each chamber must have an opening to receive blood and another one to allow blood to exit. Heart valves prevent blood backflow.
Valves between atria and ventricles are called atrioventricular valves (AV), further identified by left and right. Valves located where the blood exits are called semilunar (SV) valves, further identified by left and right. See the image to the right, courtesy of the Pearson text. The heart valves have undergone several name changes over the years. Table 10.3 shows the different names of the valves (Pearson, 2014). |
What prevents backflow into the vena cava and the pulmonary veins? Veins have flap valves that are curved in the direction of blood flow that stay open as long as blood flows in the correct direction within the blood vessel. If backflow happens, the force of blood striking the flap closes the valve.
Atrial systole does not build up very much pressure. The walls of the atria are very thin compared to the ventricles. Their force of contraction is less than the ventricles. As a result, no heart valve is necessary for blood entering the atria.
Atrial systole does not build up very much pressure. The walls of the atria are very thin compared to the ventricles. Their force of contraction is less than the ventricles. As a result, no heart valve is necessary for blood entering the atria.
Your heartbeat is the sound of one cardiac cycle and is specifically the sound of the valves closing. The first sound is the sound of the AV valves closing (stopping flow from the atria to the ventricles), and the second sound is the closing of the semilunar valves (after the blood is sent through the arteries). See below:
1. When the AV valves are open, blood moves into the ventricles. 2. The ventricle contracts, the AV valves close and the SL valves open. Blood goes to arteries and the atria are refilled from veins. 3. SL valves close. AV valves open and the ventricles fill up. Question: Describe the pressure in the atria and ventricles during each step. |
Under normal circumstances, your heart controls the frequency and internal timing of the cardiac cycle. This is called myogenic control. When your heart needs to change the rate of contraction due to increased body activity, the nervous system takes over. The sinoatrial (SA) node is a region of specialized muscle cells in the right atrium and acts as a pacemaker. It sends electrical signals to stimulate contraction from atria to ventricles.
The SA node generates an action potential on a frequent basis and they spread out instantaneously and result in the atria undergoing systole. The SA node action potential reaches a group of cells called the atrioventricular (AV) node. The delay for this is exactly 0.1 second. Then the AV node sends out its own action potential that then spread to both ventricles. |
In order for the action potential to reach all the muscle cells in the ventricles efficiently, there is a system of conducting fibers that starts at the AV node and travels down the septum. Purkinje fibers are branches of the conducting fibers. Then the gap junctions within the intercalated discs of the cardiac muscle cells finish conducting the impulse and both ventricles undergo systole at the same time. Watch the below video.
Watch the below video to learn how defibrillators work.
On your own, read about how to interpret ECG traces, artificial pacemakers, thrombosis, hypertension and coronary heart disease.