The heart has two specialized bundles of nerves that control its beating:
1. The first bundle, called the sinoatrial (SA) node,acts as a pacemaker and sets a rhythm of about 72 beats per minute. 2. The second bundle, the atrioventricular (AV) node,passes the nerve impulses to the ventricles, a message that causes them to contract in unison. The familiar lubb-dubb heart sounds are caused by the closing of the heart valves:
When the atria are relaxed, they begin to fill with blood. This relaxation is called diastole.
As the atria push blood into the ventricles, the ventricles contract, pushing blood into the arteries. This contraction is known as systole.
The increase in pressure during systole forces the AV valves shut, causing the lubb sound.
As the ventricles relax, the pressure inside decreases and blood tends to flow into them. This decrease in pressure causes the semilunar valves to close, preventing blood from flowing back from the arteries into the ventricles. The closing of the semilunar valves creates the dubb sound.
Every time the heart beats, blood surges through the arteries. In response to this increase in pressure, the arteries stretch and increase in diameter. Where the arteries are near the surface of the body, this increase in diameter can be felt as the pulse.
Blood pressureis the force of the blood on the walls of the arteries. It can be measured with an instrument called a sphygmomanometer.A cuff with an air bladder is wrapped around the arm, and the air bladder is inflated with a pump. This cuts off blood flow to the major arteries of the arm. A stethoscope is used to listen for the sound of blood entering the artery as air is gradually released from the cuff. The pressure on the gauge of the sphygmomanometer at which the sound is first heard is called the systolic blood pressure. Normal systolic blood pressure for young adults is about 120 mm Hg. The cuff is then deflated even more until the sound disappears as a result of blood flowing into the artery as the ventricles fill. At this point, the pressure is called the diastolic blood pressure, which is normally around 80 mm Hg for young adults. This normal blood pressure would be reported as 120/80 (120 over 80): a systolic pressure of 120 mm Hg and a diastolic pressure of 80 mm Hg. There are many factors that can raise or lower blood pressure from its normal range. Low blood pressure reduces the body's capacity to transport blood around the body. High blood pressure is equally serious because it can weaken an artery, which might result in its rupture.
Lesson 3: Heart Rhythms, Sounds, and Pressure
The heart has two specialized bundles of nerves that control its beating:1. The first bundle, called the sinoatrial (SA) node,acts as a pacemaker and sets a rhythm of about 72 beats per minute.
2. The second bundle, the atrioventricular (AV) node,passes the nerve impulses to the ventricles, a message that causes them to contract in unison.
The familiar lubb-dubb heart sounds are caused by the closing of the heart valves:
Every time the heart beats, blood surges through the arteries. In response to this increase in pressure, the arteries stretch and increase in diameter. Where the arteries are near the surface of the body, this increase in diameter can be felt as the pulse.
Blood pressureis the force of the blood on the walls of the arteries. It can be measured with an instrument called a sphygmomanometer.A cuff with an air bladder is wrapped around the arm, and the air bladder is inflated with a pump. This cuts off blood flow to the major arteries of the arm. A stethoscope is used to listen for the sound of blood entering the artery as air is gradually released from the cuff. The pressure on the gauge of the sphygmomanometer at which the sound is first heard is called the systolic blood pressure. Normal systolic blood pressure for young adults is about 120 mm Hg. The cuff is then deflated even more until the sound disappears as a result of blood flowing into the artery as the ventricles fill. At this point, the pressure is called the diastolic blood pressure, which is normally around 80 mm Hg for young adults. This normal blood pressure would be reported as 120/80 (120 over 80): a systolic pressure of 120 mm Hg and a diastolic pressure of 80 mm Hg.
There are many factors that can raise or lower blood pressure from its normal range. Low blood pressure reduces the body's capacity to transport blood around the body. High blood pressure is equally serious because it can weaken an artery, which might result in its rupture.