The heartbeat originates in the cardiac muscle

Cardiac muscle, as we saw in Chapter 47, has some unique properties that allow it to function as an effective pump. First,

49.5 Measuring Blood Pressure Blood pressure in the major artery of the arm can be measured with a device called a sphygmomanometer, which combines an inflatable cuff and a pressure gauge. A stethoscope is also used to detect sounds created by the blood vessels in response to changes in pressure during the cardiac cycle.

^Artery

Sphygmomanometer

^Artery

Sphygmomanometer

No sounds

Stethoscope

Pressure in the cuff is increased to close both the arteries and veins. No sound is audible.

Systolic pressure

No sounds

Stethoscope

Systolic pressure

Pulsing sounds

Pressure in the cuff is gradually lowered until the sound of a pulsing flow of blood through the constriction in the artery is heard. At this time, pressure in the cuff isjust below the peak systolic pressure in the artery.

Diastolic pressure

Pulsing sound

Pulsing sounds

Diastolic pressure

Pulsing sound

Pressure is further lowered until the sound becomes continuous. At this time, the cuff is just below the diastolic pressure in the artery. This person's blood pressure is 120/70.

Pressure in the cuff is increased to close both the arteries and veins. No sound is audible.

Pressure in the cuff is gradually lowered until the sound of a pulsing flow of blood through the constriction in the artery is heard. At this time, pressure in the cuff isjust below the peak systolic pressure in the artery.

Pressure is further lowered until the sound becomes continuous. At this time, the cuff is just below the diastolic pressure in the artery. This person's blood pressure is 120/70.

cardiac muscle cells are in electrical contact with one another through gap junctions, which enable action potentials to spread rapidly from cell to cell. Because a spreading action potential stimulates contraction, large groups of cardiac muscle cells contract in unison. This coordinated contraction is essential for pumping blood effectively.

Second, some cardiac muscle cells are pacemaker cells. These cells have the ability to initiate action potentials without stimulation from the nervous system. When they fire action potentials, they stimulate neighboring cells to contract. The primary pacemaker of the heart is a nodule of modified cardiac muscle cells, the sinoatrial node, located at the junction of the superior vena cava and right atrium. The resting membrane potentials of these cells are not stable, but gradually becomes less negative (more positive) until they reach the threshold for initiating an action potential (Figure 49.6). The action potentials of pacemaker cells result from the opening of voltage-gated calcium channels, so they look different from the sodium action potentials graphed in Figure 44.10: They are slower and last longer.

The nervous system controls the heartbeat (speeds it up or slows it down) by influencing the rate at which pacemaker cells gradually depolarize between action potentials. Acetyl-choline released by parasympathetic neurons onto the pacemaker cells slows their rate of depolarization and thereby slows the heart rate. Norepinephrine released by sympathetic neurons onto the pacemaker cells increases their rate of depolarization and thereby speeds the heart rate (see Figure 49.6).

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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