The heart is a muscular pump located between your lungs. A sac called the pericardium encloses the heart. As in other mammals, the human heart has four chambers. The two upper chambers, which receive blood returning to the heart, are called atria. The two lower chambers are called ventricles, which pump blood out of the heart.
Anatomy of the heart. Figure 30-6 Blood enters the heart through the atria and exits through the ventricles. Valves keep the blood from flowing in the wrong direction. (Note that the left and right sides of the heart refer to the person's left and right sides, causing them to appear reversed in this face-on view.)
When you run up a flight of stairs or are startled by a sudden noise, your heart is likely to beat faster than it does when you are at rest. A specific region of your heart muscle, known as the pacemaker, sets the rate at which your heart contracts.
The pacemaker is located in the wall of the right atrium. It generates electrical impulses that spread rapidly over the walls of both atria, making them contract. The impulses then spread to a region of the heart called the AV node From there, the electrical impulses spread to the ventricles, causing them to contract. The contracting ventricles propel blood to the rest of the body.
The pacemaker sets the rate at which the heart contracts. Its signals generate electrical changes in the skin that can be measured as an electrocardiogram (ECG). The yellow color in the sample ECG below indicates the different stages of the heartbeat.
When the ventricles contract, they increase pressure on the blood. The resulting force that blood exerts against the artery walls is called blood pressure. This force drives blood through the arteries and into the capillaries. Artery walls are elastic, which enables them to stretch in response to this force.
Figure 30-8 To measure blood pressure, an inflatable cuff is wrapped around a person's arm. As air is pumped in, the cuff squeezes the arm and closes a large artery. As air is slowly released from the cuff, the caregiver listens for the sounds of blood flowing through the artery.