Circulatory systems, the heart and heart rate

Last time

Before we start: last time, in Blood vessels and blood, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A pump that never stops

Your heart beats about a hundred thousand times a day, every day of your life. It pumps blood through two circuits: one to the lungs and one to the rest of the body. Today: the double circulation, the parts of the heart, and why your heart speeds up when you exercise.

The circulatory system

The circulatory system is a system of blood vessels, with a pump, and valves to make sure blood flows one way. Fish have a single circulation: blood goes through the heart once for each circuit of the body. Mammals have a double circulation: blood goes through the heart twice in each complete circuit.

Why double is better

In a double circulation, the pulmonary circuit takes blood to the lungs, and the systemic circuit takes it to the body. Blood returns to the heart after the lungs, so it can be pumped out again at high pressure. So oxygen reaches the body tissues faster. Low pressure to the lungs also protects their delicate capillaries.

The heart

Here is the human heart. Two atria at the top receive blood; two ventricles below pump it out. The right side carries deoxygenated blood, and the left side oxygenated blood. A wall called the septum keeps them apart.

The path through the heart

Deoxygenated blood from the body enters the right atrium through the vena cava. It passes to the right ventricle, which pumps it to the lungs through the pulmonary artery. Oxygenated blood returns in the pulmonary vein to the left atrium, then the left ventricle. The left ventricle pumps it to the body through the aorta.

Walls and valves

The ventricles have thicker muscle walls than the atria, because they pump blood out of the heart. The left ventricle wall is thickest, because it pumps blood all around the body at high pressure. Valves between atria and ventricles, and at the exits, stop blood flowing backwards. The coronary arteries supply the heart muscle itself with oxygen and glucose.

One heartbeat

The atria contract, pushing blood into the ventricles. The valves between them open. The ventricles then contract. The valves between atria and ventricles close, causing the first heart sound. Blood is forced out into the arteries. Then the valves at the exits close, and the heart relaxes and refills.

Monitoring the heart

Measure the pulse rate at the wrist or neck, in beats per minute. Listen to the valves closing with a stethoscope. Record the electrical activity with an E C G.

Heart rate and exercise

During exercise, muscles contract more, so they respire faster and need more oxygen and glucose. They also make more carbon dioxide, which must be removed. So the heart beats faster and more strongly, delivering more blood to the muscles.

Coronary heart disease

Coronary heart disease happens when fatty deposits block the coronary arteries. Less oxygen reaches the heart muscle, which can cause a heart attack. Risks include a diet high in saturated fat, smoking, stress, lack of exercise, age and genes.