12.2 Monitoring the Human Circulatory System Within the heart, the sinoatrial (SA) node (the pacemaker) stimulates the muscle cells to contract and relax rhythmically. The SA node is in the right atrium. It generates an electrical signal that spreads over the atria so they contract simultaneously. The signal then reaches the atrioventricular (AV) node, which transmits the signal through a bundle of fibres known as the bundle of His. These relay the signal to Purkinje fibres; these fibres initiate the almost simultaneous contraction of the ventricles. UNIT 4 Chapter 12: The Circulatory System Section 12.2

The Heartbeat UNIT 4 Chapter 12: The Circulatory System Section 12.2 The SA node sends out an electrical stimulus that causes the atria to contract. Continued…

The Heartbeat The normal sound of the heartbeat, heard with a stethoscope, is described as “lub-DUB.” These sounds are made as valves close: “lub” is the closing of the atrioventricular valves “DUB” is the closing of the semilunar valves Sound variations can indicate heart problems, such as a stenosis murmur, or narrowing of a heart valve or artery. UNIT 4 Chapter 12: The Circulatory System Section 12.2

The Electrocardiogram (ECG) Most heart function is analyzed by measuring the electrical pulses generated by the nerve signals that stimulate contractions. These create milliVolt charges that can be detected with electrodes on the skin. An ECG allows doctors to determine if the heart is generating signals of normal strength, frequency, and duration. UNIT 4 Chapter 12: The Circulatory System Section 12.2

Blood Pressure Blood pressure is the pressure that the blood exerts on the blood vessel walls as it travels through the body. Changes in blood pressure correspond to the phases of the heartbeat. Systolic pressure: maximum pressure in arteries when the ventricles contract and push blood through Diastolic pressure: lowest pressure in arteries when the ventricles are not contracting UNIT 4 Chapter 12: The Circulatory System Section 12.2 (A) Small muscles surrounding the veins contract and relax to squeeze blood along the veins. (B) One-way valves inside the veins prevent blood from flowing backward due to the pull of gravity.

Measuring Blood Pressure A sphygmomanometer (blood pressure cuff) placed on an artery in the arm measures blood pressure. Pressure is recorded in mmHg (millimeters of mercury) as systolic over diastolic. The average blood pressure of a healthy young person is below 120 mmHg and over 80 mmHg. Blood pressure is affected by genetics, activity, stress, body temperature, diet, and medications. Continuous high blood pressure (hypertension) can cause damage to the arteries and increase the risk of heart attack, stroke, and kidney failure. UNIT 4 Chapter 12: The Circulatory System Section 12.2 Using a stethoscope, the doctor or nurse can hear the flow of blood as the pressure in the sphygmomanometer is released and the blood starts to flow again.

Cardiac Output Cardiac output: the volume of blood pumped by the heart, expressed as mL per minute. It can indicate: UNIT 4 Chapter 12: The Circulatory System Section 12.2 the total level of work the body’s muscles can perform how easily the heart fills with blood the distensibility (stretchiness) of the ventricles Heart rate and stroke volume (the amount of blood pumped out of the heart with each heartbeat) are used to calculate cardiac output (heart rate X stroke volume). Average heart rate = 70 bpm, stroke volume = 70 mL Average cardiac output = 70 X 70 = 4900 mL/minute

Cardiovascular Fitness Cardiovascular fitness is the capacity of the lungs, heart, and blood vessels to deliver oxygen to working muscles. In the table below, individual C is considered to be the most fit. Another good indicator of cardiovascular fitness is the length of time it takes the heart to return to its resting heart rate after strenuous exercise. UNIT 4 Chapter 12: The Circulatory System Section 12.2 Why would individual C be considered to be the most fit? What factors determine this?

Section 12.2 Review UNIT 4 Chapter 12: The Circulatory System Section 12.2