Begin this section with Activity 6-1: Pressure, Resistance, and Flow.
Draw students' attention to the key ideas using posters or overhead transparencies.
Review the vocabulary terms listed in this section and the following:
Review the Apply Your Knowledge and the Review Questions.
Throughout and at the end of the section, refocus students' attention to the key ideas.
Blood pressure is the result of two forces. One force is created by the heart as it pumps blood into the arteries. The other force is created by the arterial blood vessels as they create resistance to the blood flow from the heart. The arterioles maintain the blood pressure in the arteries to keep the flow moving and to regulate the distribution of blood to where it is needed.
The nervous system and hormones adjust the diameters of the arterioles to control how much blood goes to each tissue in the body. Arterioles keep the upstream blood pressure in arteries higher, while distributing lower-pressure blood to capillaries. The walls of the arterioles can contract or expand, which changes the resistance to blood flow. Contraction reduces blood flow through the arterioles, while increasing blood pressure in the arteries upstream from them. If the arterioles remain constricted they can create a condition of hypertension or high blood pressure.
Consider the blood pressure readings in which the higher numbers indicate it is harder for blood to flow.
The chart below shows some normal pressure for adults throughout the circulatory system. Pressure ranges from 0 to 120mm Hg in the left ventricle. In the aorta and other arteries of the body, pressure ranges between systolic pressure 120 and diastolic pressure 80. The pulse pressure in arteries drops fast to just above 25 in the narrow arterioles. Blood flows “downhill,” down the pressure curve from the capillaries through the veins to a few mm Hg at the right atrium. Pressure in the right ventricle is much less than pressure in the left. Right ventricle systole only gets up to 25mm Hg. But this pressure drops to 5−12mm Hg at the left atrium. Pressures in the pulmonary circulation are 15 of the pressures in the body (systemic) circulation.
Most physiological systems utilize negative feedback in which the system counteracts a change with a response in the opposite direction that brings the system back to a set point. A sensor recognizes a rise or fall from the set value of the variable being measured. The controller compensates for the change.