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7.1: Pressure, Flow, and Resistance

Difficulty Level: At Grade Created by: CK-12
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How is the Right Amount of Blood Directed to Each Part of the Body?

You've investigated the heart and how it pumps blood through the body. You also investigated the vessels that carry the blood to all parts of the body and back to the heart again. But how does the body make sure the right amount of blood is sent to all the different parts of the body? Have you ever had your blood pressure taken? In this section you investigate what blood pressure is and how pressure affects the flow of blood.

What is pressure? Pressure is a force exerted over an area. You feel pressure on your skin when you touch it. If you shake a can of soda and make a small hole in the can pressure squirts soda out of the hole, as shown in Figure 6.1.

Figure 6.1 When you shake up a can of soda, pressure builds up inside. Then if you open the can, soda comes squirting out.

In the soda can example, the size of the hole will affect how the pressure is released. If the hole is narrow and “resists” the squirt, the squirting lasts longer. If the hole in a similar can is very large, a lot more soda squirts out all at once. The big hole doesn't resist the flow as much as the small hole does.

Remember how a siphon pump works. When pressure builds up in the outlet tube, fluid squirts out. The pressure buildup in the outlet tube is called a pressure head or a head of pressure. In a siphon pump the head of pressure comes from the squeezing of your hand. In your heart the pressure head comes from the squeezing of the ventricle. The pressure forces water through the tubes in a siphon. In the same way, the pressure forces blood through the vessels in your body. Vessels, especially the arterioles, can create pressure heads by squeezing the vessel opening smaller. This creates a slowing of the blood flow within the vessel. The slowing of the blood flow is called resistance. This kind of resistance can be affected by the thickness of the blood, by the length of the vessel, and by the diameter (the width across) of the vessel.

Figure 6.2 Arterioles help regulate the pressure and flow of blood. When arterioles are relaxed, blood flows freely. This is similar to the free flow of water in a garden hose when the spray nozzle is removed, as seen on the left. The spray nozzle adds resistance and a pressure head builds up, as seen on the right.

Blood Pressure

Your blood pressure tells how hard your circulatory system is working. Blood pressure measures the force with which blood travels through your blood vessels. High blood pressure means your circulatory system is working too hard. Low blood pressure occurs when your circulatory system is not working as hard as normal. Low blood pressure can signal that your body tissues may not be getting enough of what they need. Blood pressure is really a measure of how much resistance the blood meets when traveling through the vessels. Let's take a closer look at resistance.


Have you ever used a very long straw to drink your milk? Is it hard or easy? A short straw is easier to drink milk through than a long straw of the same diameter. You know your heart pumps blood through long narrow tubes called arteries. The arteries from the left side of your heart (aorta and its branches) are longer than the arteries connected to the right side (the pulmonary arteries.) So the left side of the heart has to be stronger to push blood through the longer blood vessels that reach all parts of your body.

Why is it harder to push blood through long, narrow tubes? The blood rubs against the wall along the entire length of the tube. This rubbing is called friction. Friction is the rubbing of any two things together. There is more friction in long tubes than in short ones when the tubes have the same inside diameters. Of course there is more friction in small diameter tubes than in ones with larger diameters.

Activity 6-1: Pressure, Flow, and Resistance Introduction

Does blood flow at the same rate through vessels of different diameters? In this activity you investigate how pressure and resistance influence the rate of flow.


  • 2 large containers or beakers
  • 2 equal lengths of flexible tubing of different inside diameters
  • Clamp
  • Metric ruler
  • Plastic bottle or large metal can
  • Masking tape
  • Large nail
  • Water
  • Sink, container, or large tray to collect water
  • Activity Report


Part A

Step 1 Obtain 2 large beakers or containers. Label one beaker A. Label the other beaker B. Fill each beaker 3/4 full of water. Make sure you use the same amount of water in each container.

Step 2 Obtain 2 pieces of tubing and 2 clamps.

Step 3 Fill the tubes with water. Close off one end of each tube with a clamp.

Step 4 Submerge one piece of tubing in the beaker marked A. Submerge the other piece of tubing in the beaker marked B.

Step 5 Allow the closed ends of the tubing to hang over the side of the container as shown in Figure 6.3.

Step 6 Organize the team tasks. Decide the following: Who will be the timer? Who will start the flow for beaker A? Who will start the flow for beaker B? Who will record the results? Who will observe the distance of the water stream from each beaker?

Step 7 Make sure the closed ends of the tube are pointing to the sink or large tray. At the timer's signal, remove the clamps. This will start the tubes flowing from beakers A and B into the sink or large tray.

Step 8 Answer the questions on the Activity Report.

Figure 6.3 Let the closed ends of the tubing hang over the side of the container.

Part B

Step 1 Using the large nail make three holes at different heights in the side of the plastic bottle.

Step 2 Cover the holes with tape.

Step 3 Fill the bottle with water.

Step 4 Place the bottle near a sink or large tray, and pull the tape off from all three holes.

Step 5 Compare the flow from each of the holes.

Step 6 Answer the questions on the Activity Report.

Friction and Resistance

More friction makes the heart work harder to push the blood. What factors increase friction in your arteries? Think about the straw example. If the straw is longer, you have to do more work to suck the liquid up. In a similar way, the heart has to work harder to move blood through longer blood vessels. A longer vessel has more resistance than a shorter one. Is it easier to drink through a narrow straw or a straw with a larger diameter? Think about the diameter of the blood vessels. How does the diameter of vessels affect friction?

How does friction affect the function of your heart? Let's try to find out. First think about how much blood the heart pumps each minute. For example, when you exercise, your heart beats about 100 times each minute. Each heartbeat pumps 70 milliliters of blood from each ventricle. So the total volume of blood pumped per minute per ventricle is 7000 milliliters, or 7 liters.

Remember that your heart is two pumps working together. Which pump is stronger? You know both pumps have to pump the same amount of blood. Why does one have to be stronger than the other?

Did You Know?
During exercise, muscles get 88 percent of the blood pumped through the heart. At rest, muscles....get only 20 percent of the blood. When exercising, your stomach gets only 1 percent of blood compared to the 24 percent it needs to digest a meal.

Figure 6.4 Blood flowing inside tubes is slowed by friction when the blood rubs against the vessel wall.

Figure 6.5 Cross-section of a human heart.

Cold Toes

When your feet, hands, ears, or nose get cold the blood vessels to those areas close down. When the tissues in those areas get very cold, they can freeze. Now what do you think frostbite is? How do you treat frostbite? What is gangrene? How is circulation involved? Do library/internet research to find out.

Look at Figure 6.5. Notice that the left and right sides of the heart are different from each other. Resistance makes the left side of the heart work harder than the right side. The left side of the heart pushes blood through longer blood vessels that have more resistance. The muscle of the left ventricle is thicker and stronger to do this added work. The right ventricle doesn't need to be as strong because it pumps blood to the lungs through a shorter system of blood vessels with less resistance.

The beating of the heart squirts blood under pressure into the arteries. Look at Figure 6.6. Blood pressure in your arteries is like the water under pressure in the outlet tube. The arterioles keep blood pressure in arteries high to keep the flow going.

The arterioles regulate the flow of blood to where it is needed. After you eat a big meal, such as Thanksgiving dinner, the arterioles of the stomach and intestines open wide. This allows the blood to flow in to absorb food nutrients. Since large muscles, such as your leg muscles, do not need as much blood, the arterioles connecting to those areas constrict. When you exercise, the reverse happens. Running too soon after Thanksgiving dinner is uncomfortable because our bodies aren't set up to run and digest food at the same time.

Measuring Blood Pressure

Have you been to the doctor and had your blood pressure taken? Your doctor wraps an inflatable cuff around your upper arm. Then the doctor pumps the cuff until it's tight, squeezing your arm. The doctor uses a stethoscope to listen to the big artery in your arm as the pressure in the cuff is released. Your arm may feel tingly as the pressure releases. Afterward, the doctor removes the cuff and writes a couple of numbers on your chart. What the doctor writes may look like this-BP 110/70.

Figure 6.6 The muscles in the walls of arterioles can contract or pinch causing pressure to build up behind the pinch.

The letters BP stand for blood pressure. The number to the left or above the bar, 110, is the systolic blood pressure. Remember that systole is when the heart is pumping blood into your arteries. So the systolic blood pressure is the pressure in your arteries when the heart is pumping. The number on the right or below the bar, 70, is the diastolic blood pressure. Remember that diastole is when the heart is resting between beats. So the diastolic blood pressure is the pressure in your arteries when your heart is filling, not pumping.

Blood pressure is measured in units called Torr, or millimeters of mercury mm/Hg. The tool usually used to measure blood pressure has a column of mercury in a glass tube connected to the rubber cuff. This device is called a sphygmomanometer (sfig-moh-man-AHM-ih-tur). The height of the mercury column in millimeters gives the blood pressure reading. Mechanical versions with a circular dial and electronic versions that give a digital readout are also in use.

In which blood vessels is the rate of blood flow the slowest? In which blood vessels is blood pressure the lowest?

Importance of Blood Pressure

Your blood pressure measurement helps tell how well your heart and circulation are working. High blood pressure is also called hypertension (hi-per-TE -shun). Hypertension, or high blood pressure, is a common problem. One out of every five people in the United States either has or will get high blood pressure.

Did You Know?
Blood pressure varies a lot depending on what you're doing. Compare the blood pressures in the table below.
Systolic Diastolic



120 80
Stress 140 90



180 100

Figure 6.7 This device is called a barometer. A barometer measures the changes in air pressure. The mercury moves up or down in the tube depending on the air pressure. In a similar way blood pressure is measured in millimeters of mercury. Asphygmomanometer works a lot like a barometer.

Figure 6.8 This drawing shows how the cuff and stethoscope should be positioned to read the blood pressure.

People who have high blood pressure don't always notice the symptoms. But if high blood pressure goes untreated it can cause problems with the heart, brain, and kidneys. So it is important to catch and treat me problem early. Sometimes high blood pressure is inherited in a family. people who have family members with high blood pressure should have periodic blood pressure checks.

Controlling Blood Pressure and Maintaining Homeostasis

Remember that homeostasis is the balance in all of the components of your internal environment such as temperature and blood pressure. To maintain homeostasis, your body must respond to changes both inside and outside your body24 hours a day. How does it do this? Your body has a system of sensors which sense changes in your environment and in your body. The sensors send messages to the brain where controllers send back commands to your muscles and other organs that can respond to the change. For example, think about what happens when someone right behind you drops a pile of books. Your ears send a message to your brain. Your brain sends a message to your heart. Your heart beats faster and begins to pump more blood. If this were a real emergency, the increased blood flow to your muscles would help you escape.

Both the nervous system and the endocrine system work to control and coordinate the body's activities. Generally, the body's control systems respond to changes and make adjustments to keep everything normal. For example, in a snowstorm our control systems may start our bodies shivering from the cold. The control system also makes us feel cold and miserable. This is a way the control systems tell us to get out of the snow or put warmer clothes on.

Controllers use negative feedback in deciding what adjustments to make. Feedback is information about something that happened. Negative feedback tells a controller to counteract or make up for an earlier change. For example, a home thermostat uses negative feedback. Suppose you set the thermostat on the wall for 60°F (14.5°C). If the room temperature drops below 60°F, the thermostat senses the cooler temperature and turns on the furnace. When the room reaches the set temperature again, the thermostat turns the furnace off. The difference between the thermostat's temperature setting and the room temperature acts as negative feedback. Thus, changes in the temperature information received by the thermostat causes responses that will cancel or reverse the change. That is why it is called negative feedback.

Activity 6-2: How a Controller Works Introduction

The controller for blood pressure is located in the medulla (muh-DOOL-uh) at the base of your brain. It regulates the heart and blood vessels to keep your blood pressure within safe limits. In this activity you investigate how a controller works by becoming the thermostat for a temperature control system. You will keep the temperature at 37°C (98.6°F).


  • Water bath: 1000 ml
  • Thermometer
  • Crushed ice in container
  • Hot plate or other heat source
  • Paper towels
  • Resources 1, 2, and 3
  • Activity Report


Step 1 There are four jobs in your group.

  • Student 1 watches the thermometer and tells the others to add ice or heat in order to keep the temperature at 37°C.
  • Student 2 adds ice water to the bath if the thermometer reads greater than 37°C.
  • Student 3 turns on the hot plate if the thermometer reads less than 37°C.
  • Student 4 is the recorder and completes the temperature table.

Step 2 Use the data table on Resource Sheet 2 for recording data.

Step 3 Heat the water in the beaker to a temperature of 37°C. Adjust the temperature with ice or the burner. When the temperature is exactly 37°C, use this as time zero and start recording. CAUTION -hot plate is HOT!

Step 4 Maintain the water temperature at 37°C for 20 minutes. Add ice or heat as necessary.

Step 5 Take the temperature every 2 minutes for 20 minutes and record it in your table on Resource Sheet 2.

Step 6 Plot the data from your data table on the line graph on Resource Sheet 3.

Step 7 Ask your teacher for cleanup instructions.

Pressure Sensors

Now, let's see how a negative feedback system works in our bodies by exploring blood pressure. Blood pressure is one indicator of how hard the circulatory system has to work. If blood pressure is kept normal 120/80(mm/Hg) blood moves through all vessels of the circulatory system at the proper rate and the body's cells thrive.

Pressure sensors in the aorta and carotid (kuh-ROT-ihd) arteries help the body maintain a safe blood pressure. If the pressure sensors sense a drop in blood pressure, they send a signal to the brain. The brain responds as if saying, “Well, seems the blood pressure in the aorta is low. If this keeps up I'll get too little blood and oxygen and pass out. So, I'd better fix it!” The brain fixes it by sending a message to the heart to beat faster and more forcefully. If the heart increases its squeezing force and the number of systoles (beats) each minute, more blood passes into the aorta. When the aorta fills with more blood, the blood pressure rises, then the brain stops getting the “low blood pressure” messages. What might happen if the brain got the message that the pressure in the aorta was too high? To answer that question, just go through the steps in the opposite way.

Figure 6.9 Blood pressure is maintained through a negative feedback system.

Why might doctors watch diastolic pressure more carefully than systolic pressure?

Figure 6.9 shows how blood pressure is regulated. The brain receives messages about the blood pressure in the arteries. The brain sends out messages to change how fast and how strong the heart beats. This keeps the pressure at about 120/80(mm/Hg). Remember that the pressure must be strong enough to move enough blood from the aorta to supply all the arteries, arterioles, and capillaries. If the pressure in the arteries falls too low, the blood cannot reach everywhere it needs to go.

Suppose you find a person who has cut himself and lost a lot of blood. He may be unconscious. He's pale. His heart beats fast. But his pulse is weak and hard to find. Put all these observations together to explain what you have observed.

The You in You

Lie down on a large sheet of paper and have a friend or family member use a marker to trace your body outline on the paper. Imagine what your circulatory system looks like. Then use colored markers to draw it in. Make sure to include a four-chambered heart, arteries, arterioles, capillaries, venules, veins, the lungs, and the lymph system. Include any other aspects of the circulatory system that you can think of, such as the pacemaker. Have some fun illustrating what's going on in your body as you draw.

Review Questions

  1. Describe two factors that affect cardiac output.
  2. Why is the muscular wall of one side of the heart thicker than the other side?
  3. What is the difference between systolic and diastolic pressure?
  4. How does a controller work?
  5. What does the term homeostasis mean? How does it relate to blood flow?

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Date Created:
Feb 23, 2012
Last Modified:
Jan 30, 2016
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