Blood Vessels [Part 2]

Capillaries

Eventually, blood flows from small arteries into the tiny capillaries. In the capillaries, materials are exchanged between the blood and the body's cells. Capillary walls are only one cell thick.Thus, materials can pass easily through them. Materials such as oxygen and glucose pass from the blood, through the capillary walls, to the cells. Cellular waste products travel in the opposite direction - from cells, through the capillary walls, and into the blood.
One way that materials are exchanged between the blood and body cells is by diffusion. Diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration. For example, glucose is more highly concentrated in blood than it is in the body cells. Therefore, glucose diffuses from the blood into the body cells.

Diffusion - the process by which molecules move from an area of higher concentration to an area of lower concentration.

Multiple Choice

True or False

In the capillaries, materials are exchanged between the blood and body's cells.

• True

• False

Multiple Choice

How thick are capillary walls?

• One cell thick

• Two cells thick

• Three cells thick

Multiple Choice

What is the process by which molecules move from an area of higher concentration to an area of lower concentration?

• Diffusion

• Fusion

• Pulse

Veins

After blood moves through capillaries, it enters larger blood vessels called veins, which carry blood back to the heart. The wall of veins, like those of arteries, have three layers, with muscle in the middle layer. However, the walls of veins are generally much thinner than those of arteries.

By the time bloodflows into veins, the pushing force of the heart has much less effect than it did in the arteries. Several factors help move blood through veins. First, because many veins are located near skeletal muscles, the contraction of the muscles help push the blood along. For example, as you run or walk, the skeletal muscles in your legs contract and squeeze the veins in your legs. Second, larger veins in your body have valves in them that prevent blood from flowing backward. Third, breathing movements, which exert a squeezing pressure against veins in the chest, also force blood toward the heart.

Multiple Choice

Like arteries, vein walls have how many layers?

• One

• Two

• Three

• Four

Multiple Choice

Where does blood enter after it moves through capillaries?

• Arteries

• Aorta

• Veins

Multiple Choice

True or False

Skeletal muscles help move blood in veins.

• True

• False

Blood Pressure

Suppose that you are washing a car. You attach the hose to the faucet and turn on the faucet. The water flows out in a slow, steady stream. Then, while your back is turned, Dad turns the faucet on all the way. Suddenly, the water spurts out rapidly, and the hose almost jumps out of your hand.

As water flows through a hose, it pushes against the walls of the hose, creating pressure on the walls. Pressure is the force that something exerts over a given area. When your Dad turned on the faucet all the way, the additional water flow increased the pressure exerted on the inside of the hose. The extra pressure made the water spurt out of the nozzle faster.

Blood Pressure (continued)

What Causes Blood Pressure?

Blood traveling through blood vessels behaves in a manner similar to that of water moving through a hose. Blood exerts a force, called blood pressure, against the walls of blood vessels. Blood pressure is caused by the force with which the ventricles contract. In general, as blood moves away from the heart, blood pressure decrease. This change happens because the farther away from the ventricle the blood moves, the lower its force is. Blood flowing through the arteries exerts the highest pressure. Blood pressure in arteries farther from the heart is much lower.

blood pressure - the force by which the ventricles contract

Multiple Choice

What is the force at which ventricles contract?

• Heartbeat

• Pulse

• Blood Pressure

Blood Pressure (continued)

Measuring Blood Pressure

Blood pressure can be measured with an instrument called a sphygmomanometer (sfig moh muh NAHM uh tur). A cuff is wrapped around the upper arm. Air is pumped into the cuff until the blood flow through the artery is stopped. As the pressure is released, the examiner listens to the pulse and records two numbers. Blood pressure is expressed in millimeters of mercury. The first number is a measure of the blood pressure while the heart's ventricles contract and pump blood into the arteries. The second number, which is lower, measures the blood pressure while the ventricles relax. The two numbers are expressed as a fraction: the contraction pressure over the relaxation pressure.

Multiple Choice

What is used to measure blood pressure?

• Tape

• Hammer

• Sphygmomanometer

• Notebook

Lesson Complete

Great Job!