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# 13.28: Blood Vessels

Difficulty Level: At Grade Created by: CK-12
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How does blood travel around the body?

This color-enhanced image was made with an electron microscope, so the objects it depicts are extremely small. This incredible photo shows red blood cells leaking out of a ruptured blood vessel. Blood vessels are part of the circulatory system, the “highway” system of the human body that transports materials to all of its cells. And the red blood cells that carry some of these materials are a little like trucks on the highway.

### Blood Vessels

Blood vessels form a network throughout the body to transport blood to all the body cells. There are three major types of blood vessels: arteries, veins, and capillaries. All three are shown in Figure below and described below.

Blood vessels include arteries, veins, and capillaries.

• Arteries are muscular blood vessels that carry blood away from the heart. They have thick walls that can withstand the pressure of blood being pumped by the heart. Arteries generally carry oxygen-rich blood. The largest artery is the aorta , which receives blood directly from the heart.
• Veins are blood vessels that carry blood toward the heart. This blood is no longer under much pressure, so many veins have valves that prevent backflow of blood. Veins generally carry deoxygenated blood. The largest vein is the inferior vena cava , which carries blood from the lower body to the heart. The superior vena cava brings blood back to the heart from the upper body.
• Capillaries are the smallest type of blood vessels. They connect very small arteries and veins. The exchange of gases and other substances between cells and the blood takes place across the extremely thin walls of capillaries.

#### Blood Vessels and Homeostasis

Blood vessels help regulate body processes by either constricting (becoming narrower) or dilating (becoming wider). These actions occur in response to signals from the autonomic nervous system or the endocrine system. Constriction occurs when the muscular walls of blood vessels contract. This reduces the amount of blood that can flow through the vessels (see Figure below ). Dilation occurs when the walls relax. This increases blood flows through the vessels.

When a blood vessel constricts, less blood can flow through it.

Constriction and dilation allow the circulatory system to change the amount of blood flowing to different organs. For example, during a fight-or-flight response, dilation and constriction of blood vessels allow more blood to flow to skeletal muscles and less to flow to digestive organs. Dilation of blood vessels in the skin allows more blood to flow to the body surface so the body can lose heat. Constriction of these blood vessels has the opposite effect and helps conserve body heat.

#### Blood Vessels and Blood Pressure

The force exerted by circulating blood on the walls of blood vessels is called blood pressure . Blood pressure is highest in arteries and lowest in veins. When you have your blood pressure checked, it is the blood pressure in arteries that is measured. High blood pressure, or hypertension , is a serious health risk but can often be controlled with lifestyle changes or medication. You can learn more about hypertension by watching the animation at this link: http://www.healthcentral.com/high-blood-pressure/introduction-47-115.html .

### Summary

• Arteries carry blood away from the heart, veins carry blood toward the heart, and capillaries connect arteries and veins.

### Practice

Use this resource to answer the questions that follow.

• http://www.hippocampus.org/Biology $\rightarrow$ Biology for AP* $\rightarrow$ Search: The Vascular System
1. Describe arteries and arterioles, veins and venules.
2. Describe the capillary and capillary bed.
3. What happens in the capillaries?

### Review

1. How do arteries differ from veins?

2. What is blood pressure? What is hypertension?

3. To take your pulse, you press your fingers against an artery near the surface of the body. What are you feeling and measuring when you take your pulse? Why can’t you take your pulse by pressing your fingers against a vein?

Feb 24, 2012

Aug 25, 2014