7.5 The Circulatory System

Learning Objectives

• Identify the main structures of the cardiovascular system.
• Identify three types of blood vessels.
• Describe the differences between the pulmonary and the systemic circulations.

Introduction

Every cell in your body depends on your circulatory system. How can this be?

Your circulatory (cardiovascular) system has many jobs. It acts as a message delivery service, a pump, a heating system, and a protector of the body against diseases. Every cell in your body depends on your circulatory system. In this section, you will learn how your circulatory system works and how it helps to maintain homeostasis.

Guided Learning

Functions of the Circulatory System

The circulatory system shown in Figure below is the organ system that is made up of the heart, the blood vessels, and the blood. It is a closed system meaning that the blood is contained at all times within the vessels. It moves nutrients, hormones, gases (such as oxygen), and wastes (such as carbon dioxide) to and from your cells. It also helps to keep you warm by moving warm blood around your body. To do these tasks, your circulatory system works with other organ systems, such as the respiratory and nervous systems.

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The cardiovascular system moves nutrients and other substances throughout the body.

The Movement of Gases

The movement of gases, especially oxygen and carbon dioxide, is one of the most important jobs of the circulatory system. The circulatory system cannot do this alone. It must work with other organ systems, especially the respiratory system, to move these gases throughout your body.

Oxygen is needed by every cell in your body. You breathe in oxygen and breathe out carbon dioxide through your respiratory system. Once oxygen enters your lungs, it must enter your bloodstream in order to move around your body. Oxygen is moved in your blood by attaching to a protein called hemoglobin. The oxygen moves from the blood into the tissues, while carbon dioxide travels in the opposite direction. Carbon dioxide is transported back to the lungs, where it moves out of the blood and into your lungs for release from your body.

Parts of the Circulatory System

Your heart pushes the blood around your body through the blood vessels. The heart, shown in Figure below, is made of cardiac muscle. The heart is connected to many blood vessels that bring blood all around the body. The cardiac muscle contracts and pumps blood through the blood vessels.

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Blood is collected in the heart and pumped out to the lungs, where it releases carbon dioxide and picks up oxygen before it is pumped to the rest of the body.

Blood Vessels

The job of the blood vessels is to move the blood around the body. There are three main types of blood vessels in the body.

1. Arteries are blood vessels that carry blood away from the heart. Arteries have thick walls that have a layer of smooth muscle, as shown in Figure below. Arteries usually carry oxygen-rich blood around the body. The blood that is in arteries is under pressure. The contractions of the heart muscle causes blood to push against the walls of the arteries. This "push" is referred to as blood pressure. Blood pressure is highest in the arteries and decreases as the blood moves into smaller blood vessels. Thick walls help prevent arteries from bursting under the pressure of blood.
2. Veins are blood vessels that carry blood back to the heart. Veins have thinner walls than arteries do, as you can see in Figure below. The blood in veins is not under pressure. Veins have valves that stop blood from moving backward. Blood is moved forward in veins when the skeletal muscles squeeze the veins. Blood that is carried by veins is usually low in oxygen. The only veins that carry oxygen-rich blood are called the pulmonary veins, which carry blood to the heart from the lungs.
3. Capillaries these are the tiniest blood vessels in the body. Every cell in the body needs oxygen, but arteries are too large to bring oxygen and nutrients to single cells. Further from the heart, arteries form capillaries. The walls of capillaries are only as thick as a single layer of cells. Capillaries connect arteries and veins together, as shown in Figure below. Capillaries also send water, oxygen and other substances to body cells, while they collect carbon dioxide and other wastes from cells and tissues. Capillaries are so narrow that blood cells must move in single file through them. A capillary bed is the network of capillaries that supply an organ with blood. The more active a tissue or organ is, the more capillaries it needs to get nutrients and oxygen. The water, oxygen, and other substances carried by the capillaries are delivered to the surrounding cells by diffusing through the capillaries and into or out of the cells.

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Arteries are thick-walled vessels with many layers, including a layer of smooth muscle.

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The walls of veins are not as thick as artery walls; veins have valves that stop blood from flowing backward.

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Capillaries connect arteries and veins.

Blood

Blood is a body fluid made of blood cells and a liquid called plasma. The main types of cells found in blood are red blood cells and white blood cells.

• Red blood cells carry oxygen. Oxygen-rich blood is bright red and oxygen-poor blood is dark red. Human blood is never blue!
• White blood cells fight against infection and disease.

The circulatory system of humans is "closed." That means the blood never leaves the blood vessels inside of the body. Some other organisms have blood vessels that interact with the environment.

Two Blood Circulation Systems

The blood is pumped around in two large “loops” within the body. One loop moves blood around the body to the head, limbs, and internal organs. The other loop moves blood to and from the lungs where carbon dioxide is released and oxygen is picked up by the blood. A simple version of these two “loops” is shown in Figure below.

Systemic circulation is the part of the cardiovascular system that carries oxygen-rich blood away from the heart, to the body, and returns oxygen-poor blood back to the heart. Pulmonary circulation is the part of the cardiovascular system that carries oxygen-poor blood away from the heart to the lungs, and returns oxygen-rich blood back to the heart.

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The double circulatory system. Trace the systemic circulation. Where is the path of pulmonary circulation?

Summary

• Table below summarizes the structures and functions of the circulatory system.

• The circulatory system includes the heart, the blood vessels, and the blood.
• There are three main types of blood vessels in the body: arteries, veins, and capillaries.
• Systemic circulation is the part of the circulatory system that carries oxygen-rich blood away from the heart, to the body, and returns oxygen-poor blood back to the heart.
• Pulmonary circulation is the part of the circulatory system that carries oxygen-poor blood away from the heart to the lungs, and returns oxygen-rich blood back to the heart.

Vocabulary

arteries

Blood vessels that carry blood away from the heart; have thick walls that have a layer of smooth muscle.

blood

A body fluid that is made of blood cells and a liquid called plasma.

blood pressure

The "push" that is created by the contractions of the heart muscle, causing blood to push against the walls of the arteries; blood pressure is highest in the arteries and decreases as the blood moves into smaller vessels.

capillaries

The tiniest blood vessels in the body that allow an exchange between blood and cells in tissue.

Closed System

        Blood is contained within the blood vessels at all times.

hemoglobin

A protein that attaches to oxygen, which allows it to move throughout the body.

pulmonary circulation

The part of the circulatory system that carries oxygen-poor blood away from the heart to the lungs, and returns oxygen-rich blood back to the heart.

red blood cell (RBC)

A type of blood cell that carries oxygen.

systemic circulation

The part of the circulatory system that carries oxygen-rich blood away from the heart, to the body, and returns oxygen-poor blood back to the heart.

veins

Blood vessels that carry blood back to the heart.

white blood cell (WBC)

A type of blood cell that fights against infection and disease.