READ: Cardiovascular and Respiratory Systems

What does the above image show? It could be any of a number of things. A bowl of melted strawberry ice cream. Some chewed bubble gum. But the above image is actually a close-up of cardiac muscle, the muscle that makes up your heart. If you recall, cardiac muscle is one of three muscle types in the human body. Is cardiac muscle found anywhere else in the body? No. Cardiac muscle is only found in the heart!

Why is cardiac muscle only found in the heart? Why does the heart need its own special muscle? What does blood do, anyway? If a heart pumps blood, how does the heart get the blood it needs to keep pumping? What happens if the heart does not get enough blood?

You may have heard of a heart attack - but what is actually happening in the heart when that happens?

Consider these questions about the heart and cardiac muscle as you read about one of the most important and intriguing systems in the body, the cardiovascular system.

Lesson 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.
• Identify the main structures of the lymphatic system.
• Outline how the cardiovascular and the lymphatic systems work together.
• Identify the parts of the respiratory system.
• Identify the main function of the respiratory system.
• Describe how breathing works.
• Outline how the respiratory system and the cardiovascular system work together.
• Identify how breathing and cellular respiration are connected.

Vocabulary

Functions of the Cardiovascular System

Your 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 cardiovascular system. In this chapter, you will learn how your cardiovascular system works and how it helps to maintain homeostasis.

The cardiovascular system shown in Figure below is the organ system that is made up of the heart, the blood vessels, and the blood. 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 cardiovascular system works with other organ systems, such as the respiratory, endocrine, and nervous systems.

The Movement of Gases

The movement of gases, especially oxygen and carbon dioxide, is one of the most important jobs of the cardiovascular system. But the cardiovascular 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 blood stream 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 Cardiovascular 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.

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.

Blood

Blood is a body fluid that is a type of connective tissue. Blood is 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.
• White blood cells fight against infection and disease.

The cardiovascular system of humans is "closed." That means the blood never leaves the blood vessels inside of the body 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.

The Heart

What is the heart? How does it pump blood? The heart is divided into four chambers, or spaces: the left and right atria, and the left and right ventricles. An atrium (singular for atria) is one of the two small, thin-walled chambers on the top of the heart where the blood first enters. A ventricle is one of the two muscular V-shaped chambers that pump blood out of the heart. You can remember they are called ventricles because they are shaped like a "V." The four chambers of the heart are shown in Figure below.

The atria receive the blood, and the ventricles pump the blood out of the heart. Each of the four chambers of the heart has a specific job.

• The right atrium receives oxygen-poor blood from the body.
• The right ventricle pumps oxygen-poor blood toward the lungs.
• The left atrium receives oxygen-rich blood from the lungs.
• The left ventricle pumps oxygen-rich blood out of the heart to the rest of the body.

Where is the Heart?

The heart is closer to the center of the body than you may think. It is usually found in the left to middle of the chest, with the largest part of the heart slightly to the left. It always feels like the heart is on the left side of the body because the left ventricle is bigger and stronger than the right ventricle. The heart is also surrounded by the lungs.

Blood Flow Through the Heart

Blood flows through the heart in two separate loops. You can think of them as a “left side loop” and a “right side loop." The right side of the heart collects oxygen-poor blood from the body and pumps it into the lungs, where it releases carbon dioxide and picks up oxygen. The left side carries the oxygen-rich blood back from the lungs into the left side of the heart, which then pumps the oxygen-rich blood to the rest of the body.

The above image shows the tissue found inside of the lungs. The lungs contain alveoli. Alveoli absorb oxygen and send it to the blood vessels. They also move carbon dioxide from the blood vessels back to the lungs to be exhaled.

Alveoli look like clumps of grapes. Why do you think it is important that your body have many alveoli? What would happen if your alveoli lost their ability to function? Why do you think alveoli are shaped like spheres?

The respiratory system is important because it brings oxygen to cells in your body. But it also removes a waste, carbon dioxide. Another system in your body, the excretory system, also removes wastes. The excretory system moves waste from your digestive system and from your blood out of your body.

How do the respiratory system and excretory system work together? How does damage in one system affect the other? Consider these questions about respiration and waste removal as you read the following chapter.

How We Breathe

Most of the time, you breathe without thinking about it. Breathing is mostly an involuntary action that is controlled by a part of your brain that also controls your heart beat. If you swim, do yoga, or sing, you know you can also control your breathing. Taking air into the body through the nose and mouth is called inhalation. Pushing air out of the body through the nose or mouth is called exhalation. The man in Figure below is exhaling before he surfaces from the pool water.

How do lungs allow air in? As mentioned above, air moves into and out of the lungs by the movement of muscles. The diaphragm and rib muscles contract and relax to move air into and out of the lungs. During inhalation, the diaphragm contracts and moves downward. The rib muscles contract and cause the ribs to move outward. This causes the chest volume to increase. Because the chest volume is larger, the air pressure inside the lungs is lower than the air pressure outside. This difference in air pressures causes air to be sucked into the lungs. When the diaphragm and rib muscles relax, air is pushed out of the lungs. Exhalation is similar to letting the air out of a balloon.

The walls of the alveoli are very thin and allow gases to enter into them. The alveoli are lined with capillaries. These capillaries are shown in Figure below. Oxygen moves from the alveoli to the blood in the capillaries that surround the alveoli. At the same time, carbon dioxide moves in the opposite direction, from capillary blood to the alveoli.

Breathing and Respiration

When you breath in, oxygen is drawn in through the mouth and down into the lungs. The oxygen then passes across the thin lining of the capillaries and into the blood. The oxygen molecules are carried to the body cells by the blood. Carbon dioxide from the body cells is carried by the blood to the lungs where it is released into the air. The process of getting oxygen into the body and releasing carbon dioxide is called respiration.

Sometimes breathing is called respiration, but there is much more to respiration than just breathing. There are actually two parts to respiration, external respiration and internal respiration. External respiration is the movement of oxygen into the body and carbon dioxide out of the body. Internal respiration is the exchange of oxygen and carbon dioxide between the blood and the cells of the body.

The Journey of a Breath of Air

Breathing is only part of the process of bringing oxygen to where it is needed in the body. After oxygen enters the lungs, what happens?

1. The oxygen enters the blood stream from the alveoli. Then, the oxygen-rich blood returns to the heart.
2. Oxygen-rich blood is then pumped through the aorta.
3. From the aorta, oxygen-rich blood travels to the smaller arteries and finally to the capillaries.
4. The oxygen molecules move out of the capillaries and into the body cells.
5. While oxygen moves from the capillaries and into body cells, carbon dioxide moves from the cells into the capillaries.

Breathing and Cellular Respiration

The oxygen that arrives at the cells from the lungs is used by the cells to help release the energy stored in molecules of sugar. Cellular respiration is the process of breaking down glucose to release energy (see the Cell Functions chapter). The waste products of cellular respiration include carbon dioxide and water. The carbon dioxide molecules move out of the cells and into the capillaries that surround the cells. As explained above, the carbon dioxide is removed from the body by the lungs.