Lesson Explainer: Circulatory Pathways Biology

In this explainer, we will learn how to describe the different circulatory pathways within the human body.

We know that the heart is the central and most crucial organ of the circulatory system. The heart is always functioning, whether one is asleep or doing some form of exercise. We can feel our hearts beating all the time—but what function is the heart performing when it beats?

The heart pumps blood to all the other parts of the body, providing them with the oxygen and nutrients they need to function and carrying away their waste products such as carbon dioxide. This process is called circulation. As you can see in Figure 1, the heart acts as the central organ, with an intricate network of tubes called blood vessels to carry the blood to and from the various parts of the body. Organs that contribute to the same function are organized in systems called organ systems. In total, the human body has 11 organ systems (for example, the digestive system or the nervous system)! Together, the heart and the blood vessels make up the circulatory system.

As with all the other organ systems, the circulatory system is highly organized and structured, so as to perform its functions efficiently. There are a few different types of blood vessels in the body, each of which perform a very specific role in the circulatory system.

Let’s take a look at the structures of each of these components and understand how they are designed to perform their functions, beginning with the main organ: the heart. You can see the structure of the heart depicted in Figure 2.

The heart is made of muscular tissue and nervous tissue. It sits between the two lungs, tilted slightly to the left. If you clench your fist, this is approximately how large your heart is! The heart has four chambers—two upper chambers called atria and two lower chambers called ventricles. As the heart performs its functions, the right and left atria receive the blood entering the heart. The blood then flows to the right and left ventricles, which pump blood out of the heart and to the organs of the body.

Before we begin learning more about the structures of the heart, there is an interesting point we need to discuss! Take a look at the labels for the right and left chambers of the heart in Figure 2. You might notice that the labels for the right side of the heart are actually on the left side of the page, and vice versa! This is because when we study the heart, we look at it as if it were the heart of a person facing us from the opposite side. This person’s left side corresponds to the heart’s left side, and their right side corresponds to the heart’s right side! This is why, on paper, we label the chambers of the heart in the way you can see in Figure 2.

Atria (singular: atrium) are enclosed by thin walls made of muscular tissue, whereas ventricles have thicker and more muscular walls, which are useful for the role they play in pumping blood.

On each side of the heart, the openings from the atria to the ventricles are guarded by structures called valves. Valves consist of flaps made of dense connective tissue, and they help make sure that the blood in the heart does not flow in the wrong direction. As you can see in Figure 2, the opening from the right atrium to the right ventricle is guarded by a valve called the tricuspid valve. It is named the tricuspid valve because it is made of three flaps of dense connective tissue. On the left side, the opening from the left atrium to the left ventricle is guarded by the mitral valve, which is also called the bicuspid valve because it is generally made of two flaps of dense connective tissue.

The left atrium is separated from the right atrium by a thin, muscular wall called the interatrial septum. The left and right ventricles are separated by a thicker muscular wall called the interventricular septum. This separation, as we will see, is crucial to the role played by each of the chambers of the heart.

There are three main types of blood vessels in the body: arteries, veins, and capillaries. Arteries are responsible for carrying blood away from the heart to the different tissues of the body. On the other hand, veins carry blood from the body tissues back to the heart. Both arteries and veins branch into smaller blood vessels called arterioles and venules, which reach every cell in the human body. These thin, tiny blood vessels that connect the venules and arterioles in a continuous circuit are called capillaries, as you can see in Figure 3.

Warm-blooded animals like birds and mammals, including humans, need high quantities of oxygen and other nutrients for their bodies to function. This is because they require large amounts of energy for the activities they perform, such as fast movement and temperature regulation. As we will see now, the circulatory system of these organisms is organized in a very efficient way that enables the body’s high needs for oxygen and nutrients to be met.

When blood is pumped from the heart to the different organs of the body, it contains the oxygen that the organs need to function. On the other hand, blood being transported away from the body parts and toward the heart contains waste products and carbon dioxide. In birds and mammals, the chambers of the heart are separated, as mentioned previously. As shown in Figure 4, this prevents the clean, oxygen-rich blood from mixing with the carbon dioxide-rich blood, enabling the body to supply oxygen to its organs in a highly efficient manner.

This separation involves two main circulatory pathways—the pulmonary circulation pathway and the systemic circulation pathway. Together, these two pathways form a system called the double circulation system.

The pulmonary circulation pathway, shown in the top part of Figure 5, involves the transport of blood from the heart to the lungs, where the process of oxygenation takes place. This means that the blood coming from the heart is enriched with oxygen in the alveoli of the lungs. The blood is then carried back to the heart. The word pulmonary is used to describe anything relating to the lungs.

The systemic circulation pathway, shown in the bottom part of Figure 5, transports the oxygen-rich blood from the heart to the different organs of the body. Here, the blood delivers oxygen to the body’s cells and collects carbon dioxide to be removed from the body. The blood is then returned to the heart. The word systemic is used to describe anything relating to the entire body.

Let’s now take a closer look at the heart chambers and the blood vessels involved in each circulatory pathway, beginning with pulmonary circulation, as represented in Figure 5.

When blood from the different parts of the body is carried by the veins to the heart, it is rich in carbon dioxide and poor in oxygen. This blood enters the right atrium of the heart. The muscles of the right ventricle relax, so that the blood flows through the valve from the right atrium and fills the right ventricle.

The muscles of the right ventricular wall then contract, pumping this blood through a vessel called the pulmonary artery. This vessel carries the carbon dioxide-rich blood to the lungs and branches out into several capillaries. Here, the blood releases its carbon dioxide into the alveoli of the lungs and takes up oxygen. The lungs, therefore, are the site at which blood gets oxygenated.

Once the blood has been enriched with oxygen, it needs to be transported back to the heart, so that it can be pumped to the organs of the body. A vessel called the pulmonary vein accomplishes this, carrying the oxygenated blood from the lungs back to the heart, where it enters the left atrium.

The flow of blood in the pulmonary circulatory system is therefore as follows:

Now that we have understood pulmonary circulation, let’s take a look at the steps involved in the systemic circulation pathway, as represented in Figure 5.

Once the oxygenated blood enters the left atrium, the muscles of the left ventricle relax, and the blood flows through the valve from the left atrium and into the left ventricle. Here, just like on the right side, the muscles of the left ventricle contract.

This time, the blood is pumped through the largest blood vessel in the human body—the aorta. This is an artery that branches into capillaries through the entire body, delivering oxygen and collecting carbon dioxide from every cell in the body.

The carbon dioxide-rich blood is carried toward the heart. The smaller streams of blood merge into two wider veins called the vena cava—one bringing blood from the upper body, called the superior vena cava, and one bringing blood from the lower body, called the inferior vena cava. These two large blood vessels deliver the blood to the right atrium of the heart.

The flow of blood in the systemic circulatory system is therefore as follows:

It is important to note that the two pathways of circulation form a closed loop of blood flow. The two pathways occur simultaneously, as the muscles of the right and left ventricles contract at the same time. These cycles of blood flow are constantly happening in our body, kept in check by the regular pumping of the heart.

Example 2: Oxygenation and Deoxygenation in the Circulatory System

Fill in the blank: In the systemic circulatory system, the blood being transported from the heart to the body is .

1. oxygenated
2. deoxygenated

Answer

The double circulation system involves two circulatory pathways—the pulmonary circulation pathway and the systemic circulation pathway. The systemic circulation pathway involves the circulation of blood from the heart to the other organs of the body, and back to the heart.

In this pathway, oxygen-rich blood is pumped from the left-lower chamber of the heart, the left ventricle, to the organs of the body. To accomplish this, the muscles in the wall of the left ventricle contract. The blood is then carried away from the heart by a blood vessel called the aorta, which is the largest blood vessel in the body. The aorta delivers blood to all the other organs of the body, supplying them with the oxygen they need to function. In the diagram, this stream of blood is represented by the red line emerging from the heart in the systemic circulation pathway.

When the blood reaches the organs of the body, it supplies them with oxygen and instead takes up carbon dioxide from the organs. This blood is then transported back to the heart through large veins called the vena cava. This is represented in the diagram by the blue line traveling back toward the heart in the systemic circulation pathway.

In the systemic circulatory system, the blood being transported from the heart to the body is, therefore, oxygenated.

Aside from pulmonary circulation and systemic circulation, there is a system called the hepatic portal system—a special connection between the digestive system and the liver. The word hepatic is used to describe anything relating to the liver.

Unlike the blood from the other organs of the body, the blood being transported away from the organs of the digestive system is not collected by one of the vena cava straight away. Instead, this blood is first taken up by the hepatic portal system, through another vein called the hepatic portal vein, as you can see in Figure 6. This vein collects blood from the capillaries of the stomach, the intestines, the pancreas, and the spleen and delivers it to the capillaries of the liver.

The liver is the largest internal organ of the human body, and it performs some extremely important functions. It eliminates toxic molecules from the body, produces bile for the breakdown of fats, and stores and releases glucose according to the body’s needs. In fact, the liver performs over two hundred different functions in the body, all of which are crucial to survival. The liver is also the only organ in the body capable of regenerating itself! If someone donates a part of their liver to a patient in need, the remaining portion will gradually grow back and return to its normal function.

The blood transported away from the digestive system contains nutrients, like glucose and amino acids, absorbed by the digestive organs. Since the liver has so many crucial functions to perform, it needs a substantial supply of these nutrients. The hepatic portal system serves this purpose, by directing the nutrient-rich blood from the digestive system straight to the liver. Once the blood has been processed by the liver, it flows through the inferior vena cava along with the blood from the rest of the body.

Aside from supplying the liver with the nutrients it needs, the hepatic portal system serves another very important function in the body.

The blood from the digestive system could also contain toxic molecules absorbed by the digestive organs. These could be derived from substances like ethanol or certain medications a person might take. The hepatic portal system ensures that these molecules are transported in the bloodstream to the liver, where the blood is detoxified. This is a crucial step in the circulatory system, as it eliminates all toxic substances from the body before the blood reenters the heart.

Let’s review and summarize what we have learnt about the circulation pathways.