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Animal Movement
Animal Movement

Animal Circulation

Basics of Circulatory Systems: Open vs. Closed

Animals are comprised of trillions of cells, and each cell in the body needs a supply of oxygen and nutrients in order to survive. To deliver these things and take away wastes, nearly all animals need a special transit system which is called the circulatory system. There are two types of circulatory systems: open and closed.

The open circulatory system is found in arthropods and mollusks. The body cavity of these animals is called a hemocoel. The heart pumps a blood-like substance directly into the hemocoel where it moves freely and the organs are able to directly access the nutrients. In the hemocoel, blood and interstitial fluid (a special fluid which surrounds cells) is combined into a substance called hemolymph. This is the goo which sometimes squishes out of bugs when they get stepped on.

A diagram of an open circulatory system. The heart pumps hemolymph out into the body cavity shaded in pink where it can directly bathe the tissues and provide nutrients and oxygen to them without the use of blood vessels.

A disadvantage to this system is that the fluid cannot be sent to a particular region of the body. Once it is pumped into the body cavity, it can be shifted when the animal moves, but precise delivery is difficult.

While the open system works fine for smaller animals, it would not be efficient in larger animals because it would require a tremendous amount of work to pump blood through a single large open body cavity. Imagine a pool filter that has to circulate every drop of pool water within a couple of minutes. Yikes.

Many large animals utilize a closed circulatory system. This means that the blood is enclosed in a series of vessels rather than flowing freely inside the body cavity.

By retaining the blood inside tiny blood vessels it can be pumped to the far reaches of larger animals. It also gives the animal the possibility of pumping more blood to some areas and not others. A disadvantage of the closed system is that blood is no longer in direct contact with the plasma membrane of the cells. Diffusion of nutrients must first take place in two steps, first out of the blood vessel and then into the cell.

A diagram of a closed circulatory system. The blood is enclosed in a series of blood vessels and it is pumped through them to different parts of the body.

Circulatory System Components


The heart is the key to any circulatory system because it provides the muscle power to move the blood throughout the entire body, even to itself. The arteries that supply blood to the heart are called coronary arteries, and they are the bright red blood vessels shown crisscrossing the top of the heart in the picture.

A human heart. Notice the red blood vessels running across the top. These are the coronary arteries, which supply blood to the powerful heart muscle.

Hearts come in many sizes; typically the size of the heart is directly proportional to the size of the animal. The heart of a tiny fairy wasp is less than 0.2mm. Otherwise, its heart would take over the full wasp length. The heaviest heart ever recorded belonged to a great blue whale and weighed 1,980 pounds. That's a ton of heart.

Apart from size, not all hearts have the same structure, either. An insect's heart consists of a tube-like structure which runs along an insect's back and contracts to move hemolymph from the posterior to the anterior. In the abdominal section there are a series of valves, called ostia, which allow hemolymph to enter the heart and get pumped into the anterior of the hemocoel.

Many other animals have hearts with several distinct chambers. Fish have a two-chambered heart. The two cavities are called the atrium and the ventricle. Blood always enters the heart in an atrium (just like an entrance to a building) and always leaves the heart through a ventricle.

For this reason, ventricles are always stronger than atriums because they provide the golden push that sends the blood on the long journey throughout the body. If atria and ventricles were Matthias Schlitte's arms, the atria would be the left and the ventricles would be his right.

Amphibians and reptiles have a three-chambered heart with two atria and one ventricle. Birds and mammals have a four-chambered heart with two atria and two ventricles. Between the heart chambers, there are heart valves, which are flaps of tissue that prevent backward blood flow. We will discuss the pattern of blood flow through the heart later.

As a muscle, a heart contraction is stimulated by an electrical signal created by a group of cells in an area of the heart called the sinoatrial node which tells the heart to beat every 1.0-1.6 seconds. A damaged sinoatrial node can be replaced with an artificial pacemaker that tells the heart when to beat. A pacemaker can be seen on the X-ray image shown below. Pacemakers can last for several years until, like all electronics devices, the batteries wear out.

A chest X-ray showing an artificial pace-maker inside a human chest. Image from here.

Blood Vessels

The system of blood vessels consists of arteries, veins, and capillaries.

Arteries carry blood away from the heart. They are composed of three layers, and the inside layer is coated with special anti-clotting factors which keep the blood from clogging. Unlike a clogged toilet, a clogged blood vessel can actually kill you. It's important to keep blood flowing smoothly. The middle layer contains muscle and elastic fibers, and the outside is comprised of connective tissue and elastic fibers. The elasticity of an artery allows it to flex and accommodate blood being pumped into it with a lot of force.

Interface where arteries split into capillaries, gases are exchanged with the nearby cells, and then the capillaries join together to form veins that take the deoxygenated blood back to the heart.

Capillaries are the tiniest blood vessel, and they are made of epithelial cells. Capillaries are the site of gas and nutrient exchange, and to facilitate diffusion the wall of a capillary is one cell thick. A capillary is approximately 1 mm in length and it is so narrow that red blood cells have to go through single file.

Veins carry blood returning to the heart, and since the blood they are carrying is under less pressure, they are built differently than arteries. Veins have thinner walls and larger diameters, which encourage the blood to flow back to the heart. Veins have one-way valves incorporated into them, which prevent blood from flowing backwards away from the heart.


In vertebrates, blood consists of cells, cell fragments, and plasma. When blood is separated in a centrifuge (or by letting it sit long enough in a test tube) you can see a yellow fluid at the top and a dark red mass at the bottom. The top fluid is plasma, which contains various nutrients, dissolved gasses, and special immune system proteins. The bottom part contains white blood cells, platelets, and red blood cells.

Three types of blood cells: red (left), platelet (middle), and white (right).

White blood cells are body's militia. They patrol the body to defend it from pathogens like viruses and bacteria. Platelets are cell fragments that are key ingredients in forming a blood clot. If you cut yourself and start to bleed, the platelets will form a net-like structure at the site of the injury and prevent blood from freely flowing out. Together with other proteins found in the plasma, they will plug the injured site until it can be properly repaired.

Red blood cells are round disc shaped cells whose primary function is carrying oxygen to the cells and carrying carbon dioxide away from them. Red blood cells are filled with hemoglobin, which is the key protein for gas exchange. It is also the reason why blood is red. Hemoglobin is high in iron, which gives the red color. When a red blood cell is carrying oxygen it is bright red in color. When it is deoxygenated it takes on more of a purple color, which is why the veins in your wrist look blue.

Hemolymph in mollusks uses hemocyanin instead of hemoglobin to carry oxygen, and oxygenated hemocyanin turns blue. Unlike hemoglobin, hemocyanin is not part of a red blood cell and is suspended on its own within the hemolymph. Insect hemolymph does not need to carry oxygen since insects have their own system for gas exchange—more on that later.

Pathway of Blood Flow

In a closed circulatory system, the blood can travel in either a single circulation pathway, as seen in fish, or it can travel in a double circulation pathway. The two pathways differ in how many times the heart pumps the blood during a single cycle of circulation.

Fish have a single circulatory system in which the blood is pumped from the heart to the gills and then to various points in the body. For every cycle though the body it goes to the heart one time.

Diagram of the differences in animal circulatory systems.

In fish, the first destination for the blood is the gills. This is the Shell gas station of the fish; it's where the blood loads up on oxygen. Blood flowing through the gills is especially slow because of the tiny narrow capillaries and the massive gas exchange taking place. This loss of momentum is the downside to a single circulation system, because when the blood exits the gills it just barely has enough pressure to make it around the rest of the body.

We've come to the portion of the guide where we compare Michael Phelps to blood cells. Imagine Mr. Phelps pushing off one wall of a pool of a small pool and trying to reach the other side without kicking. He leaves with a lot energy and expects to reach the other side with ease (don't we all?)…BUT, then he travels through a tight tunnel that slows him down. Once free of the tight tunnel, Mike has lost most of his momentum, but he is still able to slowly glide to the other end of the pool.

In larger animals it would be like trying that with a giant pool. A single pump is just not enough to send the blood to the lungs, around the body, and back again. Instead, mammals, birds, reptiles, and amphibians have a double circulation pathway. This means that the heart pumps the blood twice per circulation.

The first pump is for pulmonary circulation which specifically sends the blood to the lungs to get oxygenated and then it returns it to the heart. The second pump is for systemic circulation which sends the oxygenated blood out to the rest of the body. This teamwork provides enough pressure for the blood to get through the lungs and also make it through the rest of the body.

The movement through the heart in the double circulation system always starts in the right atrium and the proceeds to the right ventricle. The right ventricle sends the blood to the lungs, which slows it down. However, this blood returns to the heart where it enters the left atrium. The final chamber of the heart that the blood goes to is the left ventricle, which gives it a final push and sends it for systemic circulation out to the far corners of the body.

If there is only one ventricle, as in amphibians and reptiles, the same pathway is followed, except that the right and left ventricles are a single cavity. This means that oxygenated and non-oxygenated blood can mix together, and that the blood being sent out towards the body will likely only be partially oxygenated. There is sometimes a small fence-like divider of tissue called a septum, which partially divides the heart and prevents some of the blood from mixing.

Uses of the Circulatory System

As mentioned, the circulatory system is used for delivering oxygen and nutrients to the cells. When we exercise our muscles, we are using oxygen very quickly and as a result our cells need us to speed up the delivery cycle. This is why our breathing and heart rate increases when we exercise.

The circulatory system is also important for removing wastes from the cells. In many animals the waste is delivered to the kidneys and the liver where it is processed for excretion from the animal. The circulatory system is also involved in temperature regulation.

Brain Snack

Did you know that Octopi have three hearts? Two of them pump blood past the gills, and one pumps it to the rest of the body.
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