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Circulatory system

The circulatory system is comprised of the heart, the blood vessels and the blood. Also known as the cardiovascular system, it consists of two circuits that carry the blood around, the smaller being the pulmonary circuit which runs between the heart and the lungs, and the larger being the systemic circuit, which runs between the heart and the peripheral tissues. This article will discuss the function of the cardiovascular system and its pathway through the body as well as a basic overview of its components, including the heart, the blood vessels and the blood


The cardiac circuits function by providing the tissues of the body with oxygen and nutrients which are transported in the blood. The pulmonary circuit carries the blood that needs to be oxygenated into the lungs where the exchange of oxygen and carbon dioxide that the body has produced occurs before the blood returns back to the heart. Red blood cells are responsible for oxygen binding and transportation using their hemoglobin proteins which contain iron.

Part of the cardiovascular system (aorta in green) - ventral view

Part of the cardiovascular system (aorta in green) - ventral view

Once this has happened, the heart then pumps the blood around the systemic circuit of the body and delivers the oxygenated red blood cells to the tissues before collecting the deoxygenated blood and sending it back to the heart with unused nutrients and waste products. These extra substances within the blood filter into the liver for processing.


Blood that starts off in the right ventricle of the heart is pumped up into the pulmonary arteries and into the lung tissue. Once the exchange of gases has occurred between the capillaries and the alveoli, the blood runs into the pulmonary veins and back into the left atrium of the heart.

Heart - ventral view

Heart - ventral view

The heart continues to contract forcing the blood through the bicuspid valve into the left ventricle and again up and out into the aorta where it gives ascending and descending branches to the head and neck, the thoracic and abdominal cavities and the upper and lower limbs.

After oxygenating the peripheral tissues, the blood runs into the collecting veins that all end up merging into either the superior vena cava which collects blood from the head, neck, thorax and upper limb or the inferior vena cava which does the abdomen and lower limb. Both the superior and inferior vena cava drain into the right atrium of the heart where the blood is pumped through the tricuspid valve and into the right ventricle where the process starts all over again. The major arteriovenous capillary plexuses that exist are known as the upper body, the right and left lungs, the liver, the digestive system and the lower limbs.

Heart - dorsal view

Heart - dorsal view

The Heart


The heart is a muscle that acts as a pump. Through electrostimulation it beats and pushes the blood around the entire body. The amount of blood that is pumped out of the heart per minute in time is known as the cardiac output (CO). It can be calculated by multiplying the heart rate (HR) which is the number of beats per minute by the stroke volume (SV) which is the amount of blood that collects in the ventricles when they are at full capacity and about to contract. The cardiac output varies depending on whether the person is exercising or resting. The more oxygen the tissues required, for example during running when the muscles are being used, the higher the cardiac output will be in order to meet the needs of the tissues.

Recommended video: Left atrium and ventricle
Structures of the left atrium and ventricle.


The heart is comprised of four chambers that consist of two pairs. The atria collect the returning blood while the ventricles pump the blood out of the heart. The valves in the heart prevent backflow of the blood into the heart. The muscular septum divides up the two sections of the heart, creating a left and right side each containing one atrium and one ventricle. The right side of the heart pumps deoxygenated blood and the left side pumps oxygenated blood.

Recommended video: Right atrium and ventricle
Structures of the right atrium and ventricle.

The Blood Vessels


As the arterial branches go deeper into the tissues they get smaller and smaller so as to be able to access all areas. They do this by branching off into arterioles which are smaller in diameter and more numerous, and then into capillaries. This is the point where the arteries and veins merge with one another in a complex plexus of many tiny branches. This is also where the exchange of gases, nutrients and waste products takes place, because the capillary walls are thin and fenestrated. After this the capillaries collect into venules, which are the equivalent to arterioles and finally into the veins.

Pulmonary arteriole - ventral view

Pulmonary arteriole - ventral view

Major Types

There are three major classes of vessels, including the arteries, veins and capillaries and they are categorized according to their histological structure:

  • Arteries transport blood towards the tissues and away from the heart and have thick muscular walls with small internal lumina or passageways that can withstand blood under high pressure.

Muscular artery - histological slide

Muscular artery - histological slide

  • Veins carry blood away from the tissues and towards the heart and have thin walls. Their internal lumen is larger than that of the arteries due to the fact that they contain blood under low pressure. They also have valves that prevent the blood from flowing backwards.

Vein - histological slide

Vein - histological slide

  • Lastly, the capillaries which are found in the muscles and the lungs are microscopic and only one cells width thick. They can only tolerate blood under very low pressure due to the fact that it will move slower and gas exchange has a chance to take place.

Capillaries - histological slide

Capillaries - histological slide


The blood is made up of four major components:

  • The plasma is the fluid that surrounds the blood cells and helps transport carbon dioxide, hormones and metabolic waste products.
  • Red blood cells, which are also known as erythrocytes, are formed in the bone marrow and function mainly as oxygen carriers.

Erythrocyte - histological slide

Erythrocyte - histological slide

  • White blood cells or leukocytes make up the body’s immune system by producing antibodies and helping destroy harmful microorganisms. These cells are also created in the bone marrow.
  • Lastly, platelets are cells that clump together to form blood clots and help protect the body by preventing bleeding.

Platelets - histological slide

Platelets - histological slide


The circulatory system, also known as the cardiovascular system, is comprised of the heart, the blood vessels and the blood, which together form two major circuits that carry the blood around: a smaller pulmonary circuit which runs between the heart and the lungs, and a larger systemic circuit, which runs between the heart and the peripheral tissues.

The functions of the cardiovascular system is to oxygenate the blood and excrete carbon dioxide via the pulmonary circulation and to oxygenate all the tissues and remove their wastes via the systemic circulation.

The pathway of blood flow is the following:

  • Right ventricle
  • Pulmonary arteries
  • Lungs
  • Pulmonary veins
  • Left atrium
  • Bicuspid valve
  • Left ventricle
  • Aorta
  • Superior/Inferior Vena Cavae
  • Right atrium
  • Tricuspid valve
  • Right ventricle
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Show references


  • The circulatory system. GCSE Bitesize - BBC


  • Dr. Alexandra Sieroslawska


  • Part of the cardiovascular system (aorta in green) - ventral view - Begoña Rodriguez
  • Heart - ventral view - Yousun Koh
  • Heart - dorsal view - Yousun Koh
  • Pulmonary arteriole - ventral view - Paul Kim
  • Muscular artery - histological slide - Smart In Media
  • Vein - histological slide - Smart In Media
  • Capillaries - histological slide - Smart In Media
  • Erythrocyte - histological slide - Smart In Media
  • Platelets - histological slide - Smart In Media
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Related Atlas Images

Cardiovascular system

Heart in situ

Anterior view of the heart

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