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Human Circulatory System

The human body is a complex machine, requiring many processes to function efficiently. To keep these crucial processes running without any hitches, vital elements and components need to be delivered to the various parts of the body.

This role of transportation is undertaken by the human circulatory system, moving essential nutrients and minerals throughout the body and metabolic waste products away from the body. Below is a neat labelled  Circulatory system diagram.

Read more:  Human Body Anatomy

Read on to explore intricate about the human circulatory system and its components in greater detail.

Table of Contents

Double Circulation

  • Blood Cells

Blood Vessels

Lymphatic system, human circulatory system diagram.

The human circulatory system consists of a network of arteries, veins, and capillaries, with the heart pumping blood through it. Its primary role is to provide essential nutrients, minerals, and hormones to various parts of the body. Alternatively, the circulatory system is also responsible for collecting metabolic waste and toxins from the cells and tissues to be purified or expelled from the body.

Features of Circulatory System

The crucial features of the human circulatory system are as follows:

  • The human circulatory system consists of blood, heart, blood vessels, and lymph.
  • The human circulatory system circulates blood through two loops (double circulation) – One for oxygenated blood, another for deoxygenated blood.
  • The human heart consists of four chambers – two ventricles and two auricles.
  • The human circulatory system possesses a body-wide network of blood vessels. These comprise arteries, veins, and capillaries.
  • The primary function of blood vessels is to transport oxygenated blood and nutrients to all parts of the body. It is also tasked with collecting metabolic wastes to be expelled from the body.
  • Most circulatory system diagrams do not visually represent its sheer length. Theoretically, if the veins, arteries, and capillaries of a human were laid out, end to end, it would span a total distance of 1,00,000 kilometres (or roughly eight times the diameter of the Earth).

Organs of Circulatory System

The human circulatory system comprises 4 main organs that have specific roles and functions. The vital circulatory system organs include:

  • Blood (technically, blood is considered a tissue and not an organ)
  • Lymphatic system

The heart is a muscular organ located in the chest cavity, right between the lungs. It is positioned slightly towards the left in the thoracic region and is enveloped by the pericardium. The human heart is separated into four chambers; namely, two upper chambers called atria ( singular: atrium ), and two lower chambers called ventricles.


Heart, a major part of the human circulatory system

Though other animals possess a heart, the way their circulatory system functions is quite different from humans. Moreover, in some cases, the human circulatory system is much more evolved when compared to insects or molluscs.

Read More:   Human Heart

The way blood flows in the human body is unique, and it is quite efficient too. The blood circulates through the heart twice, hence, it is called double circulation. Other animals like fish have single circulation, where blood completes a circuit through the entire animal only once.

The main advantage of double circulation is that every tissue in the body has a steady supply of oxygenated blood, and it does not get mixed with the deoxygenated blood.

Further Reading: Double circulation

Double Circulation

Circulation of blood in humans – Double circulation

Blood is the body’s fluid connective tissue, and it forms a vital part of the human circulatory system. Its main function is to circulate nutrients, hormones, minerals and other essential components to different parts of the body. Blood flows through a specified set of pathways called blood vessels. The organ which is involved in pumping blood to different body parts is the heart.  Blood cells, blood plasma, proteins, and other mineral components (such as sodium, potassium and calcium) constitute human blood.

Blood is composed of:

  • Plasma  – the fluid part of the blood and is composed of  90%  of water.
  • Red blood cells, white blood cells and platelets constitute the solid part of blood.

Types of Blood Cells

The human body consists of three types of blood cells, namely:

  • Red blood cells (RBC) / Erythrocytes

Red blood cells are mainly involved in transporting oxygen, nutrients, and other substances to various parts of the body. These blood cells also remove waste from the body.

  • White blood cells (WBC) / Leukocytes

White blood cells are specialized cells, which function as a body’s defence system. They provide immunity by fending off pathogens and harmful microorganisms.

  • Platelets / Thrombocytes

Platelets are cells that help to form clots and stop bleeding. They act on the site of an injury or a wound.

Blood vessels are a network of pathways through which blood travels throughout the body.  Arteries and veins are the two primary types of blood vessels in the circulatory system of the body.

Arteries are blood vessels that transport oxygenated blood from the heart to various parts of the body. They are thick, elastic and are divided into a small network of blood vessels called capillaries. The only exception to this is the pulmonary arteries, which carries deoxygenated blood to the lungs.

Veins are blood vessels that carry deoxygenated blood towards the heart from various parts of the body.  They are thin, elastic and are present closer to the surface of the skin. However, pulmonary and umbilical veins are the only veins that carry oxygenated blood in the entire body.

Also Read:  Blood

The human circulatory system consists of another body fluid called lymph. It is also known as tissue fluid. It is produced by the lymphatic system which comprises a network of interconnected organs, nodes and ducts.

Lymph is a colourless fluid consisting of salts, proteins, water, which transport and circulates digested food and absorbed fat to intercellular spaces in the tissues. Unlike the circulatory system, lymph is not pumped; instead, it passively flows through a network of vessels.

Functions of Circulatory System

The most important function of the circulatory system is transporting oxygen throughout the body. The other vital functions of the human circulatory system are as follows:

  • It helps in sustaining all the organ systems.
  • It transports blood, nutrients, oxygen, carbon dioxide and hormones throughout the body.
  • It protects cells from pathogens.
  • It acts as an interface for cell-to-cell interaction.
  • The substances present in the blood help repair the damaged tissue.

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Heart and Circulatory System

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What Does the Heart Do?

The heart is a pump, usually beating about 60 to 100 times per minute. With each heartbeat, the heart sends blood throughout our bodies, carrying oxygen to every cell. After delivering the oxygen, the blood returns to the heart. The heart then sends the blood to the lungs to pick up more oxygen. This cycle repeats over and over again.

What Does the Circulatory System Do?

The circulatory system is made up of blood vessels that carry blood away from and towards the heart. Arteries carry blood away from the heart and veins carry blood back to the heart.

What Are the Parts of the Heart?

The heart has four chambers — two on top and two on bottom:

  • The two bottom chambers are the right ventricle and the left ventricle . These pump blood out of the heart. A wall called the interventricular septum is between the two ventricles.
  • The two top chambers are the right atrium and the left atrium . They receive the blood entering the heart. A wall called the interatrial septum is between the atria.

Two diagrams show the position of the heart in the body and a close-up, cross section of a healthy heart. Blood flows from the left atrium into the left ventricle and through a valve into the aorta.

The atria are separated from the ventricles by the atrioventricular valves:

  • The tricuspid valve separates the right atrium from the right ventricle.
  • The mitral valve separates the left atrium from the left ventricle.

Two valves also separate the ventricles from the large blood vessels that carry blood leaving the heart:

  • The pulmonic valve is between the right ventricle and the pulmonary artery, which carries blood to the lungs.
  • The aortic valve is between the left ventricle and the aorta, which carries blood to the body.

What Are the Parts of the Circulatory System?

Two pathways come from the heart:

  • The pulmonary circulation is a short loop from the heart to the lungs and back again.
  • The systemic circulation carries blood from the heart to all the other parts of the body and back again.

In pulmonary circulation:

  • The pulmonary artery is a big artery that comes from the heart. It splits into two main branches, and brings blood from the heart to the lungs. At the lungs, the blood picks up oxygen and drops off carbon dioxide. The blood then returns to the heart through the pulmonary veins.

In systemic circulation:

  • Next, blood that returns to the heart has picked up lots of oxygen from the lungs. So it can now go out to the body. The aorta is a big artery that leaves the heart carrying this oxygenated blood. Branches off of the aorta send blood to the muscles of the heart itself, as well as all other parts of the body. Like a tree, the branches gets smaller and smaller as they get farther from the aorta. At each body part, a network of tiny blood vessels called capillaries connects the very small artery branches to very small veins. The capillaries have very thin walls, and through them, nutrients and oxygen are delivered to the cells. Waste products are brought into the capillaries. Capillaries then lead into small veins. Small veins lead to larger and larger veins as the blood approaches the heart. Valves in the veins keep blood flowing in the correct direction. Two large veins that lead into the heart are the superior vena cava and inferior vena cava . (The terms superior and inferior don't mean that one vein is better than the other, but that they're located above and below the heart.) Once the blood is back in the heart, it needs to re-enter the pulmonary circulation and go back to the lungs to drop off the carbon dioxide and pick up more oxygen.

How Does the Heart Beat?

The heart gets messages from the body that tell it when to pump more or less blood depending on a person's needs. For example, when you're sleeping, it pumps just enough to provide for the lower amounts of oxygen needed by your body at rest. But when you're exercising, the heart pumps faster so that your muscles get more oxygen and can work harder.

How the heart beats is controlled by a system of electrical signals in the heart. The sinus (or sinoatrial) node is a small area of tissue in the wall of the right atrium. It sends out an electrical signal to start the contracting (pumping) of the heart muscle. This node is called the pacemaker of the heart because it sets the rate of the heartbeat and causes the rest of the heart to contract in its rhythm.

These electrical impulses make the atria contract first. Then the impulses travel down to the atrioventricular (or AV) node , which acts as a kind of relay station. From here, the electrical signal travels through the right and left ventricles, making them contract.

One complete heartbeat is made up of two phases:

  • The first phase is called systole  (pronounced: SISS-tuh-lee). This is when the ventricles contract and pump blood into the aorta and pulmonary artery. During systole, the atrioventricular valves close, creating the first sound (the lub) of a heartbeat. When the atrioventricular valves close, it keeps the blood from going back up into the atria. During this time, the aortic and pulmonary valves are open to allow blood into the aorta and pulmonary artery. When the ventricles finish contracting, the aortic and pulmonary valves close to prevent blood from flowing back into the ventricles. These valves closing is what creates the second sound (the dub) of a heartbeat.
  • The second phase is called diastole  (pronounced: die-AS-tuh-lee). This is when the atrioventricular valves open and the ventricles relax. This allows the ventricles to fill with blood from the atria, and get ready for the next heartbeat.

How Can I Help Keep My Heart Healthy?

To help keep your heart healthy:

  • Get plenty of exercise.
  • Eat a nutritious diet.
  • Reach and keep a healthy weight .
  • If you smoke, quit .
  • Go for regular medical checkups.
  • Tell the doctor about any family history of heart problems.

Let the doctor know if you have any chest pain, trouble breathing, or dizzy or fainting spells; or if you feel like your heart sometimes goes really fast or skips a beat.


Circulatory system

Actions for this page.

  • The circulatory system delivers oxygen and nutrients to cells and takes away wastes.
  • The heart pumps oxygenated and deoxygenated blood on different sides.
  • The types of blood vessels include arteries, capillaries and veins.

On this page

The right side of the heart, the left side of the heart, blood vessels, capillaries, blood pressure, common problems, where to get help, things to remember.

All cells in the body need to have oxygen and nutrients, and they need their wastes removed. These are the main roles of the circulatory system. The heart, blood and blood vessels work together to service the cells of the body. Using the network of arteries, veins and capillaries, blood carries carbon dioxide to the lungs (for exhalation) and picks up oxygen. From the small intestine, the blood gathers food nutrients and delivers them to every cell.

Blood consists of:

  • Red blood cells – to carry oxygen
  • White blood cells – that make up part of the immune system
  • Platelets – needed for clotting
  • Plasma – blood cells, nutrients and wastes float in this liquid.

The heart pumps blood around the body. It sits inside the chest, in front of the lungs and slightly to the left side. The heart is actually a double pump made up of four chambers, with the flow of blood going in one direction due to the presence of the heart valves. The contractions of the chambers make the sound of heartbeats.

The right upper chamber (atrium) takes in deoxygenated blood that is loaded with carbon dioxide. The blood is squeezed down into the right lower chamber (ventricle) and taken by an artery to the lungs where the carbon dioxide is replaced with oxygen.

The oxygenated blood travels back to the heart, this time entering the left upper chamber (atrium). It is pumped into the left lower chamber (ventricle) and then into the aorta (an artery). The blood starts its journey around the body once more.

Blood vessels have a range of different sizes and structures, depending on their role in the body.

Oxygenated blood is pumped from the heart along arteries, which are muscular. Arteries divide like tree branches until they are slender. The largest artery is the aorta, which connects to the heart and picks up oxygenated blood from the left ventricle. The only artery that picks up deoxygenated blood is the pulmonary artery, which runs between the heart and lungs.

The arteries eventually divide down into the smallest blood vessel, the capillary. Capillaries are so small that blood cells can only move through them one at a time. Oxygen and food nutrients pass from these capillaries to the cells. Capillaries are also connected to veins, so wastes from the cells can be transferred to the blood.

Veins have one-way valves instead of muscles, to stop blood from running back the wrong way. Generally, veins carry deoxygenated blood from the body to the heart, where it can be sent to the lungs. The exception is the network of pulmonary veins, which take oxygenated blood from the lungs to the heart.

Blood pressure refers to the amount of pressure inside the circulatory system as the blood is pumped around.

Some common problems of the circulatory system include:

  • Aneurysm – a weak spot in the wall of an artery
  • Atherosclerosis – a narrowing of the arteries caused by plaque deposits
  • Heart disease – lack of blood supply to the heart because of narrowed arteries
  • High blood pressure – can be caused by obesity (among other things)
  • Varicose veins – problems with the valves that stop blood from running backwards.
  • Your doctor
  • In an emergency, always call triple zero (000)
  • Lloyd, M. & Gomez, J. 1981, The Complete Illustrated Medical Handbook , New Burlington Books, London.

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High school biology

Course: high school biology   >   unit 8.

  • Meet the heart!
  • Circulatory system and the heart

The circulatory system review

  • Meet the lungs
  • The lungs and pulmonary system
  • The respiratory system review
  • The circulatory and respiratory systems

The circulatory system

  • The pulmonary circuit provides blood flow between the heart and lungs.
  • The systemic circuit allows blood to flow to and from the rest of the body.
  • The coronary circuit strictly provides blood to the heart (not pictured in the figure below).

Blood and blood vessels

Common mistakes and misconceptions.

  • Arteries usually carry oxygenated blood and veins usually carry deoxygenated blood. This is true most of the time. However, the pulmonary arteries and veins are an exception to this rule. Pulmonary veins carry oxygenated blood towards the heart and the pulmonary arteries carry deoxygenated blood away from the heart.
  • Blood is always red. Veins can appear blue as we see them through our skin, leading some people to believe that deoxygenated blood is blue. However, this is not the case! Blood only appears blue because of the way tissues absorb light and our eyes see color. Although oxygen does have an effect on the brightness of the blood (more oxygen makes a brighter red, less makes it darker), blood is never actually blue.

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InformedHealth.org [Internet]. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG); 2006-.

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How does the blood circulatory system work.

Created: March 12, 2010 ; Last Update: January 31, 2019 ; Next update: 2022.

The blood circulatory system (cardiovascular system) delivers nutrients and oxygen to all cells in the body. It consists of the heart and the blood vessels running through the entire body. The arteries carry blood away from the heart; the veins carry it back to the heart. The system of blood vessels resembles a tree: The “trunk” – the main artery (aorta) – branches into large arteries, which lead to smaller and smaller vessels. The smallest arteries end in a network of tiny vessels known as the capillary network.

There isn't only one blood circulatory system in the human body, but two, which are connected: The systemic circulation provides organs, tissues and cells with blood so that they get oxygen and other vital substances. The pulmonary circulation is where the fresh oxygen we breathe in enters the blood. At the same time, carbon dioxide is released from the blood.

Illustration: Pulmonary and systemic circulation

Blood circulation starts when the heart relaxes between two heartbeats: The blood flows from both atria (the upper two chambers of the heart) into the ventricles (the lower two chambers), which then expand. The following phase is called the ejection period, which is when both ventricles pump the blood into the large arteries.

In the systemic circulation, the left ventricle pumps oxygen-rich blood into the main artery (aorta). The blood travels from the main artery to larger and smaller arteries and into the capillary network. There the blood drops off oxygen, nutrients and other important substances and picks up carbon dioxide and waste products. The blood, which is now low in oxygen, is collected in veins and travels to the right atrium and into the right ventricle.

This is where pulmonary circulation begins: The right ventricle pumps low-oxygen blood into the pulmonary artery, which branches off into smaller and smaller arteries and capillaries. The capillaries form a fine network around the pulmonary vesicles (grape-like air sacs at the end of the airways). This is where carbon dioxide is released from the blood into the air inside the pulmonary vesicles, and fresh oxygen enters the bloodstream. When we breathe out, carbon dioxide leaves our body. Oxygen-rich blood travels through the pulmonary veins and the left atrium into the left ventricle. The next heartbeat starts a new cycle of systemic circulation.

  • Menche N (Ed). Biologie Anatomie Physiologie. Munich: Urban und Fischer; 2016.
  • Pschyrembel. Klinisches Wörterbuch. Berlin: De Gruyter; 2017.
  • Schmidt R, Lang F, Heckmann M. Physiologie des Menschen: mit Pathophysiologie. Berlin: Springer; 2017.

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Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.

  • Cite this Page InformedHealth.org [Internet]. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG); 2006-. How does the blood circulatory system work? 2010 Mar 12 [Updated 2019 Jan 31].

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