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Circulatory system: want to learn more about it?

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Circulatory system

Circulatory system

The circulatory system, also called cardiovascular system,  is a vital organ system that delivers essential substances to all cells for basic functions to occur. Also commonly known as the cardiovascular system, is a network composed of the heart as a centralised pump, bloods vessels that distribute blood throughout the body, and the blood itself, for transportation of different substances.

The circulatory system is divided into two separate loops: The shorter pulmonary circuit that exchanges blood between the heart and the lungs for oxygenation; and the longer systemic circuit that distributes blood throughout all other systems and tissues of the body. Both of these circuits begin and end in the heart.

Key facts
Functions Transport of gases, nutrients, electrolytes, wastes, hormones
Heart Layers - myocardium, endocardium, epicardium
Chambers - left and right atria, left and right ventricles
Blood vessels - arteries (oxygenated blood), veins (deoxygenated blood)
Blood vessels Arteries, veins, capillaries
Hierarchy: Heart -> arteries -> arterioles -> capillaries [gas exchange - oxygenated blood becomes deoxygenated] -> venules -> veins -> heart
Circulations Pulmonary - superior and inferior vena cava (with deoxygenated blood) -> right atrium -> right ventricle -> right and left pulmonary artery -> capillaries of each lung (oxygenation of the blood) -> pulmonary veins -> left atrium -> systemic circulation 

Systemic - left atrium -> left ventricle -> aorta and all of its branches -> capillaries -> veins -> superior and inferior vena cava -> pulmonary circulation 

Coronary - ascending aorta -> right coronary artery -> right marginal branch, posterior interventricular artery, left coronary artery -> anterior interventricular branch (anastomoses with the posterior branch), circumflex artery
Blood Plasma with cellular components:
Erythrocytes (red blood cells) - contain hemoglobine and carry oxygen throughout the blood vessels
Leukocytes (white blood cells) - immune system cells
Thrombocytes (platelets) - coagulation cells
Clinical relations Arteriosclerosis, cerebrovascular disease, peripheral artery disease, aneurysm, varices, arrhytmia, heart failure

This article will explain everything that is important about the circulatory system, as well as the clinical relations to it.


The main function of the circulatory (or cardiovascular) system is to deliver oxygen to the body tissues, whilst simultaneously removing carbon dioxide produced by metabolism. Oxygen is bound to molecules called haemoglobin that are on the surface of the red blood cells in the blood.

Beginning in the heart, deoxygenated blood (containing carbon dioxide) is returned from systemic circulation to the right side of the heart. It is pumped into pulmonary circulation and is delivered to the lungs, where gas exchange occurs. The carbon dioxide is removed from the blood and replaced with oxygen. The blood is now oxygenated, and returns to the left side of the heart.

Have you already learned the basic anatomy of the heart? Test your knowledge with our heart diagrams, quizzes and worksheets.

From there, it is pumped into the systemic circuit, delivers oxygen to the tissues, and returns again to the right side of the heart. The blood also acts as an excellent transport medium for nutrients, such as electrolytes, as well as hormones. The blood also transports waste products, that are filtered from the blood in the liver.

The heart

The heart is a muscular pump that is the central component of the circulatory system. It is divided into a right and left side by a muscular septum. The muscular component of the heart, the myocardium, is composed of involuntary cardiac muscle. It is lined by a membrane called the endocardium internally, as well as an external epicardium. Contraction of the cardiac muscle cells is stimulated by electrical impulses that are sporadically fired from the regulatory centres of the heart: the sinoatrial node in the roof of the right atrium, and the atrioventricular node in the septum between the atria and the ventricles. The sinoatrial node is widely regarded as the natural pacemaker of the heart.

Overview of the heart in situ (ventral view)

The heart is continuously going through a series of contractions and relaxations. Systole refers to when the ventricles of the heart simultaneously contract, diastole is when the ventricles relax. During systole, blood is forcibly pumped out of the ventricles into the outflow tracts of their corresponding circulation. The atria are filling with blood at the same time. During diastole, the ventricles are relaxed, and blood flows from the atria into the corresponding ventricles.

Pulmonary circulation

Deoxygenated blood from systemic circulation returns to the right atrium via the superior and inferior vena cava. The coronary sinus, returning blood from the coronary circulation, also opens into the right atrium. The blood in the right atrium flows into the right ventricle through the right atrioventricular valve (tricuspid valve) during diastole. During systole, the right ventricle contracts, directing the blood into the conus arteriosus at the base of the pulmonary trunk. Contraction of the ventricle causes the tricuspid valve to shut, preventing backflow of blood into the right atrium. Between the conus arteriosus and the pulmonary trunk is a valve; the pulmonary valve. In diastole, the valve closes to prevent backflow of blood into the right ventricle.  

The pulmonary trunk splits into a right and a left pulmonary artery, serving the right and left lung respectively. Deoxygenated blood flows into the capillaries of each lung, where it is then oxygenated. The pulmonary veins collect the newly oxygenated blood from the lung, and return it to the left atrium, where it will be passed into systemic circulation.

Systemic circulation

Oxygenated blood enters the left atrium from the pulmonary circulation via the pulmonary veins. During diastole, blood passes from the left atrium to the left ventricle through the left atrioventricular valve (bicuspid valve). In systole, the left ventricle contracts, forcing blood into the aorta. The blood passes through the aortic valve into the ascending aorta.

The ascending aorta becomes the arch of the aorta, where three large arteries branch from it: the brachiocephalic trunk, the left common carotid artery and the left subclavian artery. These arteries supply oxygenated blood to the head and neck, and to the upper limbs.

The descending aorta is the continuation of the arch of the aorta inferiorly. In the thorax it is referred to as the descending or thoracic aorta, and gives off numerous branches in the thorax.

The latter passes into the abdominal cavity through the diaphragm through the aortic hiatus at the level of T12. From there, it is referred to as the abdominal aorta. The abdominal aorta gives branches to the structures in and surrounding the abdominal cavity, and terminates by bifurcating into the common iliac arteries, which will supply the pelvic cavity and lower limbs.

The branches of the aorta passes towards their intended structures, with branching occurring along their length. The terminal branches enter the tissues, and pass towards the capillary beds of the tissues in vessels called arterioles. Gas exchange occurs between the blood and the tissues. The blood is collected from the capillaries by venules, which unite to form the veins of the systemic circulation. These veins ultimately drain to the right atrium via the superior and inferior venae cavae.

Coronary circulation

The coronary circulation refers to the blood supply to the heart itself. It is a component of the systemic circulation. The right and left coronary arteries branch directly from the ascending aorta, immediately above the aortic valve. The right coronary artery passes to the right and gives off two main branches: the right marginal branch along the right border of the heart and the posterior interventricular (posterior descending) artery, which descends along the interventricular septum on the base of the heart.

Learn everything about the coronary arteries with the following video lecture and quiz. 

The left coronary artery passes to the left, and gives off the anterior interventricular (Ieft anterior descending) artery which descends on the anterior aspect of the interventricular septum to anastamose with the posterior interventricular artery at the apex of the heart. It also gives off the circumflex artery.

Overview of coronary arteries and cardiac veins

The venous drainage of the heart is achieved by the coronary sinus, which drains the main veins of the heart:

Coronary circulation in a cadaver: This cadaveric specimen exhibits a rare variant of coronary arteries. The LCA (LMCA) does not stem from the ostium of the left aortic sinus, like it normally does. As you can see in this example, the LCA subsequently branches off into the LAD and LCx arteries. (LCA: Left coronary arter, LMCA: Left main coronary artery, LAD: Left anterior descending artery, LCx: Circumflex artery)

Portal system

The portal system is the system of veins that drain the blood from the intestines and directs it to the liver to be filtered. The superior and inferior mesenteric veins, draining the jejunum down as far as the upper rectum, along with the splenic vein draining the spleen, pancreas, and stomach, unite to form the hepatic portal vein, which empties blood into the liver. Toxins are filtered out by the liver, and the filtered blood is returned to the inferior vena cava via the hepatic veins.

Types of blood vessels


Arteries carry blood away from the heart. They have thick walls and a narrow lumen, to resist the high pressure from the blood being forced out of the heart. As the arteries travel toward the more peripheral tissues, they begin a process of segmentation, decreasing in diameter and wall thickness with each division. The major arterial outflow tracts of the heart are the aorta (systemic), and the pulmonary trunk (pulmonary). The coronary arteries are the arteries that supply oxygenated blood to the tissues of the heart itself. 

Arteries are typically divided into three types:

  • Artery (histological slide)
    conducting arteries arising directly from the heart and their main branches, whose walls have a high degree of elasticity;
  • distributing arteries that transport blood to specific organ systems, with a high muscular component in their walls;
  • the small and muscular resistance vessels or arterioles

Pressure in these arteries decrease from its highest level in the conducting arteries to the lowest in the arterioles. The walls of the arteries are divided into 3 layers: the tunica intima (internal), the tunica media (middle) and the tunica externa (external).

For descriptive purposes, it is easiest to describe the types of blood vessels in the sequence that they occur as they pass from the heart to the peripheral tissues, and form the peripheral tissue back to the heart.

How's your knowledge of the major arteries of the cardiovascular system? Our cardiovascular system diagrams, quizzes and free worksheets are the best way to find out. 

Types of arteries

Muscular artery (histological slide)

Large elastic arteries: are the conducting arteries and examples include the aorta and its main branches; the brachiocephalic trunk, the left common carotid artery, the left subclavian artery and the terminal common iliac arteries. These carry blood from the heart to the smaller conducting arteries. The pressure in the these arteries is at the highest level of the entire circulatory system. The tunica intima is lined by endothelium and the tunica media has a large elastic component.
Muscular arteries: are the distributing arteries and contain a large proportion of smooth muscle in their tunica media. They are lined internally by endothelium. The tunica externa is composed of fibromuscular connective tissue, with a larger proportion of elastic fibres than collagen contributing to the elasticity of this layer in the muscular arteries.

Arterioles: are the connecting vessels between the muscular arteries and capillary beds of the organs. They have small endothelial cells with nuclei projecting into the lumen of the vessel, a thin muscular wall about two layers thick, and a thin tunica externa. They control the flow of blood into the capillaries by contraction of the smooth muscle in the tunica media, which acts as a sphincter.
Capillaries: are the closest vessels to the organs. Their walls measure one large endothelial cell in thickness and provide the only barrier between the blood and the interstitial fluid of the tissues. They have a narrow lumen which is just thick enough to allow the passage of the largest blood cells. The permeability of capillaries varies depending on the surrounding tissues and the type of junctions between the adjacent endothelial cells in the vessels wall.


Types of veins 

Vein (histological slide)

Venules: are formed when two or more capillaries converge. They are lined by flat endothelial cells and a thin tunica externa. These are called postcapillary venules. The muscular component appears in venules as their lumen increases, producing muscular venules.
Veins: are formed with the union of muscular venules. In comparison to arteries, veins have a relatively thin wall and a larger lumen. The structure of the walls is similar to that of arteries, but a considerably smaller amount of muscle is present in the tunica media of veins. Veins are capacitance vessels, meaning they have a distensible wall and can expand to accommodate large volumes of blood.

Most peripheral veins have structures called valves, which are projections of the tunica interna into the lumen of the vessel. Valves prevent the backflow of blood through the veins, by passively closing when the direction of flow of the blood reverses. Valves are absent in the veins of the thorax and abdomen.
The overall hierarchy of blood vessels follows this order: arteries → arterioles → capillaries → venules → veins.

So now you know the types of blood vessels - but what about their histological features? Learn and test your knowledge at the same time using our blood vessels diagrams and artery and vein quizzes. 

Shunts and anastamoses

Arteries form connections between each other called anastomoses, which creates a continuous supply of blood throughout different areas. In the event of occlusion of an artery to a specific area, blood supply can be maintained to the tissue via the anastomosis with an artery of an adjacent area.

Anastamosis between superior mesenteric artery and inferior pancreatic artery (ventral view)

A direct anastomosis occurs where two arteries are joined directly to each other, such as in the radial and ulnar arteries via the palmar arches. Convergence anastomoses occur where two arteries unite to form a single artery, as in when the vertebral arteries join to form the basilar artery. A transverse anastomosis is where a small artery connects two larger arteries, for example, the anterior communicating artery connecting the right and left anterior cerebral arteries.

Connections between the arterial and venous systems are present throughout the body. For example, in the mesentery, metarterioles can connect the arterioles to venules, and blood can either flow into or bypass the capillary beds. Control of this flow is by local demand of the individual tissues.

Arteriovenous anastomoses are a direct connection between small arteries and small veins. These occur in regions such as the skin of the nose, lips and ears, in the mucosa of the alimentary canal, and nasal and oral cavities.

A portocaval anastomosis occurs where there is a connection between the systemic and portal system of veins. These occur at venous plexuses, such as around the oesophagus, the umbilicus, and the rectum.


The blood is the mobile component of the circulatory system. Blood is bright red when oxygenated and dark red/purple when deoxygenated. Blood consists of a cellular component suspended in a liquid called plasma. 

Plasma is a clear fluid that accounts for approximately 55% of blood, and is composed  of over 90% water. Plasma contains a high concentration of electrolytes, such as sodium, potassium and calcium. Also dissolved in plasma are plasma proteins. These include clotting factors, mainly prothrombin, immunoglobulin, polypeptides and other protein molecules, and hormones.

Erythrocytes (red blood cells)

Erythrocytes are the most abundant of blood cells, accounting for approximately 99% of all blood cells. They are biconcave disc shaped cells that lack a nucleus. Erythrocytes have a globulin protein called haemoglobin on their surface for oxygen to bind to. The proportion of red blood cells to plasma is called the haematocrit. Measured as a percentage, it is used as a reference point for the oxygen carrying capacity of a person; when there is a higher percentage of red blood cells present, more haemoglobin is present to carry oxygen.

Aged erythrocytes are ingested by macrophages in the liver and spleen. The iron released in the breakdown of the erythrocytes is used to synthesise new erythrocytes, or is stored in the liver as ferritin.

Blood Grouping

Antigens are present on the surface of erythrocytes, and can react with antibodies causing agglutination of the red blood cells. This is the basis of the ABO blood grouping system. Individuals inherit two alleles, one from each parent, that code for a specific blood group. Blood groups can be homozygous, where the alleles are the same, or heterozygous where alleles are different:

ABO blood grouping system
Allele Blood group

Specific blood groups have antibodies that are sensitive to the alleles absent from their erythrocytes. For example, blood group A will carry the A antigen and the anti-B antibodies.

Leukocytes (white blood cells)

These are divided in 5 groups: monocytes, lymphocytes, neutrophils, basophils and eosinophils. These groups are distinguishable from each other by cell size, shape of nucleus and cytoplasm composition. These groups can themselves be grouped into 2 groups: granulocytes and agranulocytes. This classification is based on the presence or lack of granules in the cytoplasm of the cell. Collectively, white blood cells form part of the immune response.


Neutrophils, eosinophils and basophils fall into this category of white blood cells. Leukocytes are classified into this group based on the presence of vesicles, called granules, in their cytoplasm. Granulocytes are largely involved in inflammatory and allergic responses.

Neutrophils: are the most abundant white blood cells, accounting for about 40-75% of all leukocytes. The number of neutrophils varies, and increases in response to acute bacterial infections. They have an irregular, segmented nucleus. They mainly function in the defence of the body against microorganisms, and can ingest foreign substances by phagocytosis. They are also involved in inflammation. Neutrophils have a short life span, spending 4-7 hours in circulation and a few days in connective tissue. 

Eosinophils: are similar to neutrophils, but are far fewer in number. Their nucleus is prominently bilobed, and the granules in the cytoplasm are large. Their motility mirrors that of other leukocytes, and they migrate from the circulation into the tissues. They increase in number in allergic reactions, and play a prominent role in the defense against parasites. They are only weakly phagocytotic, involved more so in the breakdown of particles too large for phagocytosis. The circulate for approximately 10 hours, and spend a few days in the tissues.

Basophils: are the smallest of the granulocytes. They are small in number, accounting for 0.5-1% of all leukocytes. They are distinguishable by the large, clearly visible granules in their cytoplasm. Their nucleus is irregular shaped, and sometimes bilobed, but is often obscured by the granules. The granules are membrane bound vesicles containing a variety of inflammatory agents. These vesicles herniate, dumping their contents and triggering immediate allergic hypersensitivity, such as seen in reactions like hay fever. The dumping of these agents also triggers the migration of other granulocytes to the area.


Monocytes and lymphocytes fall into this category due to the absence of granules in their cytoplasm. They are also referred to as mononuclear leukocytes, referring to the presence of a single lobed nucleus.

Monocytes: are the largest leukocytes in relation to physical size. They account for 2-8% of all leukocytes. They typically have large uni-lobed nuclei with a characteristic indentation on one side. Monocytes are phagocytic cells. Circulating monocytes transition into macrophages when they migrate from the circulation to the tissues.

Lymphocytes: are the second most abundant leukocyte, accounting for 20-30%. They are the only white blood cell that can re-enter circulation having migrated to the tissues. They are variable in size and lifespan: some live merely days, others are long-lived, and are involved in immunological memory. Lymphocytes are divided into two types: B-lymphocytes and T-lymphocytes.

B-lymphocytes synthesize and secrete antibodies specific to foreign molecules. They also stimulate other non-lymphocytic leukocytes to phagocytose. B-lymphocytes are involved in adaptive immunity, and produce memory B cells, which remain in the body and are activated in response to a specific antigen. 

T-lymphocytes develop and mature in the thymus, then migrate to and are stored in secondary lymphoid organs. They are involved in the ongoing immunity of the cell, with their function not solely dependent on the response to an antigen. T-lymphocytes are divided into three subgroups. Cytotoxic T cells directly target infected cells; Helper T cells direct destruction by recruitment of other immune cells; and Regulatory T cells are involved in developing the tolerance of cells to an antigen.

Thrombocytes (platelets)

Platelets are small, irregular shaped cells that lack a nucleus. They are present in large numbers and have highly adhesive properties. Platelets are highly involved in haemostasis. They are activated in the event of damage to a blood vessel. They accumulate at the site of injury and essentially plug the wound. Following adherence at the site of injury, platelets and the surrounding tissues release factors that trigger a complex sequence of events. A clot is formed to close the wound. The clot is then retracted and the edges of the wound are pulled together to close it and repair the vessel. Platelets circulate in the blood for approximately 10 days, before they are removed from the blood by macrophages.

Circulatory system: want to learn more about it?

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