Circulatory system

The circulatory system or cardiovascular system is the organ system which circulates blood around the body of most animals.

Human Heart Clipart provided by Classroom Clip Art (
Human Heart Clipart provided by Classroom Clip Art (



Following are some basic functions of the human circulatory system:

  1. Delivery of oxygen and nutrients to all parts of the body.
  2. Collection of metabolic wastes and delivery to the excretory organs, e.g. kidneys.
  3. Role in the immune system of defense against infection.
  4. Transport of hormones.

Types of circulatory systems

Open circulatory system

The circulatory system of arthropods and most mollusks is open, meaning that there are no capillaries and veins: one or more hearts pump the blood (more properly called hemolymph in this case) through the arteries to spaces called sinuses which surround the organs, allowing the tissues to exchange materials with the hemolymph. The hemolymph is drawn back into the heart as the heart relaxes.

Closed circulatory system

The circulatory systems of all vertebrates, as well as of annelids (for example, earthworms) and cephalopods (squids and octopuses) are closed, meaning that the blood never leaves the system of blood vessels consisting of arteries, capillaries and veins.

The systems of fish, amphibians, reptiles, birds and mammals show various stages of evolution. In fish, the system has only one circuit, with the blood being pumped through the capillaries of the gills and on to the capillaries of the body tissues. This is known as single circulation. The heart of fish is therefore only a single pump (consisting of two chambers). In amphibians and reptiles, a double circulation is used, but the heart is not always completely separated into two pumps. Amphibians have a three-chambered heart. Birds and mammals show complete separation of the heart into two pumps, for a total of four heart chambers; it is thought that the four-chambered heart of birds evolved independently of that of mammals.

All circulatory systems frequently employ countercurrent exchange systems to drive the diffusion of chemicals into or out of the bloodstream.

No circulatory system

An example of an animal with no circulatory system is the flatworm (class Tubellaria). They have a mouth leading into a digestive system. The digestive system is very branched, and because the worm is so flat, digested materials can be diffused to all the cells of the flat worm. Oxygen can diffuse from water into the cells of the flatworm. Thus every cell is able to obtain nutrients, water and oxygen without the need of a transport system.


The human circulatory system is comprised of the blood, the vascular system, and the heart.

The heart is the muscular organ which pumps the blood. The vascular system is made up of arteries, veins, and capillaries. Arteries are blood vessels that carry blood away from the heart. Veins are blood vessels that return blood to the heart. Capillaries are the smallest blood vessels, and are where the exchange of nutrients and gases takes place between the red blood cells and the body tissues.

Humans have a double circulatory system which consists of separate but connecting circulations: the pulmonary circulation and the systemic circulation.

Pulmonary circulation

The right ventricle pumps deoxygenated blood into the pulmonary arteries. These arteries carry the blood to the lungs, where it passes through a capillary network close to air-filled alveoli. This enables the release of carbon dioxide and the uptake of oxygen from the air. The now oxygenated blood returns to the left atrium in the pulmonary veins.

Systemic circulation

Oxygenated blood from the lungs returns to the heart via the pulmonary veins, flows into the left atrium and then into the left ventricle, which then pumps the blood through the aorta, the major artery which supplies blood to the body.

Smaller arteries branch off the aorta.

Splanchnic circulation

Also called visceral circulation, the splanchnic circulation is the part of the systemic circulation that supplies the digestive organs. The major arteries of the splanchnic circulation branch directly off the aorta and include the celiac artery (celiac axis), superior mesenteric artery, and inferior mesenteric artery.

Portal circulation

There are two exceptions to the system of double circulation.

The deoxygenated blood from the capillaries of the gastrointestinal tract drains into the portal vein which, instead of going directly back to the heart, leads to the liver. This allows the liver to take up the nutrients that were extracted by the intestines from food. The liver also neutralizes some toxins taken up by the intestines. Blood from the liver drains via the hepatic veins into the inferior vena cava and then the right side of the heart.

There is also a small portal flow from the hypothalamus to the anterior pituitary gland.

Fetal circulation

The circulatory system of the fetus is different, as the fetus does not use its lungs yet and obtains oxygen and nutrients from the placenta through the umbilical cord. After birth, the fetal circulatory system undergoes several anatomical changes, including closure of the ductus arteriosus and foramen ovale. See also: coronary circulation

Anatomy Clip Art and Pictures

History of discovery

The valves of the heart were discovered by a physician of the Hippocratean school around the 4th century BC. However their function was not properly understood then. Because blood pools in the veins after death, arteries look empty. Ancient anatomists assumed they were filled with air and that they were for transport of air.

Herophilus distinguished veins from arteries but thought that the pulse was a property of arteries themselves. Erasistratus observed that arteries that were cut during life bleed. He ascribed the fact to the phenomenon that air escaping from an artery is replaced with blood that entered by very small vessels between veins and arteries. Thus he apparently postulated capillaries but with reversed flow of blood.

Galen in the 2nd century AD knew that blood vessels carry blood and identified venous (dark red) and arterial (brighter and thinner) blood, each with distinct and separate functions. Growth and energy were derived from venous blood created in the liver from chyle, while arterial blood gave vitality by containing pneuma (air) and originated in the heart. Blood flowed from both creating organs to all parts of the body where it was consumed and there was no return of blood to the heart or liver. The heart did not pump blood around, the heart's motion sucked blood in during diastole and the blood moved by the pulsation of the arteries themselves.

Galen believed that the arterial blood was created by venous blood passing from the left ventricle to the right by passing through 'pores' in the interventricular septum, air passed from the lungs via the pulmonary artery to the left side of the heart. As the arterial blood was created 'sooty' vapors were created and passed to the lungs also via the pulmonary artery to be exhaled.

Ibn Nafis in 1242 was the first person to accurately describe the process of blood circulation in the human body. Contemporary drawings of this process have survived. In 1552 Servetus described the same and Realdo Colombo proved the concept. All these results were not widely accepted however.

Finally William Harvey, a pupil of Hieronymus Fabricius (who had earlier described the valves of the veins without recognizing their function), performed a sequence of experiments and announced in 1628 the discovery of the human circulatory system as his own and published an influential book about it. This work with its essentially correct exposition slowly convinced the medical world. Harvey was not able to identify the capillary system connecting arteries and veins; these were later described by Marcello Malpighi.

See also

Cardiovascular system
Heart - Aorta - Arteries - Arterioles - Capillaries - Venules - Veins - Venae cavae - Pulmonary arteries - Lungs - Pulmonary veins - Blood

Human organ systems
Cardiovascular system - Digestive system - Endocrine system - Immune system - Integumentary system - Lymphatic system - Muscular system - Nervous system - Skeletal system - Reproductive system - Respiratory system - Urinary system

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