Circulatory System: Anatomy, Function, and Evolutionary History

The circulatory system is the body's sophisticated transport network, responsible for delivering oxygen, nutrients, and essential chemicals to cells while removing metabolic waste. In humans, this system is a closed loop powered by the heart, ensuring that blood reaches every corner of the body to maintain homeostasis.

An average adult carries approximately five to six quarts (4.7 to 5.7 liters) of blood, which makes up about 7% of total body weight. This vital fluid is composed of plasma, red blood cells, white blood cells, and platelets. To function effectively, the circulatory system works in tandem with the digestive system to acquire the nutrients necessary to keep the heart pumping.

Diagram of the human heart showing blood oxygenation to the pulmonary and systemic circulation

Key Facts

Magnetic resonance angiography of aberrant subclavian artery

Dual Circuits: The system is divided into pulmonary (lungs) and systemic (body) circulations.
Oxygen Transport: Approximately 98.5% of arterial oxygen is bound to hemoglobin.
Vessel Scale: Muscle capillaries in a 70 kg human can stretch between 9,000 and 19,000 km.
Blood Volume: Blood accounts for roughly 7% of an adult's total body weight.
Development: Fetal circulation bypasses the lungs via the truncus arteriosus.

The Two Major Circuits

Flatworms, such as this Pseudoceros bifurcus, lack specialized circulatory organs.

The human circulatory system is organized into two primary loops that ensure blood is oxygenated before being distributed to the body's tissues.

Pulmonary Circulation

The pulmonary circulation is the loop that carries deoxygenated blood from the right side of the heart to the lungs. Once in the lungs, the blood is oxygenated and then returns to the left side of the heart.

Systemic Circulation

The systemic circulation delivers oxygen-rich blood from the left heart to the rest of the body. After delivering oxygen to the tissues, the deoxygenated blood returns to the right heart through large veins called the venae cavae. This circuit is further categorized into macrocirculation (large vessels) and microcirculation (small vessels).

Blood Vessels and Transport

Blood travels through a network of vessels categorized by their structure and the direction of blood flow. The largest arteries and veins connecting directly to the heart are known as the great vessels.

Arteries: These carry blood away from the heart. They branch into smaller passages called arterioles.
Capillaries: These are microscopic vessels that join the arterial and venous systems. They are the primary site of exchange between blood and tissues.
Veins: These return blood to the heart.

Diagram of capillary network joining the arterial system with the venous system

The efficiency of this system is highlighted by the renal circulation, which supplies the kidneys. This specialized network receives approximately 20% of the total cardiac output, branching from the abdominal aorta and returning via the ascending inferior vena cava.

Development and Function

Human anatomical chart of blood vessels, with heart, lungs, liver and kidneys included. Other organs are numbered and arranged around it. Before cutting out the figures on this page, Vesalius suggests that readers glue the page onto parchment and gives instructions on how to assemble the pieces and paste the multilayered figure onto a base "muscle man" illustration. "Epitome", fol.14a. HMD Collection, WZ 240 V575dhZ 1543.

The circulatory system begins forming in the embryo through a process called vasculogenesis. The arterial system develops primarily from six pairs of aortic arches, while the venous system arises from three bilateral veins between weeks 4 and 8 of embryogenesis.

Fetal circulation is distinct because it bypasses the lungs, as the fetus obtains oxygen and nutrients from the mother via the placenta and umbilical cord. This bypass occurs through the truncus arteriosus until birth.

Animation of a typical human red blood cell cycle in the circulatory system. This animation occurs at a faster rate (~20 seconds of the average 60-second cycle) and shows the red blood cell deforming as it enters capillaries, as well as the bars changing color as the cell alternates in states of oxygenation along the circulatory system.

In a healthy adult at sea level, the primary transporter of oxygen is the hemoglobin molecule. About 98.5% of arterial oxygen is chemically combined with hemoglobin, while only 1.5% is physically dissolved in the blood plasma.

Clinical Significance and Treatment

Diseases affecting this system are collectively known as cardiovascular disease. Modern medicine employs several surgical interventions to treat these conditions:

Angioplasty: A procedure to open narrowed blood vessels.
Coronary Artery Bypass/Stents: Used to restore blood flow to the heart muscle.
Valve Repair: Treatment for damaged heart valves.
Phlebectomy: The removal of varicose veins.

Depiction of the heart, major veins and arteries constructed from body scans

In the United States, the majority of these procedures are performed in inpatient settings, with only 28% occurring in ambulatory care settings.

Comparative Anatomy and Evolution

The vascular system evolved over millions of years to overcome the limitations of diffusion. The first blood vascular systems likely appeared over 600 million years ago, with endothelium evolving in ancestral vertebrates between 510 and 540 million years ago.

Comparison of Circulatory Systems Across Species
System Type	Characteristics	Example
No Circulatory System	Lack specialized circulatory organs	Flatworms (e.g., Pseudoceros bifurcus)
Open Circulatory System	Hemolymph pumped into a hemocoel	Grasshopper
Closed Circulatory System	Blood contained within vessels	Humans, Fish (two-chambered heart)

The open circulatory system of the grasshopper – made up of a heart, vessels and hemolymph. The hemolymph is pumped through the heart, into the aorta, dispersed into the head and throughout the hemocoel, then back through the ostia in the heart and the process repeated.

Historical Discoveries

Our understanding of the heart has evolved from ancient mysticism to precise science. The Ebers Papyrus (16th century BCE) first acknowledged a connection between the heart and arteries, though Egyptians believed arteries transported air.

Significant milestones include:

Avicenna (1025): Provided the first correct explanation of pulsation as alternate expansion and contraction.
Ibn al-Nafis (1242): Described pulmonary circulation and predicted the existence of capillaries (pores) 400 years before their discovery.
Michael Servetus (1546): The first European to describe pulmonary function, though his work was suppressed.
William Harvey (1628): Demonstrated that the heart produces a continuous circulation of blood throughout the entire body.
Marcello Malpighi (1661): Finally identified the capillary system connecting arteries and veins.

Image of veins from William Harvey's Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus, 1628

Frequently Asked Questions

What is the difference between pulmonary and systemic circulation?

Pulmonary circulation moves deoxygenated blood from the heart to the lungs to pick up oxygen, while systemic circulation delivers that oxygenated blood from the heart to the rest of the body's tissues.

How much blood does the average adult have?

An average adult contains between five to six quarts (approximately 4.7 to 5.7 liters) of blood, which represents about 7% of their total body weight.

What is the role of hemoglobin in the blood?

Hemoglobin is the primary transporter of oxygen in vertebrates; in healthy humans at sea level, it chemically combines with about 98.5% of the oxygen in arterial blood.

How does fetal circulation differ from adult circulation?

Fetal circulation bypasses the lungs via the truncus arteriosus because the fetus receives oxygen and nutrients from the mother through the placenta and umbilical cord rather than from breathing air.

Who discovered the continuous circulation of blood?

English physician William Harvey published the demonstration of continuous blood circulation in 1628, though the capillaries that connect the systems were later discovered by Marcello Malpighi in 1661.