Circulatory system
An average adult human carries five to six quarts of blood, which accounts for approximately 7% of their total body weight. This fluid consists of plasma, red blood cells, white blood cells, and platelets. The heart pumps this mixture through a network of vessels that never allows the blood to escape into open spaces within vertebrates. Large elastic arteries carry oxygenated blood away from the left ventricle via the aorta. These vessels branch into smaller arterioles before reaching capillaries where gas exchange occurs. Capillaries merge into venules and then veins to return deoxygenated blood to the right atrium. The pulmonary circulation moves blood between the heart and lungs while systemic circulation delivers it to the rest of the body. Specialized routes like coronary circulation supply the heart muscle itself without relying on the blood inside its four chambers.
The first vascular system appeared over 600 million years ago in an ancestor of triploblasts. Fish possess a single circuit with a two-chambered heart pumping blood through gills and then body tissues. Amphibians developed a three-chambered heart allowing some separation of oxygenated and deoxygenated blood. Reptiles feature an incomplete ventricular septum that lets them divert blood flow based on temperature needs. Birds and crocodilians evolved independent four-chambered hearts to completely separate pulmonary and systemic circuits. Mammals also utilize this double circulatory system for efficient oxygen delivery. In contrast, arthropods maintain an open system where hemolymph bathes organs directly within a hemocoel cavity. Flatworms lack any specialized circulatory organs entirely, relying instead on diffusion across their flattened bodies. Jellyfish use branching gastrovascular cavities to distribute fluids to outer layers since digestion begins internally.
Human arterial systems develop from six pairs of aortic arches during weeks 4 to 8 of embryonic life. The venous system arises from three bilateral veins while fetal circulation begins specifically within the 8th week. Before birth the fetus obtains oxygen and nutrients from the mother through the placenta and umbilical cord. Blood bypasses the lungs via the truncus arteriosus since gas exchange occurs in the maternal system. The first and second aortic arches regress to form maxillary arteries and stapedial arteries respectively. Arteries 3, 4, and 6 persist to become definitive structures while arch 5 disappears completely. Dorsal aortae fuse to create the basis for the adult aorta. Branches form intercostal arteries, lumbar arteries, and lateral sacral arteries along the back and sides. Vitelline arteries eventually become mesenteric arteries supplying the gastrointestinal tract. After birth the umbilical arteries transform into internal iliac arteries.
The Ebers Papyrus from the 16th century BCE acknowledges the connection between the heart and arteries in ancient Egypt. Greek physician Galen believed blood flowed from the liver to all body parts without returning to the heart or liver. He thought arterial blood contained pneuma or air originating from the heart rather than being pumped by it. Persian physician Avicenna wrote about cardiac cycles and valvular function in his Canon of Medicine published in 1025. Arabian physician Ibn al-Nafis described pulmonary circulation accurately in 1242 despite maintaining belief in vital spirit formation. Michael Servetus described pulmonary circulation function in 1546 but his work remained hidden until decades later due to religious persecution. Realdo Colombo made a better known discovery of pulmonary circulation in 1559 at Padua. William Harvey published Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus in 1628 demonstrating continuous blood circulation through minute connections. Marcello Malpighi discovered the capillary system connecting arteries and veins in 1661 completing Harvey's conceptual leap.
Atherosclerosis involves small plaques building up in walls of medium and large arteries over time. These plaques may rupture to occlude arteries causing acute coronary syndromes with sudden oxygen deficits. Aneurysm formation or splitting of arteries represents another risk associated with plaque buildup. Deep venous thrombosis creates clots mostly occurring in legs when people remain stationary for long periods. These clots can embolize traveling to other locations resulting in pulmonary embolus or stroke. Congenital heart defects represent conditions where circulatory changes fail to occur after birth as expected. Hematologic diseases like anemia affect blood composition while lymphatic diseases impact fluid drainage systems. Lifestyle choices including exercise habits diet and smoking contribute significantly to cardiovascular disease development. About 98.5% of oxygen in arterial blood combines chemically with hemoglobin molecules under normal sea-level pressure conditions.
Medical professionals measure circulatory health using pulse checks blood pressure readings via sphygmomanometers and stethoscope examinations. Electrocardiograms evaluate electrical conduction through the heart muscle during specific cardiac cycles. Angiography injects dye into arteries to visualize blockages or narrowings within the vascular tree. Catheters inserted into arteries allow measurement of pulse pressure or pulmonary wedge pressures directly. Stents fix blockages while coils manage active bleeds during these invasive procedures. Magnetic resonance imaging produces MRI angiograms to image arteries without radiation exposure. CT pulmonary angiograms evaluate lung blood supply specifically for suspected embolic events. Vascular ultrasonography investigates venous insufficiency stenosis thrombosis affecting both arterial and venous systems. Coronary artery bypass surgery remains a common procedure performed more often in inpatient settings than ambulatory care facilities. Only 28% of cardiovascular surgeries occurred in ambulatory care settings across the United States according to recent data.
Common questions
How much blood does an average adult human carry and what percentage of body weight is it?
An average adult human carries five to six quarts of blood, which accounts for approximately 7% of their total body weight. This fluid consists of plasma, red blood cells, white blood cells, and platelets.
When did the first vascular system appear in evolutionary history?
The first vascular system appeared over 600 million years ago in an ancestor of triploblasts. Fish possess a single circuit with a two-chambered heart pumping blood through gills and then body tissues.
What happened to the fifth aortic arch during embryonic development of the human arterial system?
Arches 3, 4, and 6 persist to become definitive structures while arch 5 disappears completely. The first and second aortic arches regress to form maxillary arteries and stapedial arteries respectively.
Who published Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus in 1628?
William Harvey published Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus in 1628 demonstrating continuous blood circulation through minute connections. Marcello Malpighi discovered the capillary system connecting arteries and veins in 1661 completing Harvey's conceptual leap.
How much oxygen combines chemically with hemoglobin molecules under normal sea-level pressure conditions?
About 98.5% of oxygen in arterial blood combines chemically with hemoglobin molecules under normal sea-level pressure conditions. This chemical combination occurs within red blood cells circulating throughout the vascular system.