The double circulatory system of blood flow references two distinct and separate systems. This distinction is shared by most vertebrates to varying degrees to include amphibians, birds and mammals (including humans.) In contrast, fish have a single circulation system because they lack lungs. Most animals living above the water require a double circulatory system to allow the added benefit of direct oxygenation from a developed pulmonary circuit. Embryology of the human circulatory system is an advanced study of the double circulatory system as the distinction between the right and left portions of the heart is founded.
For example, the adult human heart consists of two separated pumps or flow circuits. One side is dedicated to the right atrium and right ventricle (which pumps deoxygenated blood into the pulmonary circulation). This circuit is very low in pressure incumbent upon systemic resistance. The left circuit inclusive of the left atrium and ventricle (which pumps oxygenated blood into the systemic circulation) is better gated to high pressure. Blood in one circuit must go through the heart to enter the other circuit.
Mathematics remain murky but it appears that summation of imaged Right Ventricular Ejection Fraction plus Left Ventricular Ejection Fraction may not allow a solution of 100% in many cases. European models of Windkessel physiology appear to offer additional mathematical illumination of Diastolic Performance.
Blood circulates through the body at speeds which vary by a factor of one hundred, from 1.2 m/s in the aorta to approximately 1.1 cm/s in the capillaries. Blood velocity is similarly highly variable. Velocities generated within the large bore heart chambers vary considerable from velocities within the smaller bore vessels of the periphery and lungs. The circulatory system features numerous return paths (out to the kidneys and back, to the liver and back, to the legs and back, etc.), so it is incorrect to think of blood cells travelling the entire circulatory distance. The average adult heart pumps approximately 5 L/min of blood at rest (the cardiac output); with a total blood volume of approximately 5L in an average adult, a blood cell will run through the complete circuit about once a minute, though this depends on what tissue it circulates to. For example, blood supplying the coronary circulation of the heart muscle itself will return to the pulmonary loop much faster than blood supplying the toes. During intense exercise, the cardiac output can increase fivefold.
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