How fast blood circulates in the body

The science dedicated to understanding this flow is called hemodynamics. At its simplest, imagine a perfect, rigid tube with no resistance and with a homogeneous liquid flowing through in a perpendicular manner. Flow can be calculated using the following formula:.

While the above example is a simple calculation, in reality there are numerous factors that influence velocity and flow. Movement of blood throughout the circulatory system is created by differences in pressure generated by the pumping of the heart. Pressure is greatest immediately after exiting the heart and drops as it circulates around the body, particularly through the arterioles and capillary networks. A greater difference in pressure results in a greater velocity assuming all else remains equal, so when increased blood flow is required the heart can pump more quickly and also in larger volume.

Resistance is the force that must be overcome by pressure in order for flow to occur, and is a factor of vessel length, diameter, surface composition, and the viscosity of the liquid flowing through.

As resistance increases the difference in pressure which influences velocity decreases, which in turn reduces flow. For this reason, the narrow arterioles rapidly reduce local blood pressure and slow the flow of blood through the capillaries, a beneficial effect allowing for efficient transfer of chemicals and nutrients.

However, pathological changes in blood vessels that result in narrowing or an increase in surface resistance can lead to a reduction in pressure, velocity, and thus flow, which can in turn lead to tissue damage. Blood is a complex liquid formed from plasma and containing numerous cell types. As such, its viscosity is changeable depending on osmotic balance and cell load. Increases in viscosity such as reduced water content lead to increases in resistance and thus reduction in flow.

Blood vessels are capable of vasodilation and vasoconstriction to alter their diameter. Assuming all else remains equal, a reduction in diameter results in a reduction in flow, whereas an increase in vessel diameter results in an increase in flow.

Blood flow is regulated locally in the arterioles and capillaries using smooth muscle contraction, hormones, oxygen, and changes in pH.

The flow of blood along arteries, arterioles, and capillaries is not constant, but can be controlled depending upon the requirements of the body. For example, more blood is directed to the skeletal muscles, brain, or digestive system when they are active, and blood flow to the skin can be reduced or increased to aid with thermoregulation.

Blood flow is regulated by vasoconstriction or vasodilation of smooth muscle fibers in the walls of blood vessels, typically arterioles. This regulation can be systemic, affecting the whole of the circulatory system, or localized to specific tissues or organs. The greatest change in blood pressure and velocity of blood flow occurs at the transition of arterioles to capillaries.

This reduces the pressure and velocity of flow for gas and nutrient exchange to occur within the capillaries. As such arterioles are the main part of the circulatory system in which local control of blood flow occurs.

Arterioles contain smooth muscle fibers in their tunica media, which allows for fine control of their diameter. They are innervated and so can respond to nervous system stimuli and also various circulating hormones.

Local responses to stretch, carbon dioxide, pH, and oxygen also influence smooth muscle tone and thus vasoconstriction and vasodilation.

Generally, norepinephrine and epinephrine hormones secreted by sympathetic nerves and the adrenal gland medulla are vasoconstrictive, acting on alphaadrenergic receptors. However, the arterioles of skeletal muscle, cardiac muscle, and the pulmonary circulation vasodilate in response to these hormones acting on beta-adrenergic receptors.

Generally, stretch and high oxygen tension increase tone, and carbon dioxide and low pH promote vasodilation. Pulmonary arterioles are a noteworthy exception as they vasodilate in high oxygen. Brain arterioles are particularly sensitive to pH, with reduced pH promoting vasodilation. A number of hormones influence arteriole tone such as the vasoconstrictive epinephrine, angiotensin II, and endothelin and the vasodilators bradykinin and prostacyclin.

Blood flow to an active muscle changes depending on exercise intensity and contraction frequency and rate. Skeletal muscles are important in maintaining posture and controlling locomotion through contraction. Due to the requirements for large amounts of oxygen and nutrients, muscle vessels are under very tight autonomous regulation to ensure a constant blood flow, and so can have a large impact on the blood pressure of associated arteries.

Blood vessels are closely intertwined with skeletal muscle tissues lying between the fascicles, or bundles of muscle fibers. Each muscle is supplied by many capillaries. This close association reduces the diffusion distances, allowing for the efficient exchange of oxygen and nutrients required for contraction and the rapid removal of inhibitory waste products.

Blood flow within muscles fluctuates as they contract and relax. During contraction, the vasculature within the muscle is compressed, resulting in a lower arterial inflow with inflow increased upon relaxation.

The opposite effect would be seen if measuring venous outflow. This rapid increase and decrease in flow is observed over multiple contractions. If the muscle is used for an extended period, mean arterial inflow will increase as the arterioles vasodilate to provide the oxygen and nutrients required for contraction.

Following the end of contractions, this increased mean flow remains to resupply the muscle tissue with required nutrients and clear inhibitory waste products, due to the loss of the inhibitory contractile phase.

Skeletal muscles also play a key role in the movement of blood around the body. Veins embedded within a muscle are compressed during contraction of that muscle, causing an increase in blood pressure due to the presence of one-way valves within the veins.

This increase in pressure drives the blood towards the heart. The skeletal muscles of the legs are particularly important skeletal muscle pumps as they prevent pooling of the blood in the feet and calves due to gravity. Skeletal Muscle Pump : During contraction of the skeletal muscle the vein is compressed which increases blood pressure.

Due to the presence of one way valves the blood can pass only in one direction, back towards the heart. It is unclear whether the action of skeletal muscle pumps influences arterial flow or if this is maintained purely by the pumping of the heart. Following repeated stimulus such as through exercise, the number of capillaries present in a muscle tissue can increase.

This vascular recruitment increases the capillary surface area within a muscle, allowing for enhanced oxygen exchange with the muscle fibers, prolonging the period of aerobic respiration and thus muscle output, and facilitating a more rapid removal of inhibitory waster factors such as lactic acid, reducing fatigue. Cerebral circulation is the movement of blood through the network of blood vessels supplying the brain, providing oxygen and nutrients.

Cerebral circulation refers to the movement of blood through the network of blood vessels supplying the brain. The arteries deliver oxygenated blood, glucose, and other nutrients to the brain and the veins carry deoxygenated blood back to the heart, removing carbon dioxide, lactic acid, and other metabolic products. Since the brain is very vulnerable to compromises in its blood supply, the cerebral circulatory system has many safeguards.

Men and women blink at the same rate, too. That is, about 10 times a minute, or once every six seconds. Staring — like when reading — counterintuitively cuts that rate in half. But while extended focusing on one visual task makes us blink less, being tired does the opposite, and creates more blinking.

Each blink takes just a tenth of a second. The instigation of the eyeblink is even faster than the blink itself. In a road situation, it would take an additional three-quarters of a second for a driver to move her foot from the gas pedal to the brake.

Conscious choices may be overrated. Reflexive actions are the way to go. Consider that within each cell, protein synthesis creates new substances, each with a particular vital function. How fast? Good thing. These armies are often evenly balanced. A colony of bacteria can double its size in 9 minutes and 48 seconds. Our fastest movements are involuntary.

One of these is legendary: the sneeze. Yet it usually begins in slow motion. Or, sometimes, by a strange bright-light response called the photic sneeze reflex, when people emerge from a movie matinee into brilliant sunshine. Whatever the basis, the initial odd tingling grows until it reaches a level that triggers the far more animated second act. The reflexive sudden opening of the throat releases a supernova air rush through the mouth and nose, explosively expelling any irritants.

A sneeze can release 40, particles at high speed. What speed is it, exactly? Some claim that this is the only body event that breaks the sound barrier. In a medical setting and using trustworthy equipment, the fastest recorded sneeze was mph. For some reason, Guinness World Records lists the greatest sneeze a bit slower than this, at Definitely fast enough to count as the highest-velocity body motion. A long-standing puzzle is why sneezers are forced to close their eyes during the event.

The best guess is that we are then protecting our eyes from the ultrafast spray of sneeze germs and particles. Another possible reason is that a sneeze is a unique reflex that involves nearly the entire body. Many muscles contract in the nose, throat, abdomen, diaphragm, all the way to the back and even the sphincters.

So the eyes closing is just part of a much larger, unique display of physiological violence. It all originates in a part of the brain stem called the medulla oblongata, which is present in countless other animals that sneeze pretty much the same way we do.

Printed with permission. This article originally appeared in print as "Body of Work. Register or Log In. The Magazine Shop. Login Register Stay Curious Subscribe. Newsletter Sign up for our email newsletter for the latest science news. Sign Up. Already a subscriber? Want more? Feel your pulse by placing two fingers at pulse points on your neck or wrists. The pulse you feel is blood stopping and starting as it moves through your arteries. As a kid, your resting pulse might range from 90 to beats per minute.

As an adult, your pulse rate slows to an average of 72 beats per minute. The aorta, the largest artery in the body, is almost the diameter of a garden hose. Capillaries, on the other hand, are so small that it takes ten of them to equal the thickness of a human hair. Your body has about 5. This 5. In one day, the blood travels a total of 19, km 12, miles —that's four times the distance across the US from coast to coast.