117 Blood Flow Through the Body

Velocity of Blood Flow

Blood flow is a pulse wave that moves out from the aorta and through the arterial branches, then is reflected back to the heart.

The flow of blood around the circulatory system is modulated by numerous interacting factors. The science dedicated to understanding this flow is called hemodynamics.

Velocity vs. Flow

It’s important to understand the different between velocity and flow. Velocity refers to the distance an object moves over time; for example, in blood this measurement is often given as cm/sec. Flow refers to the movement of a volume of a liquid or gas over time; for example, in blood this measurement is often given as mL/sec

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:

[latex]F=v \cdot a[/latex]

Where F = flow, v = velocity and a = cross-sectional area.

Potential Complications

While the above example is a simple calculation, in reality there are numerous factors that influence velocity and flow.

Velocity and Pressure

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.

Velocity and Resistance

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.

Velocity and Viscosity

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.

Vessel Area

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.

Regulation

These individual elements are tightly regulated by the body to maintain sufficient flow to the body’s organs and tissues

Local Regulation of Blood 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.

Local Regulation

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 alpha-1-adrenergic 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 in Skeletal Muscle

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. For this reason, they receive approximately 20% of cardiac output at rest, which can increase up to a maximum of approximately 80% with exercise. 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.

Skeletal Muscle: Skeletal muscle: 1] Bone, 2] Perimysium, 3] Blood vessel, 4] Muscle fiber, 5] Fascicle, 6] Endomysium, and 7] Epimysium Tendon.

Blood Flow During Exercise

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 Muscle Pump

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.

Vascular Recruitment

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.

Blood Flow in the Brain

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. The circle of Willis, a circulatory anastomosis that supplies blood to the brain and surrounding structures while providing redundancy in case of any interruption, is a key protection. Failure of these safeguards results in cerebrovascular accidents, commonly known as strokes.

The amount of blood that the cerebral circulation carries is known as cerebral blood flow (CBF). In an adult, CBF is typically 750 milliters per minute or 15% of the cardiac output. CBF is tightly regulated to meet the brain’s metabolic demands. Too much blood can raise intracranial pressure (ICP), which can compress and damage delicate brain tissue. Too little blood flow (ischemia) results in tissue death. In brain tissue, a biochemical cascade known as the ischemic cascade is triggered when the tissue becomes ischemic, potentially resulting in damage to and death of brain cells. Medical professionals must take steps to maintain proper CBF in patients who have conditions like shock, stroke, and traumatic brain injury.

Brain Blood Flow: Schematic representation of the circle of Willis, arteries of the brain, and brain stem.

Blood Flow in the Skin

Blood flow to the skin provides nutrition to skin and regulates body heat through the constriction and dilation of blood vessels.

The skin contains a network of small blood vessels containing muscle fibers in their tunica media. These muscles are under the control of the sympathetic nervous system and provide an efficient means of thermoregulation through vasoconstriction and vasodilation.

When vasoconstricted, blood flow through the skin is reduced, so less core heat is lost. With restricted blood flow, the skin appears paler. When vasodilated, blood flow through the skin is increased, meaning more core heat can be lost through radiation. With increased blood flow, the skin appears red.

Arteriovenous anastomoses can be found in areas of the body exposed to maximal cooling, such as the hands, feet, nose, lips and ears. These richly innervated areas are called apical structures. The anastomoses connect cutaneous arterioles and venules directly, playing an important role in the reduction of blood flow in a cold environment.

Blood Flow in the Lungs

Pulmonary circulation in the lungs is responsible for removing carbon dioxide from and replacing oxygen in deoxygenated blood.

The pulmonary circulatory system is the portion of the cardiovascular system in which oxygen-depleted blood is pumped away from the heart to the lungs via the pulmonary artery. Oxygenated blood is then returned to the heart via the pulmonary vein.

From the right ventricle of the heart, blood is pumped through the pulmonary semilunar valve into the left and right pulmonary arteries (one for each lung) and travels through the lungs. The pulmonary arteries carry deoxygenated blood to the lungs, where they release carbon dioxide and pick up oxygen during respiration.

An Alveolus: The alveoli are the site of oxygen and carbon dioxide exchange in the lungs.

The pulmonary arteries divide into thin-walled capillaries closely associated with the alveoli, small air sacs in the lungs where gas exchange occurs. Air is inhaled through the nose or the mouth and fills the lungs. Oxygen passively flows from the air inside the alveoli into the blood in the alveolar capillaries, while carbon dioxide passively flows in the opposite direction. The air, along with the diffused carbon dioxide, is then exhaled.

The oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left atrium of the heart, completing the pulmonary cycle. This blood is pumped through the bicuspid valve into the left ventricle, then distributed to the body through the systemic circulation before returning to the right atrium.

Blood Flow in the Heart

The heart pumps oxygenated blood to the body and deoxygenated blood to the lungs.

The heart is a key organ in the circulatory system responsible for the generation of pressure and thus flow throughout the system and pulmonary circulatory systems.

The heart contains four chambers, two atria and two ventricles. The blood that is returned to the right atrium is deoxygenated and s passed into the right ventricle to be pumped through the pulmonary artery to the lungs for reoxygenation and removal of carbon dioxide. The left atrium receives newly oxygenated blood from the lungs through the pulmonary veins, which is passed into the strong left ventricle to be pumped through the aorta to the different organs of the body.

The Heart: Circulation of blood through the chambers of the heart. Deoxygenated blood is received from the systemic circulation into the right atrium, it is pumped into the right ventricle and then through the pulmonary artery into the lungs. Through association with the alveoli the blood is oxygenated in the lungs and returns to the left atrium through the pulmonary veins, before passing into the left ventricle and being pumped around the body.

Coronary Blood Supply

The heart has its own circulation system, coronary circulation, which is part of systemic circulation. The coronary arteries derive from the aorta and run along the surface of the heart and within the muscle to deliver oxygen-rich blood to the myocardium. The coronary veins remove deoxygenated blood from the heart muscle, returning it through the coronary sinus into the right atrium.

When healthy, the coronary arteries are capable of autoregulation to maintain blood flow at levels appropriate to the needs of the heart muscle. Blockage of these vessels can lead to poor oxygenation of the heart muscle, impairing its function and in severe cases leading to tissue death, resulting in a myocardial infarction or heart attack.

Coronary Circulation: The coronary circulation supplies the heart muscle with the oxygen and nutrients it requires to function.

Hepatic Portal Circulation

The hepatic portal system is responsible for directing blood from parts of the gastrointestinal tract to the liver.

Rat Liver Sinusoid: Sinusoid of a rat liver with fenestrated endothelial cells. Fenestrae are approx 100nm diameter, and sinusoidal width 5 microns. Original mag 30,000x. Note the microvilli of hepatocytes in the space of Disse external to the endothelium.