flow

Examples of flow in the following topics:

• 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.
  • 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.

• Velocity of Blood Flow

  • The science dedicated to understanding this flow is called hemodynamics.
  • It's important to understand the different between velocity and flow.
  • Flow can be calculated using the following formula:
  • Where F = flow, v = velocity and a = cross-sectional area.
  • 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.

• Bulk Flow: Filtration and Reabsorption

  • Bulk flow is one of three mechanisms that facilitate capillary exchange, along with diffusion and transcytosis.
  • Bulk flow is used by small, lipid-insoluble solutes in water to cross the the capillary wall and is dependent on the physical characteristics of the capillary.
  • Continuous capillaries have a tight structure reducing bulk flow.
  • Fenestrated capillaries permit a larger amount of flow and discontinuous capillaries allow the largest amount of flow.
  • The kidney is a major site for bulk flow transport.

• 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.
  • Blood flow is regulated by vasoconstriction or vasodilation of smooth muscle fibers in the walls of blood vessels, typically arterioles.
  • 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.

• Introduction to Blood Flow, Pressure, and Resistance

  • Resistance to flow must be overcome to push blood through the circulatory system.
  • If resistance increases, either pressure must increase to maintain flow, or flow rate must reduce to maintain pressure.
  • Flow is the movement of the blood around the circulatory system.
  • A relatively constant flow is required by the body's tissues, so pressure and resistance are altered to maintain this consistency.
  • A too-high flow can damage blood vessels and tissue, while flow that's too low means tissues served by the blood vessel may not receive sufficient oxygen to function.

• Blood Flow in the Brain

  • The amount of blood that the cerebral circulation carries is known as cerebral blood flow (CBF).
  • Too little blood flow (ischemia) results in tissue death.

• Blood Flow in Skeletal Muscle

  • Blood flow to an active muscle changes depending on exercise intensity and contraction frequency and rate.
  • Blood flow within muscles fluctuates as they contract and relax.
  • This rapid increase and decrease in flow is observed over multiple contractions.
  • 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.
  • 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.

• Factors Affecting Pulmonary Ventilation: Airway Resistance

  • The air that flows through the lungs varies considerably in the properties of the flow of air.
  • The air flow can either be turbulant, transitional or laminar based on the airway.
  • Transitional flow occurs in places that branch within smaller airways, in which the air flow becomes in between laminar and turbulent flow and has moderate resistance.
  • Laminar flow (a) has orderly layers and low resistance.
  • Turbulent flow (b) has disorganized layers and high resistance.

• Blood Flow in 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.
  • Outline the path of pulmonary circulation: blood flow in the lungs

• Varicose Veins

  • Veins have leaflet valves to prevent blood from flowing backwards; this backflow is called retrograde or reflux flow.
  • This change is called valvular incompetence, and it allows blood to flow backwards.
  • To accommodate the retrograde or reflux flow, the veins enlarge even more.
  • Figure A shows a normal vein with a working valve and normal blood flow.
  • Figure B shows a varicose vein with a deformed valve, abnormal blood flow, and thin, stretched walls.