control of blood flow Flashcards
what does TPR control ?
Increase in resistance means need to increase pressure to keep same flow.
how is TPR controlled ?
Darcy’s and Poiseulle’s laws
Myogenic response (reflexive contraction)
Blood viscosity
how does resistance affect flow and blood pressure ?
blood Flow: Higher resistance decreases blood flow, while lower resistance increases it. Resistance is mainly influenced by blood vessel diameter, length, and blood viscosity.
Blood Pressure: Increased resistance raises blood pressure because the heart has to work harder to push blood through narrower or more constricted vessels. Lower resistance leads to lower blood pressure.
what is Poiseuille’s law ?
describes how fluid flows through a long, narrow tube (like blood through a vessel) when the flow is smooth (laminar). It shows that the flow rate depends on several factors:
Pressure Difference (ΔP): A greater pressure difference between the two ends increases the flow.
Tube Radius (r): The flow rate is very sensitive to the radius; even a small increase in radius greatly increases the flow (it’s proportional to the fourth power of the radius).
Fluid Viscosity (η): Thicker or more viscous fluids flow more slowly.
Tube Length (L): A longer tube offers more resistance, reducing the flow.
what is the r4 effect and why is it important ?
means that the flow rate is proportional to the fourth power of the tube’s radius. This “r⁴ effect”
shows how crucial the size of a blood vessel is for regulating blood flow—a small change in the vessel’s diameter can lead to a very large change in the amount of blood that flows through it.
Why are arterioles and not capillaries involved in regulating TPR and blood flow?
Arterioles regulate TPR and blood flow because they contain smooth muscle that can constrict or dilate, directly altering their diameter and thus resistance (thanks to the r⁴ effect). Capillaries, on the other hand, lack smooth muscle and are primarily involved in nutrient and gas exchange rather than controlling blood flow.
what is the myogenic response ?
Increased distension of vessel
makes it constrict. Decreased distension of vessel makes it dilate.
Having a linear relationship would entail very large differences in blood flow with differences in pressure
Maintains blood flow at the same level during changing arterial pressures. Very important in renal, coronary, cerebral circulation.
Stretching of the vessel causes ion channels to open, which then depolarize, leading to smooth muscle contraction.
what are the control mechanisms of arteriole radius ?
What is viscosity, name ways to increase or reduce it, what are the clinical implications of this?
how much the blood resists flowing
Describe the properties of veins and how they are important for controlling CVP
Thin-walled, collapsible, voluminous vessels
Contain 2/3rd of blood volume
Contractile – contain smooth muscle, innervated by sympathetic nerves but thinner than arterial muscle & more compliant – so form blood reservoir
Contraction of vessels – Expels blood into central veins
– Increases venous return/CVP/end-diastolic volume
– Increases stroke volume (Starling’s law)
how does blood return to the heart ?
Cardiac output - the circulation is a closed system so the heart pushes the blood further through the vascular system via the arterial side of the capillary system into the venules and veins in the direction of the right side of the heart.
Breathing - the pressure in the chest is negative on inhalation at the same time intra-abdominal pressure rises as the diaphragm moves downwards causing the venous valves in the pelvic veins to close, and the blood moves up into the thorax. On exhalation, the intra-abdominal pressure decreases and the pelvic veins and inferior vena cava refill.
Muscle pump - the deep venous system is embedded in muscles. Due to this, every muscle contraction squeezes the veins to push the column of blood in them in the direction of the heart. When the muscle relaxes, the venous valves prevent the retrograde flow of blood towards the capillaries.
Venous tone - the blood in the veins exerts pressure on the vein’s wall generating tension & maintaing pressure. Furthermore, sympathetic vasoconstriction can mobilise more blood back to the heart.
what is Bernoulli’s law and why is it important ?
mechanical energy of flow is determined by pressure, kinetic, potential energies (ρ = fluid mass)
when Standing:
Pressure gradient against flow from heart to feet.
Ejected blood has greater kinetic energy at heart than feet (more velocity, V).
Also, greater potential energy than at heart than feet (more height, h)
Greater kinetic/potential energies overcome pressure gradient to maintain flow
But flow to feet easily compromised – clinically important
