Physiology Lecture (17) PDF

Physiology Blood Flow LECTURE (17) DR. El-Sawy 1  Physiology Blood Flow  Quantit...

Physiology Blood Flow LECTURE (17) DR. El-Sawy 1  Physiology Blood Flow  Quantity of blood that passes given point in circulation in given Definition period of time.  Expressed in milliliters or liters per minute. Value  Overall blood flow in circulation at rest is 5000 ml/minute = cardiac output. DR. El-Sawy 2  Physiology Blood Flow  The blood flow in any point of vascular system is equal to effective perfusion pressure (P) in that portion divided by resistance (R). Pressure (P) Flow (Q) = ———— Resistance (R)  So, 2 major factors can affect blood flow through an organ:  ΔP = intraluminal pressure at arterial end - mean pressure Effective at venous end. perfusion  Blood flow is directly proportional to perfusion pressure pressure (ΔP) Flow α Δ P  Difficulty faced by the blood during its flow through the blood vessels. Resistance of  Blood flow is inversely related to resistance blood vessels ↑ in resistance causes a ↓ in the blood flow. 𝟏 Flow α 𝐑 DR. El-Sawy 3  Physiology Blood Flow Factors determining the resistance: 1. Radius of blood vessel Inversely proportional 2. Viscosity of blood Directly proportional 3. Length of the blood vessels. Poiseuille equation 𝟖𝛈𝐋 𝛈𝐋 R= Rα 𝛑𝐫𝟒 𝐫𝟒 Where : R = resistance η (eta)= viscosity of blood L = Length of blood vessel r4 = 4th power of radius of blood vessels. Note The length of the blood vessels in the body is generally fixed. So, the radius and viscosity changes represent the major factors. DR. El-Sawy 4  Physiology Blood Flow  Resistance is inversely proportional to the 4th power of the radius. 𝟏 Rα 𝐫𝟒  With constant Δ P (pressure gradient): If radius is reduced to 1/2 , the flow will ↓ to be 1/16 of its Previous value.  Resistance to blood flow occurs mainly in arterioles: As its wall is rich in circular smooth Ms.  The arteriolar diameter is controlled by: 1. Sympathetic nervous system. 2. Various drugs or hormones and chemicals. DR. El-Sawy 5  Physiology Blood Flow Definition:  Ability of substance to resist a change in the shape.  Resistance is directly proportional to the viscosity (R α η) ↑ viscosity of blood → ↑ resistance → ↓ blood flow. Factors determining viscosity of blood:  Volume of RBCs relative to blood volume. ↑ hematocrit ↓ hematocrit RBCs  ↑ viscosity of blood →↑ R  ↓ viscosity of blood →↓ R (Hematocrit) and ↓ blood flow and ↑ blood flow  as in polycythemia.  as in anemia.  ↑ plasma proteins as immunoglobulins → ↑ blood viscosity → Plasma proteins ↑ resistance.  Resistance is directly proportional to length of blood vessel (L) through which blood flow takes place. RαL DR. El-Sawy 6  Physiology Blood Flow Methods for measuring blood flow : 1) Electromagnetic flow meter. Direct 2) Ultrasonic flow meter. methods 3) Venous occlusion plethysmography.  By Fick's principle: State that blood Flow (F) to organ = quantity of Indirect substance (Q) taken by organ divided by arteriovenous methods (A-V) difference in concentration of that substance Flow (F) = Q/ (Ax-Vx) DR. El-Sawy 7  Physiology Blood Flow Electromagnetic flow meter Ultrasonic flow meter Venous occlusion plethysmography DR. El-Sawy 8  Physiology Blood Flow laminar blood flow Turbulent blood flow  Blood flow is streamline (laminar)  Blood flow is crosswise  Within the vessel:  The blood flows crosswise in the 1. Very thin layer of blood in contact vessels & along the vessel → Eddy with wall of vessel does not move. currents which add to the overall 2. Next layer has a low velocity. friction of flow in the vessel. 3. Next has higher velocity & so  Thus blood flows with much greater forth. resistance.  Velocity being greatest in the center of the stream  Occurs when blood flows at velocity  Occurs when blood flows at or above up to a certain critical velocity this critical velocity.  Silent  Produce sounds (murmurs).  Reynold's number is < 2000  Reynold's number is > 3000  Occurs in following conditions: 1- Increase blood velocity. 2- Decrease blood viscosity (anemia) 3- Change in vessel diameter. DR. El-Sawy 9  Physiology Blood Flow Definition:  The probability of turbulence is related to the diameter of vessels & velocity. Reynold's Number (Re) 𝐏𝐃𝐕 Re = 𝛈 Where : P → density of blood D → Diameter of the tube V → Velocity of the flow η → Viscosity of the fluid Note The higher the Reynold's number → the greater probability of turbulence. When Re < 2000 → flow is not turbulent When Re > 3000 → flow is turbulent So the turbulence can occurs when: Increase velocity as during passage of blood at arterial constriction. Decreased blood viscosity as in anemia. DR. El-Sawy 10