Blood Flow

Blood Flow

• volume of blood flowing through a tissue during a given period of time (ml/min)
• total blood flow for all tissues/min = cardiac output

Which factors affect blood flow and cardiac output, and which adjustments are made to maintain an adequate blood supply to the tissues?

Blood volume affects flow, blood is unequally distributed in the body. Veins are 6-10x as distensible as arteries & have 3x volume of corresponding artery. Veins have higher compliance than arteries. C = increase in volume/increase in pressure. Venous system serves as a storage area for blood during circulation. Venoconstriction reduces venous volume & shifts venous reserve.

Blood Flow     blood pressure

Blood Flow     1/R

Blood flow is result of differences in pressure & is directly proportional to pressure gradient & inversely proportional to resistance (R), which opposes flow.

Peripheral resistance

force that opposes blood flow, major source is arterial systemic resistance

1. Vascular resistance - due to friction between blood & vessel walls

   1. Vessel diameter - small differences in vessel diameter cause large changes in R

      Laminar flow model: blood in center of vessel flows more rapidly than along wall. Blood adheres to wall.

      Poiseuille's Law - R 1/r⁴
      Resistance is greater & flow is reduced in vessels of smaller radius (r).

      ex. at P = 100 mmHg & T = 1 min (constants)

      radius = 2, R = 1/16 , flow = 16 ml/min

      radius = 1, R = 1/1⁴, flow = 1 ml/min

      Arterioles provide most of the resistance to blood flow, can change diameter by vasoconstriction & increase R or vasodilation & decrease R

   2. Vessel length - longer vessels increase R, ex. growth, obesity

2. Blood viscosity - result of interactions between blood components, blood is thicker than water & has greater resistance to flow, affected by hematocrit & plasma proteins

   1. Cellular component is mainly RBC, hematocrit = % of cells, as hematocrit increases, blood viscosity increases

   2. Plasma proteins - generally large proteins that don't leave circulatory system, type & concentration of plasma proteins affect viscosity

3. Turbulence - high flow rate, sharp turns, obstructions, rough areas increase R

Blood pressure

generally measures arterial pressure (BP)

• Mean arterial pressure (MAP) = diastolic pressure + (systolic - diastolic)/3
• Arteries branch & become smaller but total cross-sectional area increases causing BP to decrease from 120 (aorta) to 35 (capillaries) mmHg and blood velocity to decrease.
• Stretch in elastic arteries during systole followed by recoil maintains flow during diastole ( elastic rebound)
• Capillary hydrostatic pressure (CHP) decreases from 35 to 18 mmHg
• Veins converge and diameters enlarge, venous pressure is lower and decreases from 18 to 2 mmHg (atrium), R is low in veins and total cross-sectional area is decreasing, determines venous return

To maintain adequate blood pressure and flow (tissue perfusion) through the systemic system, homeostatic feedback mechanisms maintain flow under normal conditions or change flow when needed by adjusting pressure.

Vasomotor center - neural mechanism in medulla oblongata controlling sympathetic division

• Vasomotor tone - partial vasoconstriction always maintained in peripheral vessels
• Vasoconstriction - adrenergic postganglionic neurons release norepinephrine (NE) which stimulates smooth muscle of arterioles in most tissues to contract
• Vasodilation - cholinergic postganglionic neurons release acetycholine (ACh) which stimulates endothelial cells to release NO, which relaxes smooth musscle of arterioles in skeletal muscle & brain
  • - nitroxidergic postganglionic neurons release NO which relaxes arteriole smooth muscle

• Antidiuretic hormone (ADH) - dehydration or blood loss increases blood osmotic concentration & decreases BP. Posterior pituitary releases ADH which acts on vessels & kidneys
• Angiotensin II - decreased blood volume stimulate juxtaglomerular (J-G) cells to release rennin initiating a cascade to produce angiotensin II with multiple effects
• Erythropoietin (EPO) - low BP or low blood oxygen stimulates the kidneys to release EPO which increases RBC production in red marrow increasing blood volume & viscosity and oxygen carrying capacity
• Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) - excessive atrial or ventricular stretching during diastole stimulates release from cardiac muscle cells with multiple effects

Regulatory mechanisms for vasoconstriction reflex:

• Baroreceptors detect pressure decrease & stimulate sympathetic n.s.
  • Response: Increased cardiac output (HR), peripheral vasoconstriction & BP
• Chemoreceptors detect decreased O₂, increased CO₂ or H⁺ & stimulate sympathetic n.s.
  • Response: Increased cardiac output, peripheral vasoconstriction & BP
• Medullary ischemic reflex - low perfusion of brainstem stimulates sympathetic n.s.
  • Response: Increased heart rate & contractility, peripheral vasoconstriction & BP
• Hypothalmic osmoreceptors detect increased osmotic pressure of ECF, ADH released
  • Response: Peripheral vasoconstriction & fluid retention by kidneys, increased BP & volume
• Kidneys detect BP decrease or low blood oxygen, EPO is released
  • Response: Increased RBC production, blood volume & BP
• Kidney J-G cells detect BP decrease, angiotensin hormone released
  • Response: Increased cardiac output, peripheral vasoconstriction, fluid retention, increased BP & volume

Regulatory mechanisms for vasodilation:

• Baroreceptors detect pressure increase, inhibit sympathetic n.s. & stimulate parasympathetic vagus n. to heart
  • Response: Decreased cardiac output (HR), peripheral vasodilation & decreased BP
• Excessive atrial or ventricular stretching stimulates cardiac muscle to release natriuretic peptides
  • Response: Reduces H₂O intake, increases kidney excretion & peripheral vasodilation, decreased BP & volume
• Heavy exercise stimulates sympathetic n.s.
  • Response: Increases vasodilation in lungs, myocardium & skeletal muscles, but causes vasoconstriction in skin, digestive organs & kidneys; increases BP
• Autoregulation - increased blood flow to local tissues or region due to changes in interstitial fluid causing dilation of precapillary sphincters. Result of decreased oxygen or increased temperature, CO₂ , H⁺, K⁺ ; build-up of lactic acid or chemical mediators (histamine, NO)