Control of Blood Flow

• Define diastolic pressure.

  Lowest arterial pressure measured during a cardiac cycle

  Pressure in the arteries during ventricular relaxation when no blood is being ejected from left ventricle
• Define systolic pressure.

  Highest arterial pressure measured during a cardiac cycle

  Pressure in arteries after blood has been ejected from the left ventricle during systole

  'blip' in arterial pressure curve (in image) or dicrotic notch - produced when aortic valve closes -> produces brief period of retrograde flow

  
• Define pulse pressure.

  Difference between systolic & diastolic pressure
• Define mean arterial pressure.

  The average pressure over a complete cardiac cycle
• What is the calculation for mean arterial pressure?

  Pa = DP + 1/3 pulse pressire

  Pa = mean arterial pressure

  DP = diastolic pressure
• What is the normal range of mean arterial pressure?

  70 - 100 mmHg
• What is the distribution of the blood?

  85% in systemic circulation

  10% in pulmonary

  5% in cardiac chanbers at end of diastole
• What does velocity depend on?

  flow

  cross sectional area (of vessels)
• What is the relevance of this image?

  The cross sectional area of vessel type (collectively) increases from aorta -> arteries -> capillaries

  This decreases the velocity, and vice versa

  
• What does flow depend on?

  Pressure difference (between 2 ends of vessel)

  Resistance of vessel
• What is the equation for blood flow?

  Q = ΔP/R

  Q = flow

  ΔP = pressure difference

  R = resistance
• Define total peripheral resistance.

  Resistance of the entire systemic vasculature (TPR) or the systemic vascular resistance (SVR)
• Where is series resistance seen in the body?

  Arrangement of blood vessels within a given organ

  
• Where is parallel resistance seen in the body?

  Distribution of blood flow among various major arteries branching off aorta

  
• What is Poiseuille's Law?

  Resistance is directly proportional to 1/r^4

  -> if you halve the radius, resistance increases 16 fold

  This means that small changes in the radius of the lumen of blood vessels can have significant effects on the resistance of the vessels
• What is the flow in the CVS?

  Laminar
• Describe laminar flow.

  Velocity of blood flow is highest in the centre of the vessel & lowest towards vessel walls

  Smooth parabolic (shape)

  
• Describe turbulent flow.

  Irregularity occurs in blood vessel -> laminar stream is disrupted & blood flow may become turbulent

  Laminar flow is silent, turbulent flow is audible

  
• What are the major influences on Reynolds number?

  Changes in blood viscosity

  Changes in velocity of blood flow
• What is the clinical significance of Reynolds number?

  If Reynolds number (NR) is less than 2000, blood flow will be laminar

  If NR is more than 3000, blood flow will be turbulent
• What are the different vascular controls?

  Local (metabolic or myogenic)

  Central (neural)

  Hormonal

  Endothelial
• What is the local metabolic control of blood flow?

  Intrinsic factors (high K+, adenosine, lactate, H+, CO2)

  Increased metabolic activity -> increased release of metabolic byproducts -> increase resistance -> vasodilation -> increase blood flow -> increase supply of O2 & nutrients

  CHECK
• What is the local myogenic control of blood flow?

  Vessels constrict reflexively when intraluminal pressure increases

  • mediated by stretch activated Ca+2 channels in smooth muscles
  • protects capillaries from surges in arterial pressure
• What are hormones that can control blood flow?

  ADH

  Angiotensin II

  ANP

  Adrenaline
• How does ADH impact blood flow?

  Released from posterior pituitary

  Binds to V1a receptors

  Cause vasoconstriction
• How does ang II impact blood flow?

  Acts on AT1 receptors

  Causes vasoconstriction
• How does ANP impact blood flow?

  From atrial myocytes

  Binds to NPR1 receptor

  Causes vasodilation
• How does adrenaline impact blood flow?

  Released from adrenal medulla

  Binds to…

  • a1 adrenergic receptors -> vasoconstriction
  • b2 adrenergic receptors -> vasodilation
• What is blood flow to tissues driven by?

  Difference in pressure between the arterial & venous sides of the circulation
• What is the central control of blood flow?

  Sympathetic adrengeric nerve fibres

  a1 adrenergic receptors (noradrenaline acts) -> vasoconstriction -> decreases diameter of arteriole -> increases resistance to blood flow

  b2 adrenergic receptors (adrenaline) -> vasodilation -> increases diameter -> decreases resistance to blood flow
• What is active hyperaemia?

  Normal vasodilatory response to increased tissue activity & rising metabolic levels
• What is post-exercise hyperaemia?

  A period of increased blood flow that persists even after activity has ceased
• What is reactive hyperaemia?

  Period of increased blood flow that follows transient ischaemia
• What are the physiologic consequences of local control of blood flow?

  Autoregulation (intrinsic ability of an organ to maintain stable blood flow in changing perfusion pressures)

  Hyperaemia (increased blood flow)
• What is the process of active hyperaemia?

  Increased tissue metabolism -> increased release of metabolic vasodilators into ECF -> vasodilation -> decrease resistance -> increase blood flow -> O2 & nutrient supply to tissue (as long as metabolism is increased)
• What is the process of reactive hyperaemia?

  Decreased tissue blood flow due to occlusion -> metabolic vasodilators accumulate in ECF -> dilation of arterioles but occlusion prevents blood flow

  Remove occlusion -> decreased resistance -> increase blood flow

  As vasodilators wash away, arterioles constrict & blood flow returns to normal
• Fill in the blanks

  A) Tyrosine

  B) L-dopa

  C) Dopamine

  D) Noradrenaline

  E) Adrenaline

  
• What molecules cause contraction/constriction of blood vessels by acting on endothelium?

  Endothelin

  PGF

  Thromboxane
• What molecules cause relaxation/dilation of blood vessels by acting on endothelium?

  PGI2

  PGE

  Trauma

  NO

  Toxins from bacteria & stress
• Where is the vasoconstrictpr centre (C1)?

  Upper medulla, lower pons