coronary&cutaneous circulation

• Coronary blood flow at rest

  200-250 ml/min

  
• Coronary arteries


  • 2 coronary arteries
  • 20% humans left coronary art. predominates
  • 50% right coronary art. predominates
  • 30% equal supply by both
• Coronary arteries


  • Main arteries lie on the surface, while small arteries penetrates the muscle mass

  
• Venous drainage systems


  • Superficial system - Ends in coronary sinus & anterior cardiac vein
  • Deep system - opens directly in to the cardiac chambers
  • Via 3 sets of vessels (arteriosinusoidal, arterioluminal,Thebesian)

  
• Measurement of coronary blood flow

  1. Ultrasound studies such as Doppler ultrasounds evaluate the blood flow in major arteries

  2. Nuclear cardiology is a non-invasive method of imaging to assess myocardial blood flow and contractility
• Special features (Peculiarities or characteristics) of Coronary circulation

  • 1st branch to arise from aorta
  • Blood flow is more during diastole (Phasic blood flow)
  • Only organ to generate its own perfusion pressure
  • Coronary arteries are functional end-arteries
  • Shortest circulation - Mean transit time. 6 – 8 seconds
  • The veins drain directly in to the chambers adding deoxygenated blood to the oxygenated blood (Physiological shunt)
  • The diameter of the cardiac muscle fiber is smaller than the skeletal muscle fiber & the capillary density is more than the skeletal muscle
  • Autoregulation in coronary blood flow is present between 60 – 150 mmHg of mean arterial pressure
  • Hypoxia – Powerful vasodilator, Acts via adenosine
  • Oxygen extraction is nearly maximal even at resting heart rate
• Cardiac myocytes contain numerous mitochondria
• During systole the pressure in the subendocardial muscle is more than the outer layers (Epicardial)
• Regulation is mainly by metabolites, not neural
• Compensatory protective mechanism for the left ventricular subendocardium is capillary density and Myoglobin content
• Phasic flow of coronary blood flow

  1. Decreases during systole & increases during diastole

  2. During systole, arteries are compressed, decreasing CBF

  3. During diastole, cardiac muscle relaxes, no obstruction, increasing CBF

  4. Extravascular Pressure –resistance to the flow of blood by the contracting muscle
• During onset of systole, blood flow sharply declines due to strong compression of the intramuscular blood vessels

  
• During onset of diastole, blood flow increases due to decreased myocardial pressure
• Right ventricle also undergoes phasic changes but is only partial because the force of contraction of the right ventricular muscle is far less than that of left ventricle
• Tachycardia – increased heart rate


  Period of diastole decreases, blood flow decreases
• In aortic stenosis – pressure in left ventricle increases


  Intense compression of coronary vessels during systole increases myocardial ischemia
• Sub-endocardial region of left ventricle is the most vulnerable to ischemia as this part receives almost no blood supply during systole and is the common site of Myocardial Infarction
• During isovolumetric contraction, ventricles contract without any change in the length of the muscle fiber, blood is not leaving the chamber, thus Blood flow to LV muscle reduces to zero
• Sub-endocardial muscle has a higher oxygen consumption & extreme difficulty in adequate blood flow due to intense compression of blood vessels in the sub-endocardial region of left ventricle during systole
• Regulation of coronary blood flow

  • Mechanical factors
  • Chemical factors
  • Neural influences
• Mechanical factors


  Blood flow is determined by pressure head provided by the aortic pressure and extravascular pressure (resistance to the flow of blood by the contracting muscle)
• Chemical regulation


  • Oxygen demand as a major factor in local coronary blood flow regulation
  • Increased myocardial contractility, oxygen demand, and consumption leads to hypoxia and vasodilators which increase blood flow
  • Increase in blood flow parallels increase in myocardial metabolism, leading to increased adenosine which causes vasodilation
  • CO2, H+, K+, Lactic acid, PG, NO also increase coronary blood flow
• Neural regulation


  • Sympathetic nervous system has direct effects on coronary arteries with both alpha and beta adrenergic receptors, with beta receptors being more predominant and causing vasodilation
  • Sympathetic nervous system also has indirect effects on coronary blood flow by changing heart rate and contractility, which affects oxygen demand and metabolites
• If there is sudden changes in aortic pressure, coronary vascular resistance will adjust itself within a few seconds so that a constant blood flow is maintained - AUTOREGULATION
• Mechanical compression of the blood vessels during systole in the cardiac cycle causes a brief period of occlusion and reduction of blood flow
• Coronary artery disease causes

  • Vascular spasm
  • Formation of atherosclerotic plaques
  • Thromboembolism
• Myocardial infarction


  • Common cause is rupture of an atherosclerotic plaque leading to thrombus formation
  • The area of muscle that has either zero flow or so little flow that it cannot sustain cardiac muscle function is said to be infarcted
• Angina Pectoris (Cardiac Pain)


  Appears whenever the load on the heart becomes too great in relation to the available coronary blood flow, especially during exercise
• Cutaneous circulation


  • Blood flow to the skin
  • Functions are temperature regulation and nutritive blood supply to the skin
  • Average normal 15ml/100g of skin/min
• Blood flow to the skin can vary from 1ml to as much as 150 mL/100 g of skin/min because blood can be shunted through the arteriovenous anastomoses
• Subdermal capillary and venous plexus is a blood reservoir
• Primary function of cutaneous circulation

  Maintenance of constant body temperature
• Resistance vessels in cutaneous circulation

  Arterioles and arteriovenous (AV) anastomoses (shunt blood from arterioles to venules and venous plexuses bypassing capillary bed)
• Some parts contain well-innervated arteriovenous anastomosis, like fingers, palms, earlobes, toes
• Arteriovenous anastomosis


  • Allow the blood to bypass the capillaries
  • Control blood flow by constriction and dilation
  • Sympathetic activity closes surface bed during cold and opens in heat & exercise
• Regulation of cutaneous blood flow

  1. Local control - Direct effect of skin temperature on vascular smooth muscles

  2. Reflex control - by thermoregulatory center in hypothalamus that controls vascular smooth muscle contraction and sympathetic activity
• Increased sympathetic discharge


  Vasoconstriction
• Decreased sympathetic discharge


  Vasodilation
• Regulation of cutaneous blood flow is mainly by neural rather than metabolic control, local vasodilator metabolites have little effect
• Regulation is mainly for thermoregulation and less for the metabolic needs