Cardiac physiology part 1

• The primary function of the cardiovascular system is to deliver blood to the tissues, which provides essential nutrients to the cells for metabolism and removes waste products from the cells
• Heart


  Serves as the pump, which, by contracting, generates the pressure to drive blood through a series of blood vessels
• Arteries


  Carry blood from the heart to the tissues, are under high pressure and contain a relatively small percentage of the blood volume
• Veins


  Carry blood from the tissues back to the heart, are under low pressure and contain the largest percentage of the blood volume
• Capillaries


  Thin-walled blood vessels interposed between the arteries and veins, where exchange of nutrients, wastes, and fluid occurs across the capillary walls
• The cardiovascular system also is involved in several homeostatic functions: It participates in the regulation of arterial blood pressure; it delivers regulatory hormones from the endocrine glands to their sites of action in target tissues; it participates in the regulation of body temperature; and it is involved in the homeostatic adjustments to altered physiologic states such as hemorrhage, exercise, and changes in posture
• Components of the cardiovascular system

  • Left heart
  • Right heart
  • Systemic circulation
  • Pulmonary circulation
• Cardiac output


  The rate at which blood is pumped from either ventricle
• Venous return


  The rate at which blood is returned to the atria from the veins
• In the steady state, cardiac output from the heart equals venous return to the heart
• The blood vessels serve as a closed system of passive conduits, delivering blood to and from the tissues where nutrients and wastes are exchanged
• The blood vessels also participate actively in the regulation of blood flow to the organs. When resistance of the blood vessels, particularly of the arterioles, is altered, blood flow to that organ is altered
• Steps in one complete circuit through the cardiovascular system

  1. Oxygenated blood fills the left ventricle

  2. Blood is ejected from the left ventricle into the aorta

  3. Cardiac output is distributed among various organs

  4. Blood flow from the organs is collected in the veins

  5. Venous return to the right atrium

  6. Mixed venous blood fills the right ventricle

  7. Blood is ejected from the right ventricle into the pulmonary artery

  8. Blood flow from the lungs is returned to the heart via the pulmonary vein
• Hemodynamics


  The principles that govern blood flow in the cardiovascular system
• Types of blood vessels

  • Arteries
  • Arterioles
  • Capillaries
  • Venules
  • Veins
• Arteries


  • Thick-walled structures with extensive development of elastic tissue, smooth muscle, and connective tissue
  • Function is to deliver oxygenated blood to the organs
• Arterioles


  • Smallest branches of the arteries, walls have an extensive development of smooth muscle, site of major resistance to blood flow and regulated by vasoactive metabolites produced in the tissues
• Venules and veins


  • Thin-walled structures, walls contain much less elastic tissue than arteries, have a large capacitance (capacity to hold blood), contain the largest percentage of blood in the cardiovascular system
• Velocity of blood flow

  The rate of displacement of blood per unit time
• Velocity of blood flow is inversely proportional to the cross-sectional area of the blood vessel
• As the diameter of a blood vessel decreases, the velocity of blood flow increases
• As the diameter of a blood vessel increases, the velocity of blood flow decreases
• Velocity (v)


  Volume per unit time (e.g. mL/s)
• Area (A)


  Cross-sectional area of a blood vessel or group of blood vessels, calculated as A = πr^2 where r is the radius
• Changes in diameter


  Alter the velocity of flow through a vessel
• Arteries


  • Receive blood directly from the heart and are under the highest pressure in the vasculature
  • The volume of blood contained in the arteries is called the stressed volume (meaning the blood volume under high pressure)
• Arterioles


  • The smallest branches of the arteries
  • Their walls have an extensive development of smooth muscle, and they are the site of highest resistance to blood flow
  • The smooth muscle in the walls of the arterioles is tonically active (i.e., always contracted)
  • They are extensively innervated by sympathetic adrenergic nerve fibers
  • α1-Adrenergic receptors cause contraction/constriction of the vascular smooth muscle, decreasing diameter and increasing resistance
  • β2-Adrenergic receptors cause dilation/relaxation of the vascular smooth muscle, increasing diameter and decreasing resistance
• Capillaries


  • Thin-walled structures lined with a single layer of endothelial cells
  • Site where nutrients, gases, water, and solutes are exchanged between the blood and the tissues
  • Lipid-soluble substances cross by dissolving in and diffusing across the endothelial cell membranes
  • Water-soluble substances cross through water-filled clefts between the endothelial cells or through large pores
  • Not all capillaries are perfused with blood at all times, selective perfusion is determined by the degree of dilation or constriction of the arterioles and precapillary sphincters
• Velocity in the aorta

  800-fold that in the capillaries
• Blood flow (Q)


  Determined by pressure difference (ΔP) and resistance (R), analogous to Ohm's law in electrical circuits
• Total peripheral resistance (TPR) or systemic vascular resistance (SVR)

  Resistance of the entire systemic vasculature
• Calculating resistance in a single organ

  1. Use blood flow (Q), pressure difference (ΔP) between artery and vein

  2. Resistance (R) = ΔP / Q
• Poiseuille equation


  Describes the relationship between resistance, blood vessel diameter/radius, and blood viscosity
• Series resistance


  • Total resistance is the sum of individual resistances, with arteriolar resistance being the greatest
  • Total flow is the same at each level in the series, but pressure decreases progressively
• Parallel resistance


  • Total resistance is less than any of the individual resistances
  • Flow is distributed simultaneously among the parallel vessels
• Venous efflux from organs

  1. Collects in vena cava

  2. Returns to heart
• Total resistance in a parallel arrangement is less than any of the individual resistances
• Resistances of circulations


  • Cerebral
  • Coronary
  • Renal
  • Gastrointestinal
  • Skeletal muscle
  • Skin
• When blood flow is distributed through parallel resistances, the flow through each organ is a fraction of the total
• There is no loss of pressure in the major arteries and mean pressure in each major artery will be the same and approximately the same as mean pressure in the aorta