LECTURE

• General Properties of Circulatory Systems

  • A circulatory system has
  • A circulatory fluid
  • A set of interconnecting vessels
  • A muscular pump, the heart
  • The circulatory system connects the fluid that surrounds cells with the organs that exchange gases, absorb nutrients, and dispose of wastes
  • Circulatory systems can be open or closed, and vary in the number of circuits in the body
• Cardiovascular System Function

  • Functional components of the cardiovascular system:
  • Heart
  • Blood Vessels
  • Blood
  • General functions these provide
  • Transportation
  • Everything transported by the blood
  • Regulation
  • Of the cardiovascular system
  • Intrinsic v extrinsic
  • Protection
  • Against blood loss
  • Production/Synthesis
• Cardiovascular System Function

  • To create the “pump” we have to examine the Functional Anatomy
  • Cardiac muscle
  • Chambers
  • Valves
  • Intrinsic Conduction System
• Open and Closed Circulatory Systems

  • In insects, other arthropods, and most mollusks, blood bathes the organs directly in an open circulatory system
  • In an open circulatory system, there is no distinction between blood and interstitial fluid, and this general body fluid is called hemolymph

  
• Open and Closed Circulatory Systems

  • In a closed circulatory system, blood is confined to vessels and is distinct from the interstitial fluid
  • Closed systems are more efficient at transporting circulatory fluids to tissues and cells
  • Annelids, cephalopods, and vertebrates have closed circulatory systems

  
• Organization of Vertebrate Circulatory Systems

  • Humans and other vertebrates have a closed circulatory system called the cardiovascular system
  • The three main types of blood vessels are arteries, veins, and capillaries
  • Blood flow is one way in these vessels
• Organization of Vertebrate Circulatory Systems

  • Arteries branch into arterioles and carry blood away from the heart to capillaries 
  • Networks of capillaries called capillary beds are the sites of chemical exchange between the blood and interstitial fluid
  • Venules converge into veins and return blood from capillaries to the heart
• Organization of Vertebrate Circulatory Systems

  • Arteries and veins are distinguished by the direction of blood flow, not by O2 content
  • Vertebrate hearts contain two or more chambers
  • Blood enters through an atrium and is pumped out through a ventricle
• Single Circulation

  • Bony fishes, rays, and sharks have single circulation with a two-chambered heart
  • In single circulation, blood leaving the heart passes through two capillary beds before returning

  
• Double Circulation

  • Amphibian, reptiles, and mammals have double circulation
  • Oxygen-poor and oxygen-rich blood are pumped separately from the right and left sides of the heart

  
• Reptiles, Mammals, Amphibians

  • In reptiles and mammals, oxygen-poor blood flows through the pulmonary circuit to pick up oxygen through the lungs
  • In amphibians, oxygen-poor blood flows through a pulmocutaneous circuit to pick up oxygen through the lungs and skin
  • Oxygen-rich blood delivers oxygen through the systemic circuit
  • Double circulation maintains higher blood pressure in the organs than does single circulation
• Reptiles, Mammals, Amphibians

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• Mammals and Birds

  • Mammals and birds have a four-chambered heart with two atria and two ventricles
  • The left side of the heart pumps and receives only oxygen-rich blood, while the right side receives and pumps only oxygen-poor blood
  • Mammals and birds are endotherms and require more O2 than ectotherms

  
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• Functional Anatomy of the HeartCardiac Muscle

  • Characteristics
  • Striated
  • Short branched cells
  • Uninucleate
  • Intercalated discs
  • T-tubules larger andover z-discs
• Functional Anatomy of the Heart Chambers

  • 4 chambers
  • 2 Atria
  • 2 Ventricles
  • 2 systems
  • Pulmonary 
  • Systemic
• Functional Anatomy of the Heart Valves

  • Function is to prevent backflow
  • Atrioventricular Valves
  • Prevent backflow to the atria
  • Prolapse is prevented by the chordae tendinae
  • Tensioned by the papillary muscles
  • Semilunar Valves
  • Prevent backflow into ventricles
• Functional Anatomy of the HeartIntrinsic Conduction System

  • Consists of “pacemaker” cells and conduction pathways
  • Coordinate the contraction of the atria and ventricles
  • Additional notes: The sinus node is sometimes called the heart's "natural pacemaker." Each time the sinus node generates a new electrical impulse; that impulse spreads out through the heart's upper chambers, called the right atrium and the left atrium
• Myocardial Physiology Autorhythmic Cells (Pacemaker Cells)

  • Characteristics of Pacemaker Cells
  • Smaller than contractile cells
  • Don’t contain many myofibrils
  • No organized sarcomere structure
  • do not contribute to the contractile force of the heart
• Myocardial Physiology Autorhythmic Cells (Pacemaker Cells)


  Cells that generate the heart's intrinsic rhythm
• Pacemaker Cells


  • Unstable membrane potential
  • "Bottoms out" at -60mV
  • "Drifts upward" to -40mV, forming a pacemaker potential
  • Myogenic (able to generate their own electrical impulses)
• Pacemaker potential generation

  1. Slow leakage of K+ out & faster leakage Na+ in (causes slow depolarization)

  2. Ca2+ channels opening as membrane approaches threshold (causes more rapid depolarization)

  3. Slow K+ channels open as membrane depolarizes (causes repolarization)
• If channels

  Channels that open at negative membrane potentials and start closing as membrane approaches threshold potential
• The If channels, with a mixed sodium and potassium inward current, have been identified in the sinoatrial node of the heart, which mediates the slow diastolic depolarization of the pacemaker of the spontaneous rhythmic cells
• Myocardial Physiology Autorhythmic Cells (Pacemaker Cells)

  • Characteristics of Pacemaker Cells
  • Funny' (f) channels are activated by intracellular cyclic adenosine monophosphate (cAMP) concentrations according to a mechanism mediating regulation of heart rate by the autonomic nervous system, as well as by voltage hyperpolarisation.

  
• Myocardial Physiology Autorhythmic Cells (Pacemaker Cells)

  • Altering Activity of Pacemaker Cells
  • Sympathetic activity
  • NE and E increase If channel activity
  • Binds to β1 adrenergic receptors which activate cAMP and increase If channel open time
  • Causes more rapid pacemaker potential and faster rate of action potentials

  
• Myocardial Physiology Autorhythmic Cells (Pacemaker Cells)

  • Altering Activity of Pacemaker Cells
  • Parasympathetic activity
  • ACh binds to muscarinic receptors
  • Increases K+ permeability and decreases Ca2+ permeability = hyperpolarizing the membrane
  • Longer time to threshold = slower rate of action potential
• Myocardial Physiology Autorhythmic Cells (Pacemaker Cells)

  • Altering Activity of Pacemaker Cells
  • Parasympathetic activity
  • ACh binds to muscarinic receptors
  • Increases K+ permeability and decreases Ca2+ permeability = hyperpolarizing the membrane
  • Longer time to threshold = slower rate of action potentials
  • The M2 muscarinic receptors are located in the heart, where they act to slow the heart rate down to normal sinus rhythm after negative stimulatory actions of the parasympathetic nervous system, by slowing the speed of depolarization.

  
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• Myocardial Physiology Contractile Cells

  • Special aspects
  • Intercalated discs
  • Highly convoluted and interdigitated junctions
  • Joint adjacent cells with
  • Desmosomes & fascia adherens
  • Allow for synticial activity
  • With gap junctions
  • More mitochondria than skeletal muscle
  • Less sarcoplasmic reticulum
  • Ca2+ also influxes from ECF reducing storage need
  • Larger t-tubules
  • Internally branching
  • Myocardial contractions are graded!
• Contractile cells


  Special aspects
• Action potential of a contractile cell

  • Ca2+ plays a major role
  • Longer in duration than a "normal" action potential due to Ca2+ entry
• Phases of the action potential

  1. 4 - Resting membrane potential @ -90mV

  2. 0 - Depolarization (due to gap junctions or conduction fiber action, voltage gated Na+ channels open... close at 20mV)

  3. 1 - Temporary repolarization (open K+ channels allow some K+ to leave the cell)

  4. 2 - Plateau phase (voltage gated Ca2+ channels are fully open)

  5. 3 - Repolarization (Ca2+ channels close and K+ permeability increases as slower activated K+ channels open, causing a quick repolarization)
• Myocardial Physiology Contractile Cells (Plateau phase)

  This plateau phase allows for a longer muscle contraction and gives time for the nearby cardiac muscle cells to depolarize. This is important in allowing the heart to contract in a steady, uniform and forceful manner. Following the plateau phase is phase 3, also known as the repolarization phase.
• Myocardial Physiology Contractile Cells

  • Skeletal Action Potential vs Contractile Myocardial Action Potential

  
• Myocardial PhysiologyContractile Cells

  • Plateau phase prevents summation due to the elongated refractory period
  • No summation capacity = no tetanus
  • Which would be fatal

  
• Myocardial PhysiologyContractile Cells

  NO SUMMATION IN CARDIAC MUSCLE- The muscles of the heart have a long absolute refractory period which prevents the heart from undergoing summation of contractions. An absolute refractory period refers to the amount of time after a muscle contracts that it is not able to contract again from an action potential.
• Myocardial PhysiologyContractile Cells

  • Initiation
  • Action potential via pacemaker cells to conduction fibers
  • Excitation-Contraction Coupling

  1.  Starts with CICR (Ca2+ induced Ca2+ release)

  • AP spreads along sarcolemma
  • T-tubules contain voltage gated L-type Ca2+ channels which open upon depolarization
  • Ca2+ entrance into myocardial cell and opens RyR (ryanodine receptors) Ca2+ release channels
  • Release of Ca2+ from SR causes a Ca2+ “spark”
  • Multiple sparks form a Ca2+ signal
• Contractile Cells

  Myocardial Physiology
• Excitation-Contraction Coupling

  Ca2+ signal (Ca2+ from SR and ECF) binds to troponin to initiate myosin head attachment to actin