circulation

• Every organism must exchange materials with its environment
• Diffusion


  The process by which molecules move from an area of high concentration to an area of low concentration without the input of energy
• Diffusion is only efficient over small distances, because the time (t) it takes to diffuse is proportional to the square of the distance (x)
• Diffusion coefficient (D)


  Flux/gradient; ~10 micron2/s for proteins in cell cytoplasm
• For most cells in multicellular organisms, exchange is generally facilitated by specialized structures
• Gastrovascular cavities


  • Help minimize diffusion distances for digestion and circulation
  • Found in protosomes with simple body plans (cnidarians, sponges, flatworms, nematodes)

  
• Circulatory system


  A system with a circulatory fluid, a set of interconnecting vessels, and a muscular pump (heart)
• Circulatory systems


  • Can be open or closed
  • Connect the fluid that surrounds cells with organs that exchange gases, absorb nutrients, and dispose of wastes

  
• Open circulatory system


  • Internal fluid is circulated through the body cavity
  • No distinction between blood and interstitial fluid (hemolymph)
  • Pressure rapidly dissipates requiring faster heart rate and/or accessory hearts

  
• Closed circulatory system


  • Blood is confined to vessels and is distinct from the interstitial fluid
  • Substances move from blood => interstitial fluid; interstitial fluid => cells
  • More efficient at transporting circulatory fluids to tissues and cells (maintain pressure gradients)

  
• Vertebrate hearts emerged ~500 Mya in our biological history, contain two or more chambers
• Single circulation


  • Blood enters through an atrium and is pumped out through a ventricle
  • Blood leaving the heart passes through two capillary beds before returning
  • Disadvantage: no 'fresh' blood
• Double circulation


  • Oxygen-rich blood is delivered from the heart to the rest of the body through the systemic circuit
  • Blood travels separately between the heart and the respiratory surface (pulmonary circuit)

  
• Heart rates vary greatly across vertebrates, according to mass-specific metabolic rates
• Cardiac output


  The volume of blood pumped into the systemic circulation per minute, depends on both heart rate and stroke volume

  
• Humans: ~70 bpm x 70 ml = ~4.9L/min x 60 min X 24 = ~7056 L/Day!!
• Heart wall


  • Muscle (myocardium), encapsulated between an inner lining (endocardium) and a fibrous, protective sheath (pericardium)
• Atria


  • Relatively thin walls, serve as collection chambers for blood returning to the heart
• Ventricles


  • Thicker walls, contract much more forcefully, ejecting blood to distal sites
• Heart valves


  • Atrioventricular (AV) valves (tricuspid and bicuspid/Mitral)
  • Semilunar valves (aortic and pulmonary)
  • Prevent backflow of blood
• Systole


  The contraction, or pumping, phase of the cardiac cycle

  
• Diastole


  The relaxation, or filling, phase of the cardiac cycle
• Cardiac cycle


  • Heart fully relaxed, atria fill, AV valves open, ventricles begin to fill
  • Ventricles ~80% full, atria contract to fully fill ventricles
  • Ventricles contract, forcing AV valves closed, SL valves open
• Cardiac muscle cells are autorhythmic, they contract without any signal from the nervous system
• Sinoatrial (SA) node


  • The pacemaker, sets the rate and timing of cardiac muscle contractions (70 beats/min)
• Atrioventricular (AV) node


  • Impulses from SA node are delayed here before traveling to Purkinje fibres that make ventricles contract
• Electrocardiogram (ECG)


  A recording of the electrical activity of the heart

  
• Pacemakers


  • Devices that regulate the heart's rhythmic beat
• Regulation of heart rate

  • Sympathetic division speeds up pacemaker (Norepinephrine)
  • Parasympathetic division slows down pacemaker (Acetylcholine)
  • Also regulated by hormones and temperature
• Baroreceptors


  • Located in heart muscle, aorta, carotid arteries, provide blood pressure information to medulla -> SA node

  
• Arteries and veins


  • Have an endothelium, smooth muscle, and connective tissue
  • Arteries have thicker walls than veins to accommodate high pressure of blood pumped from heart
• Capillaries


  • Only slightly wider than a red blood cell, have thin walls (endothelium plus basal lamina) to facilitate exchange of materials
• Systolic pressure


  The pressure in the arteries during ventricular systole; the highest pressure in the arteries
• Diastolic pressure


  The pressure in the arteries during diastole; lower than systolic pressure
• Pulse


  The rhythmic bulging of artery walls with each heartbeat
• The recoil of elastic arterial walls plays a role in maintaining blood pressure
• The resistance to blood flow in the narrow diameters of tiny capillaries and arterioles dissipates much of the pressure
• Sphygmomanometer


  A device used to measure blood pressure
• Blood pressure and velocity depend on vessel diameter
• Velocity of blood flow is slowest in capillary beds, as a result of high resistance and large total cross-sectional area