Single-celled & simple multi-celled: all cells have contact with medium



All cells need to be capable of exchange with their environment:


      All reactions in cells depend on resources moving in, waste products moving out

     all cells must have access to this exchange

      Single-celled & simple multi-celled: all cells have contact with medium

     diffusion distances are short

      Larger multi-celled: some/most cells isolated from external environment

     diffusion distances are too long

     circulatory systems connect those cells to the outside

     bring resources to the cell, carry waste away


Circulatory systems vary in complexity:


      Gastrovascular cavities: lack specialized circulatory system

     have highly branched gut/body cavity

     high SA:V, short diffusion differences

     e.g., cnidarians, flatworms

      Specialized circulatory system: 3 basic components

     circulatory fluid: carries resources/wastes

     interconnecting tubes: through which fluid travels

     heart: muscular pump

      Open circulatory systems: e.g., arthropods, most mollusks

     circulatory fluid (hemolymph) in direct contact with organs; same as interstitial fluid

     advantages: lower pressures, can use fluid as hydrostatic skeleton

      Closed circulatory systems: e.g., annelids, vertebrates

     circulatory fluid (blood) in vessels, separate from interstitial fluid

     advantages: faster delivery of O2, easier to regulate


Even among vertebrates, there is variation in circulatory systems:


      Parts of the vertebrate circulatory system = cardiovascular system:

     pumping heart with 2+ chambers

      atrium: receives blood

      ventricle: pumps blood away

     arteries: carry blood away from heart; branch into arterioles

     branch further into capillaries: where exchange takes place; capillary bed: network of capillaries

     capillaries converge into venules; converge into veins: carry blood back to heart

     remember: arteries & veins distinguished by direction they carry blood

     more energy organism/organ needs, the more complex circulation

      Single circulation: in fishes with 2-chambered hearts

     blood passes through 2 capillary beds during circuit

     Runs at lower pressure, so lower velocity

      Aided by swimming muscles

      Double circulation: in tetrapods with 3- or 4-chambered hearts (4 in mammals)

     Blood pumped through two separate circuits

      right side pulmonary circuit: to lungs

      left side systemic circuit: to body

     maintains higher pressure/velocity of blood

      Variation across major groups: amphibians and (non-bird) reptiles have 3-chambered heart


11 Steps in the flow of blood through both circuits:


  1. right ventricle pumps blood to lungs
  2. via the pulmonary arteries
  3. blood flows through capillary beds of the left & right lungs (gas exchange)
  4. blood returns to left atrium via pulmonary veins (blood is oxygenated)
  5. left ventricle pumps blood out to body
  6. via the aorta (including coronary arteries to the heart)
  7. one branch leads to capillary beds in the head & arms
  8. another branch leads to capillary beds in the abdomen & legs
  9. deoxygenated blood drains from the head & arms via superior vena cava
  10. deoxygenated blood drains from the abdomen & legs via the inferior vena cava
  11. both empty to the right atrium

    and the process continues...

The cardiac cycle alternates pumping and filling:


      Cardiac cycle: complete sequence of contraction/pumping (systole) and relaxation/filling (diastole)

     heart rate: 72 beats per minute (average resting rate)

     stroke volume: 70 mL per ventricle

     cardiac output: ca. 5 L/minute (per ventricle)

      4 valves keep blood from flowing wrong direction

     one-way flaps, bigger than the opening they cover

     atrioventricular valve (AV): between chambers

     semilunar valves: between ventricles and arteries

     heart murmur: defective valve leads to back-flow


The heart provides its own “pacemaker”:


      Pacemaker: autorhythmic cells of heart; contraction based upon own electrical impulses

     begins at sinoatrial node: cause atria to contract

     relayed by atrioventricular node: after 0.1 s delay, ventricles contract

     nervous system can speed-up or slow-down rate with activity level

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