circulation and gas exchange

• Similar to the xylem and phloem of vascular plants, circulatory systems in animals help transport materials throughout a complex multicellular organism
• One of the advantages of having a circulatory system

  It compensates for the slow rate at which diffusion occurs over large distances
• Internal transport (circulation) and gas exchange (respiration) are anatomically and functionally related
• Gases (O2 and CO2) must reach all cells
• How does circulation occur?

  1. Circulatory fluid, interconnected vessels, and pump(s)

  2. Open vs. closed; single vs. double

  
• How does gas exchange occur?


  1. Specialized organs: gills, tracheae, lungs, skin

  2. Ventilation/breathing
• Why does gas exchange occur?


  Chemistry and homeostatic mechanisms
• Some animals do not have a circulatory system (gastrovascular cavity)
• General properties of circulatory systems


  • A circulatory system has circulatory fluid, set of interconnecting vessels, and muscular pump(s), the heart

  
• Open versus closed circulatory systems

  Arthropods and most molluscs have open circulatory systems

  Annelids, cephalopods, and vertebrates have closed circulatory systems

  
• Vertebrates have closed circulatory systems


  Circulatory fluid: blood

  Interconnected vessels: arteries → arterioles → capillaries → venules → veins

  Pump: heart

  Gas exchange mechanism: gills/lungs

  Blood must be delivered to two general locations: respiratory tissue (e.g. pulmonary circuit) and other organ systems (e.g. systemic circuit)

  
• Single versus double circulation in vertebrates

  Arteries carry blood away from the heart to the pulmonary (deoxygenated blood) and systemic circuits (oxygenated blood)

  Veins carry blood back to the heart from the pulmonary (oxygenated blood) and systemic circuits (deoxygenated blood)

  Pulmonary circuit: pulmonary arteries, pulmonary veins

  Systemic circuit: aorta, superior and inferior vena cava

  
• Circulatory fluid: blood


  Blood: connective tissue; consists of cells suspended in a liquid matrix called plasma

  By volume, plasma ~ 55%; cells ~ 45%

  
• Respiratory pigments


  Respiratory pigments: proteins that transport oxygen; greatly increase the amount of oxygen that blood can carry

  Hemoglobin, myoglobin, hemocyanin

  Hemoglobin: respiratory pigment with high affinity for O2

  CO2 decreases pH and the affinity of hemoglobin for O2 (Bohr shift)

  Hemoglobin can bind CO2, but most is transported in plasma

  
• Gas exchange and respiratory surfaces

  Gas exchange: supplies O2 for cellular respiration and disposes of CO2

  Occurs via diffusion

  Accomplished via large, moist respiratory surfaces between cells and respiratory medium (air or water)

  Respiratory surfaces vary by taxa: skin, gills, tracheae, lungs
• Ventilation


  Breathing: process that ventilates the lungs; alternate inhalation and exhalation of air

  Mechanisms vary across taxa
• Disorders of the circulatory system: cardiovascular disease


  Account for >50% of all deaths in the U.S.

  Cardiovascular disease: disorder of the heart and/or blood vessels

  Atherosclerosis: buildup of plaques in arteries

  Heart attack (myocardial infarction): blockage of one or more coronary arteries

  Stroke: rupture or blockage of arteries to the brain

  Hypertension (high blood pressure): increases risk of atherosclerosis and heart attacks