Ch 44

• Circulatory system


  Consists of a heart, vessels, and fluid (blood) that move cells and important molecules (such as oxygen, nutrients, hormones, carbon dioxide, and other wastes) from one tissue to another

  
• Lymphatic system


  An accessory system of vessels and organs that helps balance the fluid content of the blood and surrounding tissues and participates in the body's defenses against invading disease organisms

  
• The cells of all organisms take in nutrients and O2 from the external environment, and eliminate wastes such as CO2 to the environment
• For very small or very thin animals, movement of these molecules occurs primarily by diffusion
• For larger animals, circulatory systems bring fluids close to the cells so that gases and nutrients can be exchanged
• Basic elements of circulatory systems

  • Fluid
  • Heart
  • Vessels

  
• Sponges, cnidarians, and flatworms have no distinct circulatory system
• Substances diffuse between cells and the external environment through internal or external surfaces
• In sponges, water is pumped in through pores in the body wall and exits through a large, central cavity
• Cnidarians have a central gastrovascular cavity with a mouth through which water enters and leaves
• Open circulatory system


  Vessels leaving the heart release bloodlike fluid (hemolymph) directly into body spaces (sinuses) that surround organs - hemolymph reenters the heart through valves in the heart wall, no distinction between hemolymph and interstitial fluid

  
• Closed circulatory system


  The fluid (blood) is confined to blood vessels and is distinct from the interstitial fluid - substances are exchanged between blood and interstitial fluid, then between interstitial fluid and cells

  
• Arthropods and most mollusks have open circulatory systems with one or more muscular hearts
• Most of the fluid pressure generated by the heart dissipates when the hemolymph is released into the sinuses
• Most of these animals are relatively sedentary - their tissues do not require large amounts of O2 and nutrients
• The more active insects and crustaceans have specialized air passages (tracheae) that aid in gas exchange
• Annelids, cephalopod mollusks, and all vertebrates have closed circulatory systems
• Arteries


  Conduct blood away from the heart at relatively high pressure

  
• Capillaries


  Nutrients and wastes are exchanged between the blood and body tissues by diffusion across the thin walls

  
• Veins


  Blood returns to the heart at relatively low pressure

  
• Sharks and bony fishes have one atrium and one ventricle
• The ventricle pumps blood to capillaries of the gills, where blood releases CO2 and picks up O2
• Oxygenated blood is delivered to capillary networks in other body tissues, where it delivers O2 and picks up CO2
• Deoxygenated blood returns to the atrium in veins
• Turtles, lizards, and snakes also have 2 atria and 1 ventricle
• A flap of tissue in the ventricle (incomplete septum) keeps oxygenated and deoxygenated blood almost completely separate

  
• The systemic circuit carries oxygenated blood to body tissues

  
• The pulmonary circuit carries deoxygenated blood to the lungs
• Crocodilians, birds, and mammals have a double heart consisting of two atria and two ventricles
• Each half of the heart operates as a separate pump, restricting the blood circulation to completely separate pulmonary and systemic circuits

  
• Blood is pumped by a ventricle in each circuit, so both operate at relatively high pressure
• Blood components


  • Erythrocytes
  • Leukocytes
  • Platelets
• Plasma


  The clear liquid matrix in which the blood cells are suspended

  
• An average adult human has 4 to 5 L of blood, with the liquid plasma making up 55-58% and the remaining solid (cellular) components making up the hematocrit

  
• In humans, blood cells develop in red bone marrow - primarily in the vertebrae, sternum, ribs, and pelvis
• Blood cells originate from pluripotent stem cells, which retain the embryonic capacity to divide
• Blood cell types originating from stem cells

  • Myeloid stem cells give rise to erythrocytes, platelets, neutrophils, basophils, eosinophils, and monocytes
  • Lymphoid stem cells give rise to B lymphocytes and T lymphocytes, which function in the immune system

  
• Functions of blood


  • Transports cells and molecules
  • Stabilizes pH and salt composition of body fluids
  • Regulates body temperature by transferring heat
• Components of blood plasma

  • Water (91%–92%)
  • Glucose and other sugars
  • Amino acids
  • Plasma proteins
  • Dissolved gases mostly O2, CO2, and N2
  • Ions - Na+, K+, Ca2+, Cl–, and HCO3–
  • Lipids
  • Vitamins
  • Hormones and other signal molecules
  • Metabolic wastes, including urea and uric acid
• Erythrocytes (red blood cells)


  Disc-like cells containing large quantities of the O2-carrying protein hemoglobin, mammalian erythrocytes lose their nucleus, cytoplasmic organelles, and ribosomes - limiting their life span