Chapter 19 - Cardiovascular System: The Blood

• Blood:

  • a unique, fluid connective tissue (ground substance, cells and fibers
  • constitutes about 8% of body weight
  • in a healthy adult, volume is 5-6L for a male and 4-5L for a female
  • about 4-5 times as viscous as water
  • has pH of 7.35-7.45
  • colour varies from scarlet (O2-rich) to dark red (O2-poor)
• Where we get oxygen from?

  Lungs (O2) and gut (nutrients) --> blood --> interstitial fluid --> body cells --> CO2 and waste --> back in interstitial fluid --> blood --> lungs, kidney and skin
• Blood functions to:

  1. transport
  2. regulate
  3. protect
• Transport:

  • O2 and nutrients to cells
  • CO2 and metabolic wastes away from cells
  • hormones and enzymes
• Regulate:

  • body temperature
  • pH of body tissues
  • water content of tissues
• Protect:

  • against blood loss (by clotting mechanisms)
  • against infection (by defense mechanisms of white blood cells)
• Blood is composed of 2 portions:

  1. plasma
  2. formed elements
• Plasma = ground substance
• Plasma (55%)

  • watery fluid that contains dissolved substances
• Formed elements (45%)

  • red blood cells (erythrocytes)
  • white blood cells (leukocytes)
  • platelets (thrombocytes) are cell fragments
• Hematocrit value = total blood value of red blood cells
• Blood plasma:

  • fluid portion of blood; straw-coloured
  • composed of 90% water and 10% solutes
• Solutes of blood plasma include:

  • electrolytes
  • nutrients
  • enzymes and hormones
  • gases
  • metabolic wastes
  • plasma proteins
• Electrolytes: Na+, K+, Ca2+, Mg2+, Cl-, HCO3-
• Nutrients: amino acids, glucose, fatty acids, glycerol
• Gases: CO2-, O2
• Metabolic wastes: urea, ammonia, bilirubin
• Plasma proteins: produced mainly by liver
• Plasma proteins: account for 8% of the 10% of dissolved solutes in plasma

  3 main categories: albumin, globulins and fibrinogen
• Albumin (smallest size) - 54% of plasma protein

  • helps with the maintenance of osmotic pressure (helps keep water in blood plasma)
  • acts as a blood buffer (maintains pH)
  • carrier molecule of fatty acids and steroid hormones
• Globulins - 38% of plasma protein

  1. alpha and beta globulins - transport iron, metals, lipids, fat-soluble vitamins
  2. immunoglobulins (antibodies) - released by plasma cells during immune response
• Fibrinogen - 7% of plasma protein

  • soluble precursor of fibrin
  • acts in blood clotting
• Why no nucleus?

  This lack of nucleus increases the surface area for oxygen transport 

  • Lack of mitochondria means the red blood cell need to generate ATP anaerobically = without O2 that way, they do not use the O2 they transport
• Red blood cells:

  • function in the transport of respiratory gases - O2 and CO2
  • biconcave disc - lacks nucleus and other organelles when mature
  • contain about 250 million molecules of hemoglobin (Hb) per cell
• Hb molecule consists of:

  • a globin molecule composed of four polypeptide chains (2 alpha and 2 beta)
  • an iron-containing 'heme' pigment bound to each of the four chains
• • Each hemoglobin molecule can transport 4 molecules of O2 
  • 250 million molecules of Hb/1 RBC x 4 O2 each = 1 billion 
  • Each RBC can transport 1 billion molecules of O2 so we rarely run short of O2
• Red blood cells:

  • each heme binds one O2 molecule reversibly
  • globin binds CO2
  • lifespan of RBC's is about 120 days
  • formed constantly by erythropoiesis in red bone marrow (rate of 2 million per sec)
  • cells take about one week to mature
  • main stimulus for increased erythropoiesis is a decrease in the O2- carrying capacity of the blood
• Negative Feedback Loop for RBC:

  1. homeostasis is disrupted as oxygen levels in blood decrease
  2. kidney cells detect low oxygen levels and release erythropoietin (a hormone)
  3. erythropoietin stimulates increased red blood cells formation in red bone marrow
  4. increased number of red blood cells enter the circulation
  5. increase in oxygen levels in the blood and homeostasis is restored
• • number of RBCs and platelets stays fairly constant because regulated by negative feedback systems
  • number of WBCs varies because it depends on invading pathogens and other foreign antigens
• How are blood cells formed?

  • blood cell formation is called hematopoiesis
  • all blood cells arise from a Pluripotent stem cell in red bone marrow (hemocytoblast) which goes on to form 2 stem cell lines:

  1. myeloid stem cells
  2. lymphoid stem cells
• Leucopoiesis = white blood cells
• Myeloid stem cells:

  • form RBC's, Platelets, Eosinophils, Basophils, Neutrophils and Monocytes
• Lymphoid stem cells:

  • form Lymphocytes (type of WBCs)
• Erythropoiesis requires a dietary intake of iron, vitamin B12 and folic acid
• Iron:

  • needed for heme synthesis
  • stored in the liver and spleen as protein-iron complexes
  • transported in the blood by transferrin (plasma protein) to red bone marrow and is taken up by developing RBCs
• Vitamin B12:

  • found in dietary liver, eggs and meat
  • requires production of an intrinsic factor by the stomach mucosal cells to allow B12 to be absorbed in the small intestine
  • once absorbed, B12 is stored in the liver
  • transported to red bone marrow when needed
• Vitamin B12 and Folic Acid are needed for cell division and DNA synthesis that occur in RBC formation =erythropoiesis
• Fate of red blood cells includes:

  • anucleate so can't grow and divide
  • old or damaged RBC's are destroyed by marcophages in the spleen, liver and bone marrow
  • hemoglobin is salvaged and degraded to heme and globin
  • globin is broken down to amino acids which can be reused
• Fate of red blood cells includes:

  • iron from heme is released and is stored in the liver and spleen for reuse
  • pigment portion of heme is degraded to bilirubin
  • bilirubin is secreted into bile in the liver
  • if liver is not working properly, bilirubin builds up in the blood and gives a yellow appearance = jaundice
• Bilirubin: yellowish pigment that enters the blood to be picked up by liver cells and enters the bile --> enters small intestine --> it will then be metabolized by intestinal bacteria