Chapter 18: Blood Composition and Physical Characteristics

Blood Composition

• Blood is a fluid connective tissue composed of:
  • Plasma (90% water, 8% plasma proteins, 2% other solutes)
  • Formed elements (Erythrocytes, Leukocytes, Thrombocytes)

Plasma

• Plasma proteins:
  • 60% albumin (substance carrier, buffer, and osmotic pressure contributor)
  • 36% globulins
  • 4% fibrinogen
• Last 2% of plasma consists of:
  • Wastes (lactic acid, urea, creatinine)
  • Nutrients (glucose, amino acids)
  • Electrolytes (Na+, K+, Ca2+, Cl–, HCO3–)
  • Respiratory gases (O2 and CO2)
  • Hormones and enzymes

Formed Elements in Blood

• Erythrocytes (RBCs):
  • Biconcave discs, anucleate, and essentially no organelles
  • Major factor contributing to blood viscosity
  • Filled with hemoglobin (Hb) for gas transport
  • Spectrin: a membrane protein that provides flexibility for RBC to change shape
• Leukocytes (WBCs):
  • Only complete cells in blood
  • Defend against foreign invaders
• Platelets:
  • Cell fragments
  • Most formed elements survive in the bloodstream for only a few days
  • Originate in red bone marrow

Erythrocyte Function

• RBCs are dedicated to respiratory gas transport
• Hemoglobin protein (Hb) structure:
  • Globin: 4 protein chains each containing a heme group
  • Each heme group has an iron atom in the middle
  • Each iron atom can bind to one O2 molecule
  • Each Hb molecule can transport four O2
  • Each RBC contains 250 million Hb molecules

Hemoglobin

• O2 loading in the lungs:
  • Produces oxyhemoglobin (ruby red)
• O2 unloading in the tissues:
  • Produces deoxyhemoglobin or reduced hemoglobin (dark red)
• CO2 loading in the tissues:
  • Produces carbaminohemoglobin (carries 20% of CO2 in the blood)
  • Binds to the amino acid portion of the globin

Hematopoiesis

• Hematopoiesis (hemopoiesis): blood cell formation
• Occurs in red bone marrow of axial skeleton, girdles, and proximal epiphyses of humerus and femur
• On average, the marrow turns out an ounce of new blood containing 100 billion new cells each and every day

Erythropoiesis

• Erythropoiesis: red blood cell production
• The number of circulating erythrocytes remains constant and reflects a balance between RBC production and destruction
• Balance between RBC production and destruction depends on:
  • Hormonal controls
  • Adequate supplies of iron, amino acids, and B vitamins
• Too few RBCs leads to tissue hypoxia
• Too many RBCs increases blood viscosity

Hormonal Control of Erythropoiesis

• Erythropoietin (EPO):
  • Direct stimulus for erythropoiesis
  • Released by the kidneys in response to hypoxia
• Causes of hypoxia:
  • Hemorrhage or increased RBC destruction reduces RBC numbers
  • Insufficient hemoglobin (e.g., iron deficiency)
  • Reduced availability of O2 (e.g., high altitudes)
• Effects of EPO:
  • More rapid maturation of committed bone marrow cells
  • Increased circulating reticulocyte count in 1–2 days
• Testosterone also enhances EPO production, resulting in higher RBC counts in males

Development of Red Blood Cells

• Erythrocytes:
  • Life span: 100–120 days
  • Old RBCs become fragile, and Hb begins to degenerate
  • Macrophages engulf dying RBCs in the spleen

Fate and Destruction of Erythrocytes

• Heme and globin are separated:
  • Iron is salvaged for reuse
  • Heme is degraded to the yellow pigment bilirubin
  • Liver secretes bilirubin (in bile) into the intestines
  • Globin is metabolized into amino acids