5 - RBC Structure and function

Question 1

blood volume and composition

Answer 1

• Normally 5-6 liters
• Fluid (55%) and cellular (45%) components
• Fluid component = Plasma (water, proteins, solutes)
  Serum (clotted plasma)
• Cellular Component
  Erythroid (43-44%)
  Buffy coat (leukocytes and platelets, 1-2%)

difference btw plasma and serum: serum is plasma that has components which allow clotting

Question 2

erythryocytes (RBCs) overview

Answer 2

• 7-8 μm diameter (< lymphocyte nucleus)
• Biconcave disc
• Round with central area of pallor (1/3 diameter)
• Anucleate
• Eosinophilic cytoplasm (hemoglobin)
• Simple metabolism (glycolysis)
• 120 day lifespan
• Senescent cells removed by splenic macrophages

Question 3

Erythrocyte Function

Answer 3

• Transport oxygen from the lungs to the tissues
• Transport carbon dioxide from the tissues to the lungs

Question 4

Erythropoiesis - general overview

Answer 4

• Maturation features
  1) Size (decreasing)
  2) Nucleus (decreasing size, increasing chromatin
  density, extrusion of nucleus)
  3) Cytoplasm (increasing eosinophilia = hemoglobin)
• Stages of maturation
• Regulation by erythropoietin

Question 5

maturation features - size and nucleus relationship

Answer 5

cell size and nucleus size (decreasing) with maturation

Nucleus (decreasing size, increasing chromatin
density, extrusion of nucleus) with maturation

Question 6

maturation features - size and nucleus

Answer 6

Nucleus (decreasing size, increasing chromatin
density, extrusion of nucleus) with maturation

cell size decreases with maturation

Question 7

reticulocytes vs mature erythrocytes

Answer 7

Cytoplasm (increasing eosinophilia = hemoglobin)
both pictures below are reticulocytes

measurement of reticulocytes is important in anemia - anemia causes release of immature reticulocytes to compensate* Lipid bilayer (phospholipids, cholesterol)
Fluidity inversely proportional to cholesterol content

• Proteins
• Carbohydrate (glycolipids, glycoproteins)

Question 8

Erythropoiesis regulation by erythropoietin

Answer 8

The main factor regulating erythroid proliferation is erythropoietin (EPO), a 34-kD glycoprotein growth
factor, which is produced by interstitial peritubular cells of the kidney in response to hypoxia.

Tissue hypoxia leads to upregulation of hypoxia-inducible transcription factors, which in turn control
EPO production. EPO affects erythroid precursors at all stages of development and influences erythroid
lineage commitment, proliferation, and differentiation, as well as iron uptake and metabolism by red
cell precursors.

Question 9

Erythrocyte Membrane

Answer 9

• Lipid bilayer (phospholipids, cholesterol)
  Fluidity inversely proportional to cholesterol content
• Proteins
• Carbohydrate (glycolipids, glycoproteins)

Question 10

Erythrocyte Cytoskeleton

Answer 10

• Network of contractile proteins (deformability, shape)
• Attach to the cell membrane
• Spectrin (α and β chains, form heterodimers)
• Other proteins (actin, tropomyosin, band 4.1,
  ankyrin, glycophorins)
• Horizontal vs. vertical interactions

Question 11

Hemoglobin

Answer 11

• Tetrameric molecule
• Heme component (iron binding)
• Globin chains

Question 12

Heme Synthesis

Answer 12

• Mitochondria vs. cytoplasm
  -mitochondria: initial and final steps
  -intermediate steps in cytoplasm
• Synthetic pathway
• Clinical correlation:
• Ringed sideroblasts
• Lead poisoning
• Porphyrias

Question 13

heme synthesis - Mitochondria vs. cytoplasm

Answer 13

-mitochondria: initial and final steps
-intermediate steps in cytoplasm

ferrous iron (II)

Question 14

synthetic pathway for heme synthesis (simplified version) and heme synthesis - clinical correlation, porphyrias

Answer 14

Question 15

heme synthesis - clinical correlation, lead poisoning

Answer 15

blocks incoporation of iron at the protoporphyrin 9 stage causing image to the right referred to as basophilic stippling where rna fragments precipitate

Question 16

heme synthesis - clinical correlation, ringed sideroblasts

Answer 16

red cell precursors in the bone marrow with an iron stain

blue granules form around the nucleus in the image below

iron (blue) would usually be incorporated into the porphyrin ring but is blocked creating ringed sideroblasts

Question 17

Globin Chain Synthesis

Answer 17

*Genes on chromosomes 16 and 11

*Pairs of α and non-α globin chains

*α chains: α (adult) and ζ (embryonic)

*Non-α chains: β (adult), δ (adult), γ (fetal), and ε
(embryonic)

Question 18

Normal Hemoglobins

Answer 18

*Adult: A (α2β2), A2 (α2δ2)

*Fetal: F (α2γ2)

*Embryonic:
Gower 1 (ζ2ε2), Gower 2 (α2ε2), and Portland (ζ2γ2)

Question 19

Hemoglobin Function

Answer 19

• 4 heme, four globin (2 pair), 4 iron (Fe++), 4 oxygen
• Ability to bind oxygen depends on conformation
  (successive binding becomes easier)
• Hemoglobin-oxygen dissociation curve
• Right shift (acidosis, increased temperature,
  increased 2,3-DPG)
• Left shift (alkalosis, decreased temperature,
  decreased 2,3-DPG, hemoglobin F, carbon monoxide)

Question 20

Hemoglobin-oxygen dissociation curve

Answer 20

• Right shift (acidosis, increased temperature,
  increased 2,3-BPG)
• Left shift (alkalosis, decreased temperature,
  decreased 2,3-BPG, hemoglobin F, carbon monoxide)

Question 21

Erythrocyte Energy Metabolism

Answer 21

• Glycolysis (anaerobic) – 2 ATP (net) per glucose
• Hexose monophosphate shunt (G6PD)
  Reduction reactions (free radicals)
  Methemoglobin (Fe+++) – Heinz bodies
• Rapoport-Luebering shunt
  Generation of 2,3-bisphosphoglyerate (2,3-bPG)

more bpg, shift curve right, lower affinity for O2

Question 22

Hexose monophosphate shunt (G6PD)

Answer 22

Reduction reactions (free radicals)
Methemoglobin (Fe+++) – Heinz bodies

Question 23

Erythrocyte Senescence

Answer 23

• 120 day lifespan
• Splenic macrophages
  -recognize ageing/deformed red cells and remove them
• Catabolism and recycling of hemoglobin components:
  Iron
  Heme (bilirubin)
  Globin chains

Question 24

Heme recycling (bilirubin)

Answer 24

The heme portion of the hemoglobin molecule undergoes a series of reactions, which
initially result in the formation of biliverdin, which is subsequently converted to bilirubin. This bilirubin
is referred to as unconjugated (indirect) and is not soluble in water. Unconjugated bilirubin is released
into circulation and bound to albumin, which transports it to the hepatocytes for additional processing.
In the hepatocytes, bilirubin is conjugated with glucuronic acid and becomes water soluble. Conjugated
(direct) bilirubin can then be excreted in bile into the small intestine. Much (95%) of the excreted
bilirubin is reabsorbed within the small intestine and returned to the liver via the enterohepatic
circulation. The non-reabsorbed bilirubin may be metabolized to urobilinogen by colonic bacteria.

Urobilinogen may be subsequently oxidized to form urobilin and stercobilin, which are excreted and
give fecal material its brown color. Some of the reabsorbed urobilinogen is excreted by the kidneys and
further oxidized to urobilin, which imparts a yellowish color to urine. In pathologic conditions
associated with increased erythrocyte destruction (hemolysis), elevated levels of some of the products
of heme catabolism can be detected in the blood and urine. The hemoglobin synthesis and catabolism
pathways are summarized in the figure below.