Developmental Changes of RBCs

Question 1

Where is the first site of RBC production in the fetus?

Answer 1

The yolk sac, at 2 weeks of gestation.

Question 2

Where is the primary site of RBC production in the fetus from gestational week 8 to 5 months?

Answer 2

The liver

Question 3

Where does RBC production primarily take place after the 5th month of gestation?

Answer 3

The bone marrow.

Question 4

Describe the changes in location that take place in fetal RBC production.

Answer 4

RBC production starts in the yolk sac (2-8 weeks of gestation), then transitions to the liver (2-5 months of gestation), then transitions to the bone marrow.

Question 5

Compare the information obtained from mean corpuscular volume (MCV) with that from mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC).

Answer 5

MCV reflects the size of the RBCs, while MCH and MCHC reflect the amount of hemoglobin in RBCs.

Question 6

Define mean corpuscular hemoglobin (MCH).

Answer 6

The average mass of hemoglobin in each RBC.

Question 7

Define mean corpuscular hemoglobin concentration (MCHC).

Answer 7

The ratio of hemoglobin mass to the volume of RBCs.

Question 8

Define RBC distribution width (RDW).

Answer 8

A measure of the variability in RBC size. An increased RDW means that there is increased variation in RBC size.

Question 9

Define mean corpuscular volume (MCV).

Answer 9

A measure that reflects the size of the RBCs.

Question 10

How is a reticulocyte count usually reported?

Answer 10

It is often reported as a percentage of reticulocytes in the blood.

Question 11

What is a reticulocyte?

Answer 11

An immature RBC.

Question 12

How does the RBC count in a fetus at 12 weeks of gestation compare with the RBC count of an infant at term?

Answer 12

The RBC count triples in that time.

Question 13

What is the average reticulocyte count for term infants at birth?

Answer 13

~5%

Question 14

What is the expected MCV of an infant born at term?

Answer 14

Macrocytic, usually ~108 fL

Question 15

Compare the hemoglobin levels in the capillaries with levels in the venous blood for a newborn? What accounts for the difference?

Answer 15

Hgb levels are much higher in the capillaries than in the venous blood due to a loss of plasma from the capillaries.

Question 16

What is the expected change in venous hgb, hct, and RBC counts between birth and 3 days of life? What accounts for the change?

Answer 16

All three values will increase between birth and the first 3 days of life due to a postnatal decrease in plasma volume.

Question 17

What regulates the production of RBCs?

Answer 17

Erythropoietin (EPO)

Question 18

Where is EPO produced in the fetus? After birth?

Answer 18

Erythropoietin is produced in the liver in the fetus. Production switches from liver to kidney soon after birth.

Question 19

What regulates the production of erythropoietin?

Answer 19

Tissue oxygenation

Question 20

What is the average hemoglobin at birth? What causes it?

Answer 20

17 g/dL; this relative polycythemia is due to the low arterial PaO2 in utero that stimulates EPO production and thus increases erythropoiesis.

Question 21

What factors are responsible for the “physiologic anemia of infancy”?

Answer 21

Arterial PaO2 rises acutely at birth, resulting in a decrease in EPO production. Nucleated RBCs disappear from the peripheral blood and the retic count drops. Red cell lifespan in the first 6-8 weeks of life is around 90 days instead of the usual 120 days. This results in the “physiologic anemia of infancy”, which reaches its nadir around 2 months of age, with an average Hgb of 9-11 g/dL.

Question 22

What is the average lifespan of a RBC during the first 6-8 weeks of life?

Answer 22

90 days (in contrast to the usual 120 days)

Question 23

How does the RBC count change postnatally in a preterm infant compared with a term infant?

Answer 23

Both have a drop in Hgb postnatally, but it is a more severe drop in preterm infants, with severity proportional to decreasing birthweight.

Question 24

What causes anemia to be more severe in preterm compared with term infants?

Answer 24

Preterm infants have inappropriately low levels of EPO and thus cannot stimulate production of RBCs.

Question 25

When do the average hemoglobin values start to diverge for males and females? Why?

Answer 25

In adolescence, because of the erythroid-stimulating effects of androgens in adolescent males.

Question 26

Describe the normal changes in MCV from birth through childhood and adolescence.

Answer 26

MCV is usually ~108 at birth (macrocytic), then falls during the first 6-12 months of life, during which time it reaches a nadir of 77 fL. It then rises throughout childhood and adolescence.

Question 27

What is the typical reticulocyte count after 4 months of age?

Answer 27

< 2%

Question 28

At what age is it normal to have nucleated RBCs in the peripheral circulation?

Answer 28

In the first week of life.

Question 29

Describe the basic structure of a hemoglobin molecule.

Answer 29

Hgb is a tetramer made up of 2 pairs of globin chains, each attached to an iron-containing porphyrin ring (heme).

Question 30

What is the difference between fetal hemoglobin and adult hemoglobin?

Answer 30

Adult hemoglobin contains two pairs of α chains and two pairs of β chains (α2β2). Fetal hemoglobin contains two pairs of α chains and two pairs of γ chains (α2γ2).

Question 31

Describe the fetal hemoglobin makeup in a 6 month old fetus > term neonate > 4 month old > 12 month old.

Answer 31

6 month old fetus has 90% circulating fetal Hgb > Neonate has 70% > 4 month old has 20% > 12 month old has less than 2% fetal Hgb.

Question 32

What happens to the oxygen dissociation curve after birth?

Answer 32

After birth the oxygen dissociation curve shifts to the right to allow better release of oxygen to the tissues. (***picture of curve?)