Dr. Basim Al-Ka'abi
Blood Physiology
Fourth Lecture
1
Lecture Name: Vitamins Needed for RBC Formation
Lecturer Name: Dr. Basim A. Al-Ka'abi
Department: Medical Physiology
Stage: First Stage Medical Students
Lecture Objectives:
By the end of this lecture the students should be able to know the following:
Definition, meaning and metabolism of vitamin B
12
.
Meaning of intrinsic factor and its relationship to B
12
.
Folic acid and its role in RBC synthesis.
General functions of blood.
References:
Barrett, K et al. (2018). Ganong's Review of Medical Physiology. Twenty-
sixth edition. USA.
Guyton, A and Hall, J (2015). Text Book of Medical Physiology.
Thirteenth edition. Philadelphia, USA.
Dr. Basim Al-Ka'abi
Blood Physiology
Fourth Lecture
2
Vitamins needed for formation of RBCs
Vitamin B
12
(cobalamin) or (cyanocobalamin)
-Vitamin B
12
or cobalamin has a similar structure to heme in having four
pyrrole rings, but these are linked to a cobalt atom.
-It is formed mainly by bacterial synthesis and the only source to man is
dietary and mainly in foods of animal origin, such as meats, eggs, dairy
products and liver.
-Vitamin B
12
is an essential nutrient for cells of the body, and growth of
tissues in general is greatly depressed when this vitamin is lacking.
-This results from the fact that it is required for the synthesis of DNA. Since
tissues that produce RBCs are among the most rapidly growing and
proliferating of all the body’s tissues, lack of this vitamin especially
inhibits the rate of RBCs production.
-Furthermore, the erythroblastic cells of the bone marrow, in addition to
failing to proliferate rapidly, become larger than normal, developing into
megaloblasts and the adult erythrocyte called a macrocyte.
-Therefore, it is said that vitamin B
12
deficiency causes maturation failure
in the process of erythropoiesis.
-Total body vitamin B
12
is about 3000 µg and is stored mainly in the liver.
-Between 1-4 µg are required daily to replace that lost by the body.
Absorption
-Dietary vitamin B
12
is released from its binding to proteins by gastric and
intestinal enzymes.
-Passive absorption does occur but very small (1% of oral dose) and is
ineffective unless there is a very high vitamin B
12
intake.
-The alternative and physiologically effective mechanism is by binding to
gastric intrinsic factor (IF).
Dr. Basim Al-Ka'abi
Blood Physiology
Fourth Lecture
3
Intrinsic factor (IF)
-Is a glycoprotein, secreted by parietal cells of the stomach.
-Vitamin B
12
becomes firmly bound to IF in the intestine, in which one
molecule of IF binds one molecule of vitamin B
12
.
-The IF-B
12
complex then becomes bound to specific receptors on the ileal
mucosal cells that allow the absorption of B
12
.
-The role of IF in the process is to stimulate endocytosis, so B
12
is
transferred across the intestinal epithelium.
-Vitamin B
12
is released then from the ileal cell and bound to a transport
protein in the blood stream called transcobalamin II.
-In this bound state the B
12
is protected from digestion by the
gastrointestinal enzymes.
-Once vitamin B
12
has been absorbed from the GIT it is stored in large
quantities in the liver and then released slowly as needed to the bone
marrow and other tissues of the body.
-The total amount of vitamin B
12
required each day to maintain normal red
cell maturation is only about 5 µg, and the normal store in the liver and
other body tissues is about 1000 times this amount.
-Therefore, as many as 4-5 years of defective B
12
absorption are required
to cause maturation failure anemia.
-So inadequate absorption of this vitamin B
12
causes megaloblastic anemia,
which is also called pernicious anemia.
-Final lack of IF, therefore, causes loss of much of the B
12
because of both
enzyme action in the gut and failure of its absorption.
Folic Acid
-Folic acid is also concerned with the maturation of RBCs. Since folic acid,
like vitamin B
12
is required for the formation of DNA.
Dr. Basim Al-Ka'abi
Blood Physiology
Fourth Lecture
4
-If there is a deficiency of folic acid the marrow fills with megaloblasts and
the number of mature red cells in the peripheral blood is reduced and cause
megaloblastic anemia.
-Dietary deficiency results from a poor diet usually owing to alcoholism,
poverty, old age or chronic overcooking of vegetables.
-Folic acid is present in green vegetables (especially spinach), liver and
kidney.
-Total body content is 6-10 mg present in the liver. In normal pregnancy
300-500 µg are required daily.
Functions of the blood
1. Transport of nutrients from the digestive tract to the tissues.
2. Transport of metabolites (e.g. lactic acid from the muscles to the liver).
3. Transport of excretory products from the tissues to excretory organs
(e.g. urea from the liver to the kidney).
4. Transport of gases (O
2
and CO
2
) between the lungs and tissues.
5. Transport of hormones and vitamins.
6. Transport of heat from deeper organs to the surface of the body.
7. Coagulation, serves to protect against blood loss.
8. Forms antibodies that help resisting various types of infections.