5. Erythrocytes and iron

Question 1

Describe the normal measurements in a full blood count (FBC).

Answer 1

• RBC: 4.4-5.9 x 10^12 cells/L
• Hb: 120-170 g/L
• mean cell volume: 80-100 fL
• WBC: 4-11 x 10^9/L
• platelets: 150-400 x 10^9/L

Question 2

How much do RBCs normally make up of the total blood volume?

Answer 2

40-50%

Question 3

What are the functions of RBCs?

Answer 3

Deliver O2 from lungs to all tissues and carry CO2 away

• carry Hb
• maintain Hb in reduced (ferrous) state
• generate ATP (maintain membrane and osmotic equilibirum to maintain structure)

Question 4

How does the structure of RBCs improve their function?

Answer 4

• Anucleate biconcave discs ~8um in diameter with a flattened depressed centre
  i) high surface area:volume ratio - more gas exchange
  ii) optimises laminar flow properties of blood in large vessels
  iii) allows cells to deform and facilitate passage through microcirculation (min. diamter of 3.5um)
• Cell membrane contains proteins such as spectrin, Ankyrin, Band 3 and protein 4.2
  iv) facilitate vertical interactions with the cytoskeleton of cell which are essential for maintaining the RBC’s biconcave shape and deformability.

Question 5

Which disease can cause RBCs to lose their shape?

Answer 5

Hereditary spherocytosis: gene mutation in erythrocyte cell membrane proteins - lose biconcave shape

Question 6

What % of the RBC volume does Hb take up?

Answer 6

95%

Question 7

REVIEW HAEMOGLOBIN LECTURE

Answer 7

MCBG

Question 8

Why is iron an essential element in all living cells?

Answer 8

1. transports and stores oxygen
2. integral part of many enzymes inc. cytochromes (ETC and energy production), neurotransmitter production, collagen formation and immune system function

Question 9

Why is free iron very toxic to cells?

Answer 9

acts as catalyst in formation of free radicals from reactive oxygen species

Question 10

What are the different sources of available and stored iron in the body?

Answer 10

Available/functional iron:

• mainly haemoglobin (2000mg)
• myoglobin: O2 reserve in muscles
• tissue iron: enzyme systems (e.g. cytochromes)
• transported (‘serum’) iron (3mg)

Stored iron (1000mg):

• ferritin: soluble
• haemosiderin: insoluble macrophage iron

Question 11

What are the pathways for iron intake and loss from the body?

Answer 11

~1-2mg/day iron lost from body from skin and GI mucosa.

Well-balanced diet contains sufficient iron to balance this loss (~10% of the 10-20 mg dietary iron in a balanced diet is absorbed each day) from haem (animal sources) and non-haem sources.

Question 12

Are iron requirements different in different individuals?

Answer 12

Yes, greater requirement during childhood/adolescence, pregnancy, menstruation, blood loss (e.g. from cancer)…

Question 13

Which type of iron (haem/non-haem) is more readily absorbed?

Answer 13

Haem iron more readily absorbed than inorganic iron which consists of both ferric (Fe3+) and ferrous (Fe2+) iron. Ferric iron most be reduced to ferrous form before it is absorbed.

Question 14

Where does most of our active iron come from?

Answer 14

80% of active iron from breakdown and recycling of RBCs, not from gut absorption.

Via reticuloendothelial system: old RBCs phagocytosed mainly splenic macrophages and Kupfer cells of liver.

Question 15

How is iron stored in liver tissue?

Answer 15

• 95% in hepatocytes as ferritin

- 5% as haemosiderin, predominantly in Kupffer cells

Question 16

Where does dietary iron absorption occur?

Answer 16

in duodenum and upper jejenum

Question 17

Describe the process of dietary iron absorption.

Answer 17

1. Reduction of Fe3+ from non-haem iron to Fe2+ in intestinal lumen by stomach acid (duodenal cytochrome B reductase - DcytB).
2. Uptake of non-haem Fe2+ by DMT1 transporter protein on apical surface of enterocytes (uptake of haem iron remains unclear).
3. In enterocyte, degradation of haem to release ferric iron (Fe3+).
4. Fe3+ either stored as ferritin or reduced to Fe2+ and transferred to bloodstream via ferroportin protein.
5. In blood, Fe2+ bound by transferrin and mostly transported to BM for erythropoiesis or taken up by macrophages of RES as storage pool.

Question 18

How is iron in the bloodstream taken up by body cells?

Answer 18

Binding of iron-transferrin complex to transferrin receptor (TfR).
RBCs contain highest no. of TfR - for iron incorporation into Hb.

Question 19

How is the process of iron absorption regulated?

Answer 19

Depends on dietary factors, body iron stores and erythropoiesis. Dietary iron levels sensed by enterocyte villi.

1. Citrate in citrus fruit can form complexes with iron that increase absorption. Tannins (e.g. tea, coffee) can decrease absorption by causing precipitation/chelation.
2. Hepicidin (small peptide expressed by liver):
   - directly binds to ferroportin resulting in its degradation - prevents iron from leaving enterocyte or enrering macrophages
   - inhibits transcription of DMT1 gene - downregulates iron uptake
3. expression of receptors, e.g. HFE, and transferrin receptor - no cellular uptake

Question 20

When is the production of hepicidin increased or decreased? What regulates this peptide?

Answer 20

Synthesised by liver (and excreted by kidney):

• increased in iron overload
• decreased by high erythropoietic activity

Regulated by: HFE, transferrin receptor and inflammatory cytokines

Question 21

What is the most common nutritional deficiency worldwide?

Answer 21

Iron deficiency (causes at least 1/2 of anaemia)

Question 22

Why might iron deficiency occur?

Answer 22

1. insufficient intake/poor absorption (e.g. vegan diet, gut problems and diarrhoea)
2. increased use
   - physiological, e.g. pregnancy, menstruation
   - pathological, e.g. bleeding

Question 23

What are the symptoms and signs of anaemia?

Answer 23

Symptoms:

1. tiredness
2. reduced oxygen carrying capacity (pallor, reduced exercise tolerance)
3. cardiac symptoms - angina, palpitations, dev of heart failure

Signs:

a) pallor
b) tachycardia
c) increased resp rate
d) epithelial changes

Question 24

Which blood parameters and blood film features indicate iron deficiency?

Answer 24

1. hypochromic: low Hb content
2. microcytic: small RBCs, low mean cell volume (MCV)
3. anisopoikilocytosis: change in cell size and shape (pencil cells, target cells)
4. low serum ferritin, serum iron and % transferrin saturation, raised total iron binding capacity (TIBC)
5. low reticulocyte haemoglobin content (CHR)

Question 25

Which tests are used to confirm iron deficiency?

Answer 25

1. Ferritin levels
   - but ferritin = acute phase protein so is increased with acute/chronic inflammation, malignancy, liver disease and alcoholism
   - reduced levels definitely indicate iron deficiency
   - but normal or increased levels do not exclude iron deficiency
2. Reticulocyte Hb content (CHR)
   - recommended by NICE as test for functional iron deficiency (amount of iron getting to RBCs)
   - remains low during inflammatory responses etc
   - but CHR also low in Ps with thalasemia so can’t be used in this setting

Question 26

Which investigations can be used to determine cause of iron deficiency?

Answer 26

1. dietary history (vegetarian) - inadequate intake
2. GI investigations (endoscopy) - increased loss
3. pregnancy test - excessive use

Question 27

What are the treatment options for iron deficiency?

Answer 27

1- dietary advice
2- oral iron supplements (side effects: constipation or diarrhoea) - take on empty stomach with OJ
3- intramuscular iron injections
4- intravenous iron (associated with anaphylactic shock)
5- transfusion - not unless there is severe anaemia with imminent cardiac compromise

Question 28

Why is iron excess dangerous?

Answer 28

There is no system for excretion of excess iron.

1. If exceeds binding capacity of transferrin, iron becomes ‘free’ - Fe2+ can produce highly reactive hydroxyl and lipid radicals… damages lipid membranes, nucleic acids and proteins.
2. Excess iron deposited in tissues in insoluble haemosiderin form - haemochromatosis

Question 29

What is haemochromatosis?

Answer 29

Disorder of iron excess resulting in end organ damage due to iron deposition. Causes:

i) liver cirrhosis
ii) diabetes mellitus (deposition in pancreas)
iii) hypogonadism (deposition in sex organs)
iv) cardimyopathy
v) arthropathy (deposition in joints)
vi) skin pigmentation

Question 30

What are the 2 different types of haemochromatosis?

Answer 30

hereditary haemochromatosis

transfusion-associated haemosiderosis

Question 31

What causes hereditary haemochromatosis?

Answer 31

• Autosomal recessive disease characterised by excess absorption of dietary iron.
• Caused by mutation in HFE gene on chromo. 6.
• HFE protein usually competes with transferrin for binding to transferrin receptor, so HFE mutation results in greater cellular uptake of iron.

Question 32

What is the treatment for hereditary haemochromatosis?

Answer 32

therapeutic phlebotomy to remove excess iron