Erythrocytes

Question 1

Learning objectives

Answer 1

• list the components of blood
• describe the cellular constituents of blood
• describe the structure and function of erythrocytes
• describe the process of erythropoiesis and the role of iron, B12 and folic acid
• describe the control of erythropoiesis via arterial blood oxygen content and the formation of erythropoietin
• describe the structure and function of haemoglobin
• know that anaemia is common in clinical practice
• know the basic types of anaemia: blood loss, increased destruction (haemolysis) and impaired production
• understand the role of iron deficiency as a cause of anaemia
• briefly explain the breakdown of haemoglobin and the fate of its various products

Question 2

What is the composition of blood?

Answer 2

Plasma 55%- 7% p. proteins, 1% organic solutes, 92% water : transports heat, in/organic molecules and formed elements

Buffy coat= less than 1%: Leukocytes and platelets

RBCs- 45%

Question 3

Outline what is meant by complete blood count

Answer 3

Complete blood count (CBC) or full blood count (FBC) –
one of the most frequently performed blood tests
Calculates the cellular or formed elements of blood
99% of the formed elements are RBCs
Red cell count: males 4.3–5.9 x 10 tpo 12/L
females 3.5–5.0 x 10 tpo 12/L

Question 4

What is the haematocrit?

Answer 4

% of volume of the packed red cells to the total blood volume is referred to as the haematocrit or packed cell volume (PCV)
Average values – Males: 40 – 52% or 0.40 – 0.52; Females: 36 – 47% or 0.36 – 0.47

Question 5

Characteristics of RBCs

Answer 5

• Biconcave discs
• 7-8 μm diameter
• Develop in bone marrow
• No major organelles
• No DNA
• Produce ATP through glycolysis- no mitochondria
• ~ 120 day lifespan
• Broken down by reticuloendothelial system

Question 6

What are the functions of RBCs?

Answer 6

• Have haem group (poryphryin) with ferrous iron in centre associated with peptide chains that forms a complex with oxygen for transportation around body
• Allosteric protein- once one o2 molecule binds, affinity increases
• CO2 transportation
• Control of acid/base equilibria in body
  CO2 + H2O h2co3 (carbonic acid) HCO3- (bicarbonate) +H+
  Influenced by carbonic anhydrase in RBC
  -Accounts for about 70% of the total CO2
  transported from tissues to lungs

Question 7

Where does erythrocyte production occur?

Answer 7

• In fetus –
liver & bone marrow
• After birth –
bone marrow
• In adult –
marrow of membranous bones
e.g. vertebrae, ribs & pelvis

Vitamin b12 and folate (division of RBC- DNA synthesis and multiplication) and iron (haemoglobin) are important for erythropoiesis

• produced from myeloid stem cell from pluripotent stem cells, unlike lymphoid stem cell

Question 8

Regulation of erythrocyte production

Answer 8

Stimulus- hypoxia ( lung infection, poor circulation, anaemia, altitude sickness)
Detector- kidney cells- increase EPO secretion into blood, binds to EPO receptors in BM to bring about EP
Proerythroblasts in red bone marrow mature more quickly into reticulocytes
More enter circulation, larger concentration and increased oxygen delivery to tissues
Normal oxygen levels detected by kidneys, inhibit secretion of EPO

Question 9

Give examples of why there are sex differences in blood haemoglobin levels in health

Answer 9

-A loss of red cells at menses in
females
- An inhibitory effect of oestrogen
on the bone marrow
- Direct stimulatory effect of
testosterone on bone marrow
- Stimulatory effect of testosterone
on EPO production

Question 10

What is anaemia?

Answer 10

A deficiency of haemoglobin in the blood, reducing the oxygen carrying capacity of blood.

• symptoms- weakness, short of breath, tiredness, dizziness, headaches etc
• signs- pallor of skin, nails, eyes- poor condition of lips, nails, tongue- hair loss, tachycardia, heart murmurs and enlargement- possible failure

Anaemias may be caused by:

1. Abnormalities of erythropoiesis
2. Deficiencies of erythrocytes (haemolysis, blood loss)

Question 11

Red blood cell indices

Answer 11

MCV (mean corpuscular volume) fL (80–100)
MCH (mean corpuscular haemoglobin) pg (28–32)
MCHC (mean corpuscular haemoglobin concentration) g/L (320–350)

MCV = Hct/RBC count

MCHC = Hb/Hct

MCH = Hb/RBC count

Macrocytic- large rbcs
Normocytic- normal
Microcytic- small

Hyperchromic- high Hb
Normochromic- Normal Hb
Normochromic- Low Hb

Question 12

Classification of anaemia

Answer 12

Through morphology, etiology or Hb content

Hyperchromic, macrocytic- megaloblastic anaemia- Vit B12 or folate deficiency
Normochromic, normocytic- Haemolysis- acute blood loss
Hypochromic, microcytic- Iron deficiency

Etiology

1. Impaired erythropoiesis
2. RBC depletion/haemolysis
3. Loss of RBCs- acute/chronic blood loss
4. Impaired RBC destruction-hypersplenism

Question 13

Red cell breakdown

Answer 13

Hb broken down and haem + globin recycled
Porphyrin ring forms bilirubin, a component of bile (fat absorption in intenstine)
Accumulation of bilirubin leads to jaundice (yellowing of skin/sclerae)

One of the causes of jaundice is increased RBC breakdown.