Blood Composition and Functions

Blood is a living connective tissue
Contains 45% cellular elements and 55% intercellular fluid.
Considered 'fluid of life' as it carries oxygen and carbon dioxide
Color: Arterial blood is scarlet red due to higher oxygen content, Venous blood is purple red due to higher carbon dioxide content
Volume: Average 5 liters in adults, 450 ml in newborns, increases during growth and reaches 5 liters at puberty. Females typically have 4.5 liters.
Reaction and pH: Slightly alkaline with a pH of 7.4
Viscosity: Five times more viscous than water, primarily due to red blood cells and plasma proteins

Nutrient function: Transports nutrients like glucose, amino acids, lipids, and vitamins from the gastrointestinal tract to other parts of the body.
Respiratory function: Carries oxygen from the lungs to tissues and carbon dioxide from tissues to the lungs.
Defense Mechanism: Gamma globulins (immunoglobulins) act as antibodies, providing defense against pathogens.
Osmotic pressure: Maintained by albumin due to its low molecular weight, important for fluid exchange between capillaries and interstitial fluids.
Transport Mechanism: Albumin, alpha globulin, and beta globulin transport hormones, enzymes, and metals.
Reserve proteins: Plasma proteins can be utilised by tissues as a source of energy during inadequate food or protein intake.
Regulation of Acid-base balance: Plasma proteins, especially albumin, buffer the blood, helping to regulate pH.
Viscosity: Plasma proteins, particularly fibrinogen, contribute to blood viscosity, which is essential for maintaining blood pressure.

Normal values: 5 million RBCs per microliter of blood. Male: 5.4 million/L, Female: 4.8 million/L.
Higher in infants, lower in children and elderly
Increased in athletes and at high altitudes
Hemoglobin (Hb): 14-16 g/dl in the blood (male: 16±2 gram/dl, female: 14±2 gram/dl)

RBC membrane functions:
- Allows RBCs to pass through narrow capillaries due to plasticity.
- Biconcave shape provides increased surface area for gas exchange.
- Maintains carbonic anhydrase enzyme inside the RBCs, essential for carbon dioxide transport.
- Semipermeable to H+ , Cl- , and HCO3- , important for maintaining blood pH during carbon dioxide carriage.
- Keeps hemoglobin inside the RBCs, preventing precipitation in kidney tubules, increased osmotic pressure, engulfment by macrophages, and filtration into urine.
RBC content functions:
- Hemoglobin: Carries oxygen from lungs to tissues, carries carbon dioxide from tissues, and acts as a blood buffer.
- Carbonic anhydrase: Essential in carbon dioxide carriage.
- Histamine inactivation: Inactivates histamine by histamines enzyme.

In-utero:
- Early weeks: Nucleated RBCs formed in the yolk sac.
- Middle trimester: Mainly in the liver, spleen, and lymph nodes.
- Last months: RBCs formed in the bone marrow of all bones.
After birth:
- Bone marrow of both flat bones and long bones continue to produce RBCs.
- At puberty: Shaft of long bones stop producing RBCs while epiphysis and flat bones continue.
Life span: 120 days ± 7 days
Destroyed by macrophages in the spleen, liver, and bone marrow.

Oxygen Tension:
- Lower oxygen tension (hypoxia) stimulates bone marrow activity to produce more RBCs.
- Hypoxia does not act directly on bone marrow but through the release of erythropoietin.
- Causes of hypoxia: High altitude, lung disease.
Kidney:
- Juxtaglomerular cells of the kidney respond to hypoxia by forming ‘Renal Erythropoietic Factor’ (REF).
Erythropoietin:
- Glycoprotein, 90% from renal cortex, 10% liver.
- Stimulates growth of early stem cells, does not affect maturation.
- High levels in conditions like high altitude, heart failure, and lung disease, leading to polycythemia.
Food factors:
- Proteins: High biological value proteins are more effective in erythropoiesis.
- Iron: Forms part of hemoglobin. Deficiency leads to microcytic hypochromic anemia.
- Copper and Cobalt: Act as catalysts for hemoglobin synthesis but are not part of the molecule.
- Vitamins: Folic acid and vitamin B12 are crucial for RBC maturation.
Hormones:
- Erythropoietin, thyroxine, and testosterone.
Liver:
- Important for erythropoiesis, helps manufacture globin of hemoglobin and stores iron, copper, and vitamin B12.
Bone marrow:
- The only site in adults for normal erythropoiesis.

Iron from food is absorbed in the gastrointestinal lumen.
Iron is absorbed by enterocytes and transported across the membrane into capillaries.

Food-derived vitamin B12 (Extrinsic factor) is bound to proteins.
In the stomach, gastric mucosa secretes 'Intrinsic factor' to aid in the absorption of vitamin B12 from the intestine (lower ileum).
Deficiency of B12 leads to ‘Pernicious anaemia’.

Characterized by reduced RBC count and/or Hb content
Aetiological Classification:
- Excessive RBC destruction:
  - Chemical factors (e.g., lead poisoning),
  - Diseases (e.g., malaria),
  - Increased RBC fragility (e.g., spherocytosis, thalassemia, sickle cell anemia).
- Excessive RBC loss:
  - Acute hemorrhage,
  - Chronic hemorrhage (e.g., piles).
- Defective RBC formation:
  - Nutritional deficiencies (e.g., iron, copper, protein),
  - Pernicious anemia (e.g., vitamin B12 deficiency),
  - Aplastic anemia (e.g., bone marrow destruction).
Morphological Classification:
- Microcytic (<80 fL): Small RBCs.
- Normocytic (80-100 fL): Normal size RBCs.
- Macrocytic (>100 fL): Large RBCs.

Caused by decreased dietary intake, malabsorption, increased demands (e.g., pregnancy), and prolonged blood loss (e.g., piles).
Leads to the formation of microcytic hypochromic RBCs.
Treatment: Ferrous salts by mouth, injection in severe cases or poor absorption.

Lack of intrinsic factor secretion leads to poor absorption of vitamin B12 from the intestine.
Characterized by nucleated, macrocytic RBCs with higher hemoglobin content, spinal cord and peripheral nerve lesions, and gastric mucosal atrophy.
Treatment: Vitamin B12 injections.