Blood: Physical Properties and Functions

• Blood is a fluid that circulates to keep the body's environment constant for its cells.

Physical Properties of Blood

• Arterial blood is bright red because of oxyhemoglobin.
• Venous blood is dark red (bluish) because of reduced hemoglobin.
• Specific gravity of whole blood is 1060.
• Specific gravity of RBCs is 1090.
• Specific gravity of plasma is 1030.
• Total blood volume: 5L, which is 8% of the body weight.
• Blood pH of arterial blood is 7.4±0.02.
• Blood pH of venous blood is 7.38±0.02.
• Blood viscosity is five times greater than water, due to cells (mainly RBCs) and plasma proteins such as fibrinogen.
• Blood viscosity declines with anemia and increases with polycythemia.
• Blood viscosity is vital for keeping normal arterial blood pressure by stopping rapid blood flow from arteries to veins.

Functions of Blood

• Blood maintains "homeostasis," keeping the body's environment steady.
• Blood transports O2 & CO2 as part of the respiratory function.
• Blood transports nutrients for nutritive functions.
• Blood carries waste products as part of excretory functions.
• Blood transports hormones.
• White blood cells (WBCs) & antibodies enable defensive functions.
• Blood contributes to hemostasis via clotting.
• Blood regulates body temperature, acid-base balance, and H2O balance.

Blood Composition

• Blood consists of constituents of blood and suspended elements.
• Plasma makes up 55% of blood volume.
• Suspended elements make up 45% of blood volume.
• Plasma's volume is 3L.
• Suspended elements volume is 2L.
• Erythrocytes (RBCs) are part of the suspended elements.
• Leucocytes (WBCs) are part of the suspended elements.
• Thrombocytes (platelets) are part of the suspended elements.

Plasma

• Plasma is the fluid where blood cells are suspended.
• Plasma volume is 3L.
• It has a yellow color.
• Plasma viscosity is two times greater than water.
• Specific gravity is 1030.
• The osmotic pressure is 290-300 milliosmol/Kgm H2O.
• Plasma is 90% water.
• 9% of plasma is organic constituents
• Plasma contains 7% plasma proteins.
• Plasma contains inorganic constituents (1%) such as Na, Cl, Ca & K.
• Plasma contains nutrients & waste products (2%).

Serum

• Serum is the fluid left after whole blood clots and the clot is removed.
• Serum composition includes all the same components as plasma, except that fibrinogen and clotting factors are missing.
• Formula: Serum = Plasma - Clotting factors

Plasma Proteins

• Total plasma proteins are 7 gm/dl (7-9).
• Albumin: 4.8g/100ml.
• Globulins: 2.7g/100ml.
• Fibrinogen: 0.2g/100ml.
• Prothrombin: 0.02g/100ml.
• All plasma proteins are created in the liver, except for gamma globulins which are produced by lymphoid tissues.

Functions of Plasma Proteins

• Plasma proteins help metabolize other proteins because they can be used by tissues.
• Fibrinogen, prothrombin, and other clotting factors assist in blood coagulation.
• Antibodies (gamma globulins = γ globulins) produced by plasma proteins protect from pathogenic organisms and their toxins.
• Plasma proteins transport hormones from endocrine glands to target organs.
• Plasma proteins transport iron, fatty acids, amino acids, enzymes, and drugs.
• Plasma proteins regulate blood volume by exerting a colloid osmotic pressure of about 25 mmHg, mainly by albumin.
• Plasma proteins regulate blood pressure by inducing blood viscosity, especially fibrinogen.
• Plasma proteins buffer blood and account for 15% of its buffering power.
• Plasma proteins buffer acids and alkalis to maintain stable blood pH.
• Arterial blood pH is 7.4±0.02, while venous blood pH is 7.38±0.02.
• Albumin transports many substances and greatly helps colloid osmotic pressure in plasma.
• Alpha and beta globulins are clotting factors as well as substance transporters.
• Gamma globulins are antibodies.
• Fibrinogen contributes to blood clotting and blood viscosity.
• Prothrombin causes blood clotting.

Suspended Elements

• Erythrocytes: They contain Hemoglobin, essential for transporting O2 and CO2 and make up over 99%
• Leukocytes: These are mobile defense units of the immune system.
• Platelets: Play an essential role in hemostasis.

Red Blood Cells

• Shape: Flat biconcave discs & Non nucleated.
• Count in females is 4.5 - 5 millions/mm3 blood.
• Count in males is 5 - 5.5 millions/mm3 blood.
• Anemia corresponds to a decreased count.
• Polycythemia corresponds to an increased count.
• Life span: 120 days.
• Diameter: 7.2-7.8 µm (Normocytic, microcytic , macrocytic ).
• Origin: Red bone marrow in adult.
• Catabolism: Tissue macrophage system e.g. spleen & liver.
• Responsible for transport of O2 & CO2 by hemoglobin (Respiratory function).
• They help with blood viscosity which is is important in regulation of arterial blood pressure (ABP).
• Hemoglobin (The most important constituent).
• Mature RBCs have no nucleus.
• Hematocrit is a percentage volume of RBCs in relation to the whole blood.

Hemoglobin

• Hb is a reddish O2- carrying pigment inside the RBCs.
• Globin: A protein part made up of 4 polypeptide chains (2 a and 2 ẞ=adult, Hb A) (2 a and 2 γ=fetus, Hb F).
• Haem: Is a non-protein part, containing 4 iron molecules (ferrous) each iron is bound to one of the polypeptides.
• Hb + O2 = Oxy Hb
• O2 is attached to the Fe+2 (ferrous iron) in the heme part and one Hb molecule can carry 4 O2 molecules.
• The combination between Hb and O2 is loose & reversible.
• Hemoglobin catabolism breaks down into Globin (Reused into amino acids) and Haem. Haem is broken down into Biliverdin, which can further break down into Bilirubin (Bile pigments excreted in bile) and Iron (Reused).

Erythropoiesis

• Erythropoiesis is the formation or regeneration of red blood cells.

Factors Affecting Erythropoiesis

• Tissue oxygenation.
• Nutritional factors: Proteins, Iron, Vitamins (Folic acid & vitamin B12), Trace elements: (Cupper and Cobalt).
• Hormones.
• Healthy bone marrow.
• Normal functioning liver.

Tissue Oxygenation

• Erythropoiesis is mainly controlled by a hormone called "Erythropoietin".
• Decreased O2 transport to the tissues (tissue hypoxia) leads to increase in RBCs formation.
• Lack of O2 (hypoxia) stimulates the release of "Erythropoietin" which stimulates RBCs production by the bone marrow until the hypoxia is relieved.
• When O2 transport to tissues rises above normal, the rate of erythropoietin release decreases (negative feedback mechanism).
• Erythropoietin is a hormone secreted from the kidney (90%) and the liver (10%).
• Tissue hypoxia may be due to decreased blood volume (hemorrhage), decreased blood flow, Anemias, Pulmonary or cardiac disease, and High altitude.
• If bilateral renal disease or nephrectomy, severe anemia occurs due to loss of erythropoietin.

Nutritional factors

• Amino acids are required for the globin part of Hb & cell membrane.
• Copper and Cobalt act as catalysts in Hb synthesis.
• Iron is required for the formation of Hb (heme fraction) 60%, Myoglobin, Cytochrome enzymes (for intracellular oxidation) and is a Storage form "ferritin" which is present in liver & other cells.
• Non - specific vitamins like vit C can affect general metabolism including bone marrow (BM).
• Specific vitamins like Vitamin B12 and folic acid are needed
• Vitamin B12 and folic acid are "maturation factors" required for formation of nuclear DNA.
• Deficiency of vitamin B12 and folic acid leads to anemia which is called "megaloblastic anemia".
• Deficiency of these vitamins leads to ↓DNA formation and failure of nuclear division with RBCs maturation and the RBCs become large, immature and highly fragile.
• Vitamin B12 needs intrinsic factor from stomach so that it can be protected from digestion and become absorbed into the ileum.
• Gastrectomy or gastric mucosal atrophy causes deficiency of Intrinsic factor causing destruction of destruction of vit B12 by GIT enzymes with failure of vit B 12 absorption leading to Pernicious anemia/vitamin B12 deficiency.
• It is rarely due to dietary intake

Hormones

• Hormones can be specific like Erythropoietin or eNon specific.
• Non specific hormones stimulate body metabolism and erythropoiesis. Examples include thyroid hormones (thyroxin), male sex hormones (testosterone) & growth hormone

Liver

• A Healthy liver is essential for normal erythropoiesis.
• The liver forms globin.
• The liver stores Fe, vit B12, folic acid, copper.
• The liver forms small fraction of erythropoietin (10%).

Bone Marrow

• Bone marrow is the factory for RBCs formation and other blood cells and should be functioning.
• If Bone marrow is destroyed and becomes non-functioning then “Aplastic anemia" is produced with "pancytopenia".

Polycythemia

• Polycythemia is an increase in the number of circulating RBCs with increased hematocrit and hemoglobin level above the normal value.
• Polycythemia can be Primary or Secondary.
• In Secondary Polycythemia occurs because of "tissue hypoxia" leading to increased RBCs and these high altitudes and/or Pulmonary or Cardiac diseases causing a down regulation of O2 delivered.
• Primary Polycythemia is a tumorous condition of the bone marrow with excess uncontrolled production of red blood cells
• Effects of Polycythemia are increased RBCs causing high Blood viscosity leading to high Blood pressure and increased work done by the heart.