Blood

Plasma

• The fluid component of whole blood
• Yellow liquid component of blood in which the blood cells in whole blood are normally suspended
• Makes up approximately 55% of blood volume
• Contains:
• Water 
• Salts
• Positive ions (cations)
• Sodium
• Also potassium, calcium, magnesium, and hydrogen ions
• Negative ions (anions)
• Chloride
• Also bicarbonate, phosphate, sulphate and organic anions
• Proteins 
• Normally 7-9% of plasma is protein
• There are thousands of different proteins, but a few major proteins predominate
• Albumin
• A single protein made in the liver
• >90%
• Maintaining colloid osmotic pressure
• A form of osmotic pressure exerted by proteins in the blood plasma that usually tends to pull water into the circulatory system
• Transport for substances with low water solubility (keeps blood in vessels)
• Alpha and beta globulins
• A variety of proteins made in the liver
• 1-4%
• Transport of antibodies for defence
• Gamma globulins
• Produce by B-lymphocytes
• <0.5%
• Immunoglobulins eg IgA and IgG
• Antibodies which have a role in the body’s defence against infection
• Fibrinogen
• 2-5%
• Blood clotting
• Organic molecules (eg metabolites (substance involved in a metabolic reaction), carbohydrates and lipids)

Serum

Serum is the fluid left after blood clotting, so with the fibrinogen removed. Some tests require unclotted blood (use coagulant), other tests work better with serum rather than plasma. Serum can be seperated by electrophoresis into the four major protein classes listed under plasma.

Basic Blood Tests

Blood Count

A complete blood count (CBC) is a valuable screening test that is used to diagnose and manage many conditions and diseases such as acute and chronic infections, allergies and anaemia. It measures the number of erythrocytes (RBCs) and leukocytes, total amount of blood haemoglobin and fraction of blood composed of RBCs (the haematocrit). The CBC also includes information such as  MCH (mean corpuscular haemoglobin) and MCHC (mean corpuscular haemoglobin concentration) as well as the platelet count.

Haematocrit

Haematocrit is established with centrifuged blood and is the packed cell volume (PCV).

Haematocrit (Ht)
            =
Volume of Cells  
Total Volume

Normal value = 0.4 – 0.5

Smokers have a higher value because they are producing more red blood cells to make up for the carbon monoxide poisoning. Anaemics have a lower value.

Blood Volume

Normally, the total blood volume is approximately 5 - 6 litres for a 70kg male where the plasma volume is approximately 2.5 - 2.75 litres. So, in this example Ht = 2.5/5 which is 0.5. For a female is is approximately 4-5 litres.

Plasma is composed of 92% water that transports organic and inorganic molecules, cells, platelets and heat, 7% plasma proteins and 1% other solutes. The buffy coat is a mixture of platelets and white blood cells (neutrophils (60-70%), monocytes (3-10%), eosinophils (1-4%) lymphocytes (29-40%) and basophils (0-1%)).

Other common blood tests include haemoglobin level (Hb), mean red blood cell volume (MCV), white blood cell count (WBC), liver function tests (LFTs), blood glucose and urea and electrolytes (U&E).

Blood Components

Blood is a specialised type of connective tissue that consists of cells suspended in a circulating fluid known as plasma. Its main functions are transport, defence and homeostasis. Cellular elements of blood constitute about 45% of blood volume, which plasma making up the other 55%. These elements include erythrocytes (RBCs), granular and agranular leukocytes (WBCs) and circulating cytoplasmic fragments known as platelets (or thrombocytes).

Erythrocytes

• Red blood cells
• 7-10 micrometres in diameter
• Life span of 120 days
• 5x10¹² cells/litre of blood in males, 4.5x10¹² in females
• Biconcave disc
• Pink
• Transports haemoglobin that binds to O₂ and CO₂

Platelets

• Thrombocytes
• 2-4 micrometres in diameter
• Life span of 10 days
• Oval, biconvex disc
• Pale blue
• Promote blood clotting in haemostasis and plugs endothelial damage

Leukocytes (white blood cells)

Granulocytes

Neutrophil

9-12 micrometres in diameter

Segmented with 3-5 lobes

Polymorphonuclear (many shaped nucleus)

Pale and finely granular

Phagocytoses bacteria ad increases in number in acute bacterial infections

Eosinophil

12-15 micrometres in diameter

Granules stain red with eosin

Bilobed with a clumped chromatin pattern

Large homogenous red granules that are coarse and highly refractile

Phagocytoses anti-gen antibody complexes and parasites

Basophil

10-14 micrometres in diameter

Granules stain blue with basic dyes

Bilobed or segmented

Large blue specific granules

Involved in anticoagulation and increases vascular permeability

Agranulocytes

Monocytes

12-20 micrometres in diameter

Indented – kidney shaped

Pale blue cytoplasm with lysosomes

Is motile and gives rise to macrophages

Lymphocytes

Can be small (6-10 mcirometres) or large (11-16 micrometres)

Round or slightly indented

Faintly basophilic and blue to grey

Acts in humoral (B cell) and cellular (T cell) immunity

Haemoglobin

Haemoglobin is a tetramer (made up of 4 bits) of 4 polypeptide chains:

2 alpha globin chains

2 beta globin chains

Each globin chain carries a haem molecule as a prosthetic group (something added to an enzyme that is securely attached). The haem holds a ferrous iron atom. Oxygen binds reversibly to the iron atom by a coordination bond to form oxyhaemoglobin that is bright red when fully saturated with oxygen. When it has lost all of its oxygen to form deoxyhaemoglobin it is blue purple in colour. This is the basis of pulse oximetry, which measures the colour of the haemoglobin and determines if the patient is hypoxic.

Origins of Blood Cells

Mature blood cells (and platelets) are produced from stem cells in the bone marrow which contains many immature cells. Some blood diseases can be treated by bone marrow transplantation.

Two types of bone marrow occur in adults: red and yellow. Red marrow, the actively haematopoietic tissue, is abundant in prenatal life and in the young. It is red in colour due to erythrocytes and its precursors. Until age 20-25 years, progressive fatty replacement of the marrow leads to yellow marrow, which is relatively inactive and is mainly composed of adipocytes (fat cells). Newly formed blood cells migrate across sinusoid walls to enter the bloodstream and then large veins.