Composition and Functions of Blood and Plasma Proteins

Introduction

Blood is a dynamic connective tissue that sustains life by transporting respiratory gases, nutrients, hormones, and metabolic waste while maintaining homeostasis. Its cellular elements, plasma, and plasma proteins work together to support immunity, coagulation, fluid balance, and normal physiological function throughout the body.

Hematological Investigations

• Hematological investigations are routinely performed because many diseases produce early changes in blood composition.
• Blood findings provide reliable indicators of disease severity and progression.
• Blood is easily accessible, making it a convenient sample for laboratory testing.
• Many tests can be performed rapidly in clinical settings, aiding diagnosis and monitoring.
• Knowledge of blood physiology is essential for understanding normal and abnormal findings.
• Hematology involves the study of blood components, their formation, and their physiological functions. It also includes analysis of blood disorders and changes occurring in systemic diseases.
• Blood examination helps determine whether bone marrow produces adequate numbers of mature cells. It also evaluates whether cell development within each hematopoietic lineage is normal.
• Peripheral blood tests, including cell counts and smear examination, provide initial diagnostic information. However, bone marrow examination offers more detailed insight into underlying disorders.
• Combined assessment improves accuracy in diagnosis and guides appropriate clinical management.

Blood

• Blood is a specialized liquid connective tissue circulating within the heart and blood vessels. It transports gases, nutrients, hormones, and waste products.
• Normal adult blood volume is approximately five to six liters, about eight percent of body weight.

Composition of Blood

• Blood consists of formed elements and a fluid component called plasma.
• Formed elements include red blood cells, white blood cells, and platelets.
• Red blood cells transport oxygen and carbon dioxide within the body.
• White blood cells participate in immune defense mechanisms.
• Platelets play a key role in blood clotting and hemostasis.
• Plasma constitutes about fifty-five percent of total blood volume. It contains water, proteins, electrolytes, nutrients, hormones, and waste products. On centrifugation, blood separates into plasma, a thin buffy coat, and red cell mass. The buffy coat contains white blood cells and platelets.

Buffy Coat

• The buffy coat is a thin intermediate layer formed between plasma and red cell mass after centrifugation of blood. It contains white blood cells and platelets, which are essential for immunity and hemostasis. This layer is useful for microscopic examination of cellular elements.
• In disease conditions, abnormal or immature cells may appear in the buffy coat. These include atypical mononuclear cells and precursor forms such as blast cells.
• Detection of such cells aids in diagnosis of hematological disorders and systemic diseases.

Functions of Blood

• Blood performs essential respiratory transport by carrying oxygen from lungs to tissues and carbon dioxide back to lungs. It serves as a major transport medium for nutrients, hormones, vitamins, and metabolic intermediates.
• Circulating substances regulate cellular growth, metabolism, and organ function.
• Blood contributes to temperature regulation by distributing heat from internal organs to the body surface.
• Vasodilation promotes heat loss, whereas vasoconstriction conserves body heat. It supports excretory function by transporting metabolic wastes such as urea and creatinine to excretory organs.
• Blood maintains water balance by preserving plasma volume and supporting adequate tissue hydration.
• Loss of fluid reduces blood volume and may impair cellular function if not corrected.
• Blood plays a vital role in acid–base balance through buffer systems involving hemoglobin, plasma proteins, and bicarbonate. These buffers resist rapid changes in hydrogen ion concentration. It is central to immunity, containing leukocytes for cellular defense and antibodies for humoral responses.
• Blood also carries factors involved in nonspecific immune protection. It acts as a reservoir for electrolytes, nutrients, hormones, and signaling molecules.
• Blood provides nutritive support by delivering glucose, amino acids, lipids, vitamins, and minerals to tissues. It contributes to body coloration, with hemoglobin imparting the characteristic red color.
• Reduced hemoglobin levels may lead to pallor.
• Plasma proteins, especially albumin, generate oncotic pressure that regulates fluid exchange across capillaries. This pressure prevents excessive fluid leakage into interstitial spaces and helps avoid edema.

Blood Volume

• Blood volume is approximately seventy to eighty milliliters per kilogram of body weight in adults.
• Adult males typically have about five to six liters of blood.
• Adult females usually have about 4.5 to 5.5 liters of blood.
• In children, blood volume per kilogram is higher, around eighty to ninety milliliters.
• Absolute blood volume in children is lower due to smaller body size.
• Blood volume can also be expressed per body surface area, averaging about 2.8 liters per square meter.

Determination of Blood Volume

• Blood volume is estimated by measuring plasma volume and red cell volume separately.
• The normal ratio of plasma to cells is approximately fifty-five to forty-five.

Composition of plasma.

• Plasma consists of about ninety-two percent water and eight percent solutes.
• Most solids are plasma proteins, forming the major colloidal fraction.
• Inorganic components include cations such as sodium and potassium, and anions such as chloride and bicarbonate.
• Organic substances include proteins and crystalloids like glucose, lipids, urea, and uric acid.

Measurement of Cell Volume

• Cell volume is estimated by measuring the volume of red blood cells in circulation.
• The contribution of leukocytes and platelets is negligible and usually ignored.
• Red cell volume is determined using radioisotope labeling techniques.

Red cell volume

• Red cell volume represents the total volume occupied by circulating red blood cells. It can be estimated by subtracting plasma volume from total blood volume.
• Direct measurement is performed using labeled red cells introduced into circulation.
• Chromium-51 is commonly used to label red cells for accurate estimation.
• Other isotopes such as iron and phosphorus may also be used.
• Proper mixing of labeled cells ensures reliable calculation of red cell volume.

Determination of Plasma Volume

• Plasma volume is measured using dye dilution methods such as Evans blue, which binds to plasma proteins. It can also be estimated using radioiodinated serum albumin for accurate measurement.
• Average plasma volume in adults is about 3.5 liters, approximately five percent of body weight.
• Total blood volume is calculated from plasma volume and hematocrit value.
• For example, with hematocrit forty percent, blood volume is approximately 5.8 liters.

Effective Blood Volume

• Effective blood volume refers to the portion of blood actively circulating and contributing to tissue perfusion. It equals total blood volume minus blood that is sequestered in organs such as the liver and spleen. Estimation is difficult because the sequestered fraction cannot be measured precisely.

Specific Gravity and Viscosity of Blood

• Specific gravity of whole blood is approximately 1.050, reflecting its density relative to water. It depends on the number of blood cells and the composition of plasma proteins. It can be measured using a refractometer or suitable laboratory methods.
• Dehydration increases specific gravity due to reduced plasma water content.
• Viscosity is the resistance offered by blood to flow within vessels. Blood viscosity is about four to five times that of water. It is mainly influenced by red cell concentration and plasma proteins. Increased viscosity can affect circulation and increase cardiac workload.

Plasma

• Plasma is the fluid component of blood, forming about fifty-five percent of total blood volume.
• The remaining portion consists of formed elements, including cells.
• Blood must be collected with anticoagulants to obtain plasma for analysis.
• Plasma can be stored for long periods under appropriate conditions for future investigations.

Composition of Plasma

• Plasma is composed mainly of water and dissolved substances.
• Water forms about ninety-two percent of plasma volume.
• Plasma proteins constitute the major portion of solid components, around seven percent.
• Remaining solutes, about one percent, include electrolytes, nutrients, hormones, and metabolic waste products.

Serum

• Serum is the fluid obtained after blood coagulates and clotting factors are removed. It lacks fibrinogen because it is consumed during clot formation.
• Serum is widely used for biochemical investigations after separation from clotted blood. It contains higher serotonin levels due to release from platelets during clotting.
• Serum provides nutrients, proteins, and growth factors required for cell survival. It is commonly used in cell culture to support growth and maintenance of cells.
• Specialized forms such as fetal serum are used for optimal cell growth in laboratory conditions.

Plasma Proteins

• Plasma proteins are the major solute components of plasma, with normal concentration about six to eight grams per hundred milliliters. They are mainly synthesized in the liver.

Types of Plasma Proteins

• The three main types are albumin, globulins, and fibrinogen. Albumin is the most abundant, followed by globulins and fibrinogen.

Albumin

• Albumin is the most abundant plasma protein and is synthesized in the liver. It has a molecular weight of about sixty-six thousand and a half-life of nearly twenty days. It maintains oncotic pressure, preventing fluid leakage from capillaries.
• Due to its small size, it appears early in urine during glomerular damage.
• Loss or reduced synthesis causes hypoalbuminemia, leading to edema.
• Liver disease reduces albumin production, further lowering plasma protein levels.

Globulin

• Globulins are synthesized in the liver, reticuloendothelial cells, and plasma cells. They are classified into alpha, beta, and gamma fractions.
• Globulins function as transport proteins, carrying metals and lipids in circulation. They form lipoprotein complexes important for lipid transport.
• Gamma globulins act as immunoglobulins, providing immune defense.                        
• The albumin to globulin ratio is normally between 1.5 and 2.5 to one.
• Alteration of this ratio helps in diagnosis and assessment of disease progression.

Fibrinogen

• Fibrinogen is a high molecular weight plasma protein synthesized in the liver. It plays a key role in blood coagulation by forming fibrin. It also influences plasma viscosity and affects erythrocyte sedimentation rate.

Functions of Plasma Proteins

1. Plasma proteins generate oncotic pressure, approximately twenty-five millimeters of mercury, which maintains intravascular fluid volume. This pressure opposes filtration and prevents excess fluid movement into interstitial spaces.
2. Reduction in plasma proteins, especially albumin, leads to edema formation.
3. Plasma proteins contribute significantly to blood viscosity, influencing resistance to flow.
4. Fibrinogen, due to its elongated structure, increases viscosity more than albumin.
5. Immunoglobulins provide humoral immunity by neutralizing pathogens and toxins.
6. Plasma proteins play a crucial role in coagulation, with fibrinogen forming fibrin during clot formation. Other clotting factors, including prothrombin, are also plasma proteins. They act as transport proteins, carrying hormones, lipids, metals, and drugs in circulation.
7. Binding to proteins improves solubility and regulates availability of transported substances.
8. Plasma proteins function as buffers, helping maintain acid–base balance by binding hydrogen ions.
9. They serve as a protein reserve, supplying amino acids during nutritional deficiency or stress. These proteins also support synthesis of enzymes and certain regulatory molecules.
10. Plasma proteins influence erythrocyte sedimentation rate by promoting rouleaux formation.
11. Increased fibrinogen levels accelerate sedimentation, especially during inflammatory conditions.
12. Overall, plasma proteins are essential for maintaining fluid balance, defense mechanisms, and metabolic regulation.

Separation of Plasma Proteins

• Plasma proteins can be separated using techniques such as electrophoresis, salt fractionation, and ultracentrifugation. These methods identify different protein fractions, including albumin and globulins.
• Measurement of fractions helps determine the albumin–globulin ratio. This analysis aids diagnosis and prognosis of various diseases.