Blood is defined as liquid connective tissue that fills the heart and blood vessels. The normal blood volume in an average adult is 5–6 liters, which accounts for about 8% of the body weight.
Composition of Blood
Blood consists of two components: cells and fluid. The cellular component comprises of different formed elements, and the fluid component is the plasma.
- The formed elements are red blood cells (erythrocytes), white blood cells (leucocytes), and platelets (thrombocytes).
- The plasma consists of about 55% of the total blood volume which is made up of water and solid particles. When blood is collected in an anticoagulated tube and allowed to settle by centrifugation, three layers are distinctly visible. The upper plasma layer is separated from lower red cell mass by a thin buffy coat.
The thin middle layer that separates the upper plasma and lower red cell mass after centrifugation of blood is the buffy coat.
- The buffy coat contains leucocytes and platelets.
- In diseases, abnormal cells are found in the buffy coat like LE cells in systemic lupus erythematosus and atypical or primitive blood cells (atypical mononuclear cells, promyelocytes, metamyelocytes, blast cells, megakaryocytes etc.) in malignant or premalignant conditions.
Functions of Blood
- Respiratory functions: Blood transports oxygen from lungs to tissues, and carbon dioxide from tissues to the lungs. Thus, by transporting gases blood serves an important function of respiration.
- Transport medium: Blood acts as the transport medium for various hormones, chemical substances, nutrients, vitamins, etc. Hormones secreted from various endocrine tissues, and nutrients absorbed from GI tract circulate in blood and distribute to all tissues of the body. Thus, blood regulates growth and metabolism of the tissues.
- Temperature regulation: Blood plays an important role in temperature regulation as it conducts heat from the interior of body to the surface through blood vessels. Temperature is continuously produced in the body. Blood absorbs this heat and transfers to the body surface. Therefore, vasoconstriction preserves body temperature and vasodilation facilitates heat dissipation from the body.
- Excretory function: Blood helps in excretion of waste materials by transporting them from different parts of the body to the kidney.
- Water homeostasis: Fifty-five per cent of blood contains plasma and 92% of plasma contains water. Loss of water from body as occurs in diarrhoea, vomiting, excessive sweating etc. results in decreased blood volume (hypovolemia). Persistent hypovolemia results in cellular dehydration. In response to hypovolemia, mechanisms are activated to increase the water content of the blood that aims at maintaining cellular hydration. Thus, by maintaining blood volume, water homeostasis of the body is maintained.
- Acid-base balance: Blood contains plasma proteins and hemoglobins. They (protein and Hb buffers) play important role in maintaining acid-base balance of the body. Also, bicarbonate in blood buffers the acids.
- Immunity: Blood forms a critical component of body immunity. It contains cells that play primary role in cellular immunity and also contains antibodies that play major role in humoral immunity. Blood also contains cells and chemicals that take part in nonspecific defences of the body.
- Storage: Blood is the storage site of electrolytes, nutrients, chemicals, hormones, etc.
- Body color: Blood provides natural color to the body. In anemia and hypovolemia, body becomes pale.
- Nutritive functions: Blood delivers all nutrients, such as glucose, lipids, proteins, vitamins, minerals, etc. to all tissues of the body for their utilization and storage.
- Oncotic pressure: The albumin present in blood exerts osmotic pressure, known as oncotic pressure that controls capillary filtration and prevents oedema formation (discussed more in ‘functions of plasma proteins).
The total blood volume can be roughly calculated as 70–80 ml/kg of body weight. In adult males, the total volume of blood is about 5 to 6 liters and in females about 4.5 to 5.5 liters. In children, though the absolute volume of blood present is significantly less than adults, the volume expressed per kg of body weight is more (80–90 ml/ kg-BW). Blood volume is also expressed per square meter of body surface area, which is normally 2.8 lit/m2 of the body.
Determination of Blood Volume
Blood volume is determined by determining the plasma volume and cell volume separately. The normal ratio of plasma volume to cell volume is 55:45.
Composition of plasma.
- Water (92%)
- Solids or Solutes (8%): plasma proteins make 90% of solids.
- Anions: Chloride, bicarbonate, phosphates, sulphates, etc.
- Cations: Na+, K+, Ca++, Mg++, etc.
- Colloids: Plasma proteins
- Crystalloids: Glucose, lipid, urea, uric acid, etc.
Measurement of Cell Volume
Cell volume is measured by measuring the volume of red cells. The volume of leucocytes and platelets is ignored as they constitute minor fraction of the total cell volume. Red cell volume is measured by radioactive isotope study.
Red cell volume
Red cell volume is the volume occupied by all circulating red cells.
- It is usually calculated by subtracting plasma volume from total blood volume.
- It is also directly calculated by injecting tagged red cell. The radioactive isotope of chromium is usually used as tag, though isotopes of iron (59Fe) and phosphorous (32P) are also used.
- The commonly used chromium isotope is 51Cr, which is attached to red cells through incubation of cells in the chromium solution.
Determination of Plasma Volume
Plasma volume is usually measured by using dyes. The dye used is Evans blue (T-1824). Plasma volume is also measured by injection of serum albumin labeled with radioactivem iodine. An average plasma volume is about 3.5 L (5% of the body weight of a 70 kg man).
Total blood volume is calculated by multiplying plasma volume with 100/(100 – hematocrit). (Hematocrit is to be determined). If hematocrit is 40 and plasma volume is 3.5 L, then blood volume will be:
Effective Blood Volume
Effective blood volume = the total blood volume – the volume that is sequestrated.
This means the volume which is present actually in the circulation helps in perfusion of tissue. However, it is difficult to estimate the sequestered volume of blood in the visceral organs.
Specific Gravity and Viscosity of Blood
Specific gravity: Specific gravity or density of whole blood is approximately 1.050.
- This depends on the number of cells and composition of plasma
- Specific gravity is measured by the extent of light bending in a refractometer or as part of a test strip dipped into blood.
- Usually, loss of water from blood as occurs in dehydration increases specific gravity.
Viscosity: Viscosity of blood or any liquid is a measure of resistance to flow.
- Blood viscosity is 4 to 5 times that of water.
- It is mainly due to the number of cells and macromolecules like proteins present in the blood.