Blood: Composition, Function and Volume
COMPOSITION OF BLOOD
Blood is a fluid connective tissue containing 55% plasma and 45% ‘formed elements’ including Erythrocytes (red blood cells or RBCs), Leukocytes (white blood cells or WBCs), and Thrombocytes (platelets).
Figure: Composition of Blood
1. Erythrocytes or RBCs
Erythrocytes or red blood cells (RBCs) are biconcave, disc-shaped, unnucleated, 7.5μm in diameter, and 2.5μm in height. The process of production of RBCs is known as erythropoiesis. During this process, hematopoietic stem cells (HSCs) in the bone marrow undergo morphological changes to form mature RBCs. The lifespan of mature RBCs in circulation is about 120 days, after which they are engulfed by macrophages and replaced by new red blood cells. The normal range of RBCs is around 4 to 5.5 million RBCs per cubic millimeter of blood.
Figure: Structure of RBCs
✏️ Haemoglobin
RBCs contain a pigment called haemoglobin that imparts red colour to the blood and transports oxygen around the body. A single RBC contains about 300 million haemoglobin molecules. ‘Haemo-globin' is made up of protein globin bound to red heme pigment. The heme group (also called ferroporphyrin) is an organic compound with a ring-like structure that contains an iron atom at the centre. The Iron atom binds reversibly to oxygen, therefore, each haemoglobin molecule has four binding sites for oxygen molecules, i.e. the iron atoms in the four heme groups.
Figure: Structure of Haemoglobin A (HbA)
A variety of haemoglobin (Hb) molecules exists, however, the most common are HbA, HbA2, and HbF. In human adults, the predominant type of haemoglobin is haemoglobin A (HbA), constituting approximately 95-98% of the total. HbA is a tetramer composed of four heme groups bound to four globin chains, consisting of 2 alpha and 2 beta chains, denoted as α2β2. In infants, hemoglobin F (HbF or fetal haemoglobin) is predominant, containing 2 alpha and 2 gamma chains (α2γ2), which are gradually replaced by beta chains as they grow older.
2. Leukocytes or WBCs
Leukocytes or white blood cells (WBCs) are nucleated cells that play a crucial role in the immune system by fighting infections and protecting the body against foreign materials. WBCs are produced from hematopoietic stem cells (HSCs) in bone marrow by a process called leukopoiesis, and their production levels are regulated by the spleen, liver, and kidneys. WBCs are the primary source of nuclear DNA in the blood. Their lifespan can range from a few hours to a few days. The normal range of WBCs in the blood is approximately 4,000 to 11,000 per cubic millimeter.
✏️ Classification of WBCs
WBCs can be classified into two major categories- granulocytes and agranulocytes, based on the presence of granules within the cytoplasm. Granulocytes and agranulocytes can be further classified into various types, with each cell type playing a specific role in the immune system.
Figure: Classification of WBCs
a) Neutrophils: The term ‘neutrophil’ is derived from the staining characteristics of the cell. Their granules are stained in a ‘neutral colour’- pale pink to bluish purple with acidic or basic dyes. Neutrophils contain a characteristic multi-lobed nucleus (three to five lobes) joined together with a tube-like strand). Neutrophils are the most abundant WBCs found in blood. They are the first immune cells to migrate to the site of infection and destroy bacteria or viruses by a process called phagocytosis.
b) Eosinophils: The term ‘eosinophils’ means ‘eosin loving’. These cells contain a bi-lobed nucleus and contain granules that are stained a bright pink colour with the acidic dye ‘eosin’. These granules contain chemicals that are released to fight off parasitic infections and mediate inflammation.
c) Basophils: Basophils have a bi-lobed nucleus along with granules in the cytoplasm that are stained a bright purple colour with basic dyes. They play a role in triggering inflammatory and allergic reactions in response to an injury or infection by releasing histamine that causes symptoms like runny nose or sneezing. Basophils also release heparin, which acts as a blood thinner and helps in preventing blood clotting.
d) Monocytes: Monocytes are the largest type of WBCs containing a ‘U’ or ‘kidney-shaped’ nucleus. They travel through the blood and differentiate into macrophages or dendritic cells in case of tissue damage or infection.
Macrophages are also known as “big eater” cells. They can ingest pathogens, cancer cells, and cellular debris by a process called phagocytosis.
Dendritic cells are also called “professional antigen-presenting” cells. They capture, process, and present the antigens of the pathogen on their cell surface for other cells of the immune system.
e) Lymphocytes: Lymphocytes are further classified into B cells, T cells, and Natural Killer cells. B cells produce antibodies that bind specifically to different antigens and mark the pathogen for destruction. T cells recognize antigens that the body has encountered before and directly attack them. Natural Killer cells can recognize and kill the cells that have been infected with a pathogen and destroy tumor cells.
3. Thrombocytes or Platelets
Platelets are tiny, disc-shaped pieces of cells that are formed by the breakdown of megakaryocytes in the bone marrow. The process of platelet formation is called thrombopoiesis. Once in circulation, platelets have a lifespan of about 7 to 10 days. The normal platelet count of a healthy adult is 150,000 to 450,000 per cubic millimeter of blood.
Platelets play a major role in blood clotting and prevent the loss of excess blood from injured areas. At the site of injury, collagen chemically interacts with platelets and activates them. The activated platelets then start clumping together and attaching themselves to the walls of damaged blood vessels to form a platelet plug. This process is called adhesion. A chemical signal is also sent out to attract more platelets at the injury site. The additional platelets adhere to the clot by a process called aggregation.
Clotting factors such as fibrinogen (inactive form) get converted into fibrin (active form). Fibrin is a long and sticky protein that forms a mesh around the platelet plug, forming a fibrin clot. The mesh strengthens the platelet plug and traps red blood cells to form a blood clot. The platelets constrict, bringing the two sides of the injured vessel closer together to facilitate quick healing.
BLOOD VOLUME
The total amount of blood flowing through the heart's chambers, arteries, veins, and capillaries is called blood volume. The amount of blood circulating within an average human adult is about 5 liters or 8% of body weight. However, it also depends on factors other than weight, like- height, amount of adipose tissue, hydration levels, and electrolyte concentrations.
Maintenance of blood volume ensures adequate blood flow to all the tissues of the body. The kidneys are primarily responsible for regulating blood volume by balancing the concentration of solutes and water content of blood. If blood volume is too low (hypovolemia), more filtrate will be reabsorbed by the kidneys; if blood volume is too high (hypervolemia), less filtrate will be reabsorbed by the kidneys
FUNCTIONS OF BLOOD
Transport
Blood is majorly responsible for the transport of gases (oxygen and carbon dioxide) to and from the cells. It also transports nutrients absorbed from the digestive tract to all the cells and carries metabolic waste products from the cells to the kidneys and liver. Hormones released from endocrine glands are also transported through blood to their target action site.
Defense
Blood contains various types of WBCs that are a major part of our immune system and protect the body from infections caused by bacteria, viruses, and foreign pathogens.
Maintaining Homeostasis
Homeostasis is the maintenance of our internal body temperature of about 37°C by a part of the brain called the hypothalamus. The hypothalamus constantly monitors the blood temperature. If the blood temperature increases, the hypothalamus sends a signal that causes the capillaries to dilate (become wider). Dilated capillaries carry more blood near the skin surface, allowing more heat to escape. In the case of decreased blood temperature, the capillaries constrict (become narrower) to reduce heat escape.
Maintaining acid-base balance
Most of the enzymes in the human body function at an optimal pH of 7.35 to 7.45. Blood acts as a buffer that resists any change in its pH due to the imbalance of acids and bases during the metabolic process. Blood contains three buffer systems: bicarbonate buffer, phosphate buffer, and protein buffer. The bicarbonate buffer system is the most important buffer system of blood. It is regulated by increasing or decreasing the rate of reabsorption of bicarbonate by kidneys; and altering the rate of removal or retention of carbonic acid by lungs.
Maintaining Osmolarity
The osmolarity of blood is largely dependent on how much water is retained within the body. The osmolality (concentration of solutes vs water in blood) of a properly functioning body is 275 to 295 milliosmoles/kg of water. If the osmolarity gets higher, i.e. there is more solute than water (dehydration), the pituitary sends a message to the kidneys to release a high amount of ADH (anti-diuretic hormone). This hormone causes the kidneys to release less water in the urine, thus producing concentrated urine. If the osmolarity gets lower, i.e. there is more water than solutes, a lesser amount of ADH will be secreted and kidneys will release excess water in the form of diluted urine.