Blood – definition, composition, and Function

Blood is the fluid connective tissue consisting of a fluid matrix, plasma, and formed elements. It is a fluid found in humans and other organisms that transport necessary substances such as nutrients and oxygen to the cells, and it carries away metabolic waste from cells. It is pumped by the heart to various parts of the body.

  • The color of vertebrate blood is bright red when its hemoglobin is oxygenated and dark red when it is deoxygenated.
  • Some animals, such as crustaceans and mollusks use Haemocynin to carry oxygen, instead of hemoglobin.
  • Insects and some mollusks use a fluid called hemolymph instead of blood, the difference being that hemolymph is not contained in a closed Circulatory system.

Composition of blood

In vertebrates, blood is composed of blood cells suspended in blood plasma.


Plasma constitutes 55% of blood fluid. 90 to 92% of plasma is water (92% by volume). it contains 6 to 8 % proteins. Fibrinogen, globulins, and albumins are the major proteins.

  • Fibrinogens are needed for clotting or coagulation of blood.
  • Globulins primarily are involved in the defense mechanisms of the body
  • Albumin is the main protein in plasma. It functions to regulate the colloidal osmotic pressure of blood.
  • The blood plasma also contains traces of minerals like Na+, Ca++, Mg++, HCO3 –, Cl–, etc.
  • Glucose, amino acids, lipids, etc. are present in plasma as they transit in the body.
  • Without clotting factors, the plasma is called serum.
  • Plasma also contains hormones, carbon dioxide (plasma being the main medium for excretory product transportation), and blood cells themselves.
Formed Elements

The blood cells are mainly Red blood cells (RBCs also called erythrocytes), White blood cells (WBCs also called leukocytes), and platelets (also called thrombocytes) these are collectively called formed elements and they constitute nearly 45 percent of the blood.

  • By volume, the red blood cells constitute about 45% of whole blood, the plasma about 54.3%, and white cells about 0.7%.

(**The formed cells are divided into two types of blood cells or corpuscles- RBCs and WBCs, and the cell fragments are called platelets.)

Blood Cells - RBC, WBC, Platelet

Blood Cells – RBC, WBC, Platelet


Count of red blood cells: One microliter of blood contains 4.7 to 6.1 million (male), and 4.2 to 5.4 million (female) – The most abundant cells in vertebrate blood are red blood cells.

  • A healthy adult man has, on average, 5 million to 5.5 million RBCs mm–3 of blood.
  • These are formed in the red bone marrow in adults.
  • The nucleus (and other cell organelles) is absent in RBCs in most mammals and are biconcave in shape
  • RBCs are red in color and iron-containing protein hemoglobin, which facilitates oxygen transport by reversibly binding to this respiratory gas and increases its solubility in blood.
  • A healthy individual contains 12-16 gms of hemoglobin in every 100 ml of blood
  •  Approximately 2.4 million new erythrocytes are produced per second in human adults
    • Packed red blood cells, also known as packed cells, are red blood cells that have been separated for blood transfusion. 
  • RBCs play a significant role in the transport of respiratory gases.
  • The cells have an average life span of 120 days.
  • After 120 days these were destroyed in the spleen (the graveyard of RBCs).
  • The spleen act as resevoir
  • Red blood cells of non-mammalian vertebrates are flattened and ovoid in form and retain their cell nuclei

RBCs the blood components

Leucocytes: (White blood cells)
  • One microliter of blood contains 4,000–11,000 White blood cells
  • WBCs are colorless due to the lack of hemoglobin. They are nucleated and are relatively lesser in number which averages 6000-8000 mm–3 of blood
  • WBCs are part of the body’s immune system.
  • The life span of leucocytes is generally short.
  • They destroy and remove old or aberrant cells and cellular debris, as well as attack infectious agents (pathogens) and foreign substances. The cancer of leukocytes is called leukemia.
    • Leukemia is a group of blood cancers that usually begin in the bone marrow and result in high numbers of abnormal blood cells. These blood cells are not fully developed and are called blasts or leukemia cells.
White Blood cells

White blood cells

  • Leucocytes have two categories: Granulocytes and agranulocytes.
    • Granulocytes: Neutrophils, eosinophils, and basophils
    • Agranulocytes: lymphocytes and monocytes
  • Neutrophils and monocytes (6-8 percent) are phagocytic cells that destroy foreign organisms entering the body.
  • Basophils secrete histamine, serotonin, heparin, etc., and are involved in inflammatory reactions.

Types of leucocytes, their function, and other details are given in the table below:

Type Approx. % Main function and target Nucleus lifetime
Neutrophil 62% Neutrophils are phagocytic in nature. Bacteria, Fungi

They are commonly referred to as polymorphonuclear (PMN) leukocytes

Multilobed 6 hours–few days
Eosinophil 2.3% Larger parasites, inflammatory response.

It rises in response to allergies, parasitic infections, collagen diseases, and disease of the spleen and central nervous system.

They secrete chemicals that destroy these large parasites, such as hookworms and tapeworms, that are too big for any one WBC to phagocytize

Bi- lobed 8–12 days (circulate for 4–5 hours)
Basophil 0.4% They release histamine for inflammatory response and Heparin.

Heparin is an anticoagulant that inhibits blood clotting and promotes the movement of white blood cells into an area.


Bi or trilobed A few hours to a few days
lymphocytes 30% B-cells – releases antibodies and assists activation of T cells

T cell – is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response.

Deeply staining, eccentric Years for memory cells, weeks for all else.
Monocytes 5.3% Monocytes migrate from the bloodstream to other tissues and differentiate into tissue-resident macrophages, Kupffer cells in the liver Kidney shaped Hours to days
Platelets (Thrombocytes)
  • Platelets are cell fragments produced from megakaryocytes (special cells in the bone marrow).
  • Blood normally contains 1,500,00-3,500,00 platelets mm–3
  • These release a variety of substances and these mostly function for coagulation or clotting of blood.
  • The platelet cell membrane contains receptors for collagen. Following the rupture of the blood vessel wall, the platelets are exposed and they adhere to the collagen in the surrounding connective tissue
The pH of the blood

(In chemistry, pH (denoting ‘potential of hydrogen’ or ‘power of hydrogen) is a scale used to specify the acidity or basicity of an aqueous solution.) The pH of blood ranges from 7.35 to 7.45, making it slightly basic.

  • Blood that has a pH below 7.35 is too acidic
  • Blood pH above 7.45 is too basic
The Function of Blood

Blood performs many important functions within the body, including:

Production and degradation of blood cells:

In vertebrates, the various cells of the blood are made in the bone marrow, by the process named hematopoiesis, including:

  • Erythropoiesis: It is the process that produces red blood cells (erythrocytes), which is the development from erythropoietic stem cells to mature red blood cells.
  • Myelopoiesis: refers to the regulated formation of myeloid leukocytes (myelocytes), including eosinophilic granulocytes, basophilic granulocytes, neutrophilic granulocytes, and monocytes.
  • During childhood, almost every human bone produces red blood cells; as adults, red blood cell production is limited to the larger bones: the bodies of the vertebrae, the breastbone (sternum), the ribcage, the pelvic bones, and the bones of the upper arms and legs.
  • During childhood, the thymus gland found in the mediastinum is an important source of T lymphocytes.
    • (The mediastinum is the central compartment of the thoracic cavity surrounded by loose connective tissue, as an undelineated region that contains a group of structures within the thorax).
  • The proteinaceous component of blood (including clotting proteins) is produced predominantly by the liver. The liver also clears some proteins, lipids, and amino acids.
  • hormones are produced by the endocrine glands
Some deficiencies related to blood:

Thrombocytopenia: It is a condition characterized by abnormally low levels of platelets, also known as thrombocytes, in the blood. Thrombocytopenia is the most common coagulation disorder among intensive care patients and is seen in 20% of medical patients and a third of surgical patients, the disorders lead to excessive loss of blood from the body

Leukopenia: Leukopenia is a decrease in the number of leukocytes. Found in the blood, they are the white blood cells and are the body’s primary defense against infection. Thus the condition of leukopenia places individuals at increased risk of infection.

Neutrophilia: Neutrophilia is an increase in the absolute neutrophil count in the peripheral circulation.

Eosinophilia: Eosinophilia is a condition in which the eosinophil count in the peripheral blood exceeds 5×108/L (500/μL).

Anaemia-It is a decrease in the total amount of Red blood cells or hemoglobin in blood or a lowered ability of the blood to carry oxygen. The diagnosis of anemia in men is based on a hemoglobin of less than 130 to 140 g/L (13 to 14 g/dL); in women, it is less than 120 to 130 g/L (12 to 13 g/dL)

leukemia–  is a group of blood cancers that usually begin in the bone marrow and result in high numbers of abnormal blood cells. These blood cells are not fully developed and are called blasts or leukemia cells.

A myelodysplastic syndrome (MDS)– is one of a group of cancers in which immature blood cells in the bone marrow do not mature so do not become healthy blood cells, no symptoms typically are seen earlier phase.

Haemophilia – It is a mostly inherited genetic disorder that impairs the body’s ability to make blood clots, a process needed to stop bleeding. This results in people bleeding for a longer time after an injury, easy bruising, and an increased risk of bleeding inside joints or the brain


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