Introduction to Blood

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Blood is a type of circulating connective tissues. Blood is also one of the two types of liquid tissues found in the body. The other being lymph, which is derived from blood. Blood, as a circulating connective tissue differs from other connective tissues in that the cells are not fixed. They freely circulate in a liquid matrix (blood plasma). In an adult human, blood volume measures about five liters and accounts for eight percent (8%) of the body weight.

Blood brings nutrients and oxygen to the cells but also serves as the primary transport medium which carries away metabolic cellular waste in order to maintain a stable cellular environment in the human body. The heart pumps blood through a superhighway of blood vessels to all the organs in the body.

Functional Activities of Blood

Activity Description
Transport
  • Carries oxygen and nutrients to the cells.
  • Transports carbon dioxide and nitrogenous wastes from the tissues to the lungs and kidneys where these wastes can be removed from the body.
  • Carries hormones from the endocrine glands to the target tissue.
Regulation
  • Helps regulate body temperature by removing heat from active areas, such as skeletal muscles, and transporting it to other regions or to the skin where it can be dissipated.
  • Plays a significant role in fluid and electrolyte balance because the salts and plasma proteins contribute to the osmotic pressure.
  • Functions in pH regulation through the action of buffers in the blood.
Protection
  • Clotting mechanisms (by platelets) prevent fluid loss through hemorrhage when blood vessels are damaged.
  • Helps phagocytes (white blood cells/macrophages) protect the body against disease-causing microorganisms by engulfing and destroying the agent.
  • Antibodies in the plasma help protect the body by binding to antigens, which are foreign substances like bacteria.

Origin and Production of Blood

As stated previously, the production of blood cells is called hematopoiesis and starts with hematopoietic stem cells. Before birth, hematopoiesis occurs primarily in the liver and spleen of the fetus, but some cells develop in the thymus, lymph nodes, and red bone marrow. After birth, most production is limited to red bone marrow in specific regions, but some white blood cells are produced in the lymphoid tissue. New (immature) blood cells are called blasts. Some blasts stay in the marrow to mature, while others travel to various parts of the body to mature.

Hematopoietic stem cells reside in the bone marrow (femurs, hips, ribs, sternum, and other bones) and have the unique ability to give rise to all the different mature blood cells. They are also self-renewing. Some of the daughter cells remain as unspecialized, undefined or uncommitted hematopoietic stem cells, so the pool of stem cells does not become depleted. This is a unique process to stem cells and is incredibly important to our survival.

All of the formed elements originate from pluripotential cell or hemocytoblasts. From the hemocytoblast, seven distinct cell lines develop, each regulated by its own growth factor. When a stem cell divides, one daughter cell remains a stem cell, while the other differentiates into a precursor—either a lymphoid or a myeloid cell. These precursor cells then mature into the various blood cell types. Thus, all blood cell lineages ultimately arise from these two progenitor lines.

Composition of Blood

Illustration of blood composition.

When a blood sample is placed in a centrifuge, the cellular components separate from the liquid portion of the blood. The formed elements are denser than the fluid matrix and they settle to the bottom of the tube under centrifugal force. The light yellow fluid at the top is plasma, which makes up about 55% of blood volume. The red blood cells form the hematocrit, or packed cell volume (PCV), at the bottom of the tube. Between these layers is a thin whitish band called the buffy coat, which contains white blood cells and platelets.

Definitions  
Plasma
  • Watery fluid portion of blood (90%) in which the corpuscular elements are suspended
  • Transports nutrients as well as waste throughout the body.
  • Various compounds, including proteins, electrolytes, carbohydrates, minerals, and fats, are dissolved in it
Formed Elements
  • Cells and cell fragments suspended in the plasma
  • Three classes of formed elements: erythrocytes (red blood cells), leukocytes (white blood cells), and the thrombocytes (platelets)
Erythrocytes (red blood cells)
  • Most numerous of the formed elements
  • Tiny biconcave disks, thin in the middle and thicker around the periphery
  • Shape provides a combination of flexibility for moving through tiny capillaries with a maximum surface area for the diffusion of gases
  • Primary function of erythrocytes is to transport oxygen
Leukocytes (white blood cells)
  • Second formed element, larger than erythrocytes, fewer in number
  • Do most of their work in the tissues: move through the capillary walls into tissue spaces (diapedesis). In tissue, provide defense against organisms that cause disease and either promote or inhibit inflammatory responses
  • Use blood as a transport medium
  • Functions include phagocytosis, producing antibodies, secrete histamine and heparin, neutralize histamine

Leukocytes

Leukocytes in the blood are divided into two main groups: granulocytes, which contain cytoplasmic granules, and agranulocytes, which lack them.

Type of Leukocyte Name Description
Agranulocytes Lymphocytes Have a special role in the immune process. Some attack bacteria directly; others produce antibodies
Agranulocytes Monocytes Critical role in the immune system acting as the body’s cleanup crew by engulfing and digesting pathogens, cellular debris and damaged cells
Granulocyte Basophils Secrete histamine and heparin and have blue granules. Called mast tissues
Granulocyte Eosinophils Have granules that stain red under the microscope and help counteract the effects of histamine.
Granulocyte Neutrophils Most numerous leukocytes. They are phagocytic and have light-colored granules.

Platelets

Platelets, also known as thrombocytes, are not whole cells but rather small fragments derived from very large cells called megakaryocytes. These megakaryocytes originate from hemocytoblasts in the red bone marrow. Platelets become sticky and clump together to form plugs that seal breaks or tears in blood vessels, and they also play a key role in initiating blood clot formation.

Updated: December 2, 2025