Plasma has about 92\% water. Hydration is important because when plasma water decreases, blood becomes thicker and its function declines.
About 8\% of plasma is not water and is primarily protein-based.
Albumin accounts for about 54\% of the plasma proteins. Albumin helps with osmotic pressure to keep fluids in the blood. Most plasma proteins are produced by the liver.
Globulins account for about 38\%. They are subdivided into alpha, beta, and gamma globulins.
Alpha and beta globulins transport iron, lipids, and fat-soluble vitamins (these are important for transport and metabolism).
Gamma globulins are antibodies (immunoglobulins) that recognize antigens on cell surfaces. They help distinguish self from non-self and recognize foreign cells.
Fibrinogen accounts for about 7\% and will be discussed later in the context of blood clotting (hemostasis).
Other plasma components (not a large portion but present):
Nitrogenous byproducts (e.g., urea) are present but kept at low levels by kidney clearance.
Nutrients such as glucose and amino acids, plus carbohydrates, are carried in plasma and redistributed to cells as needed.
Electrolytes (Na⁺, K⁺, Ca²⁺, Cl⁻, HCO₃⁻) regulate resting membrane potential, water balance, pH, and ion balance.
Na⁺ and K⁺ are key to resting membrane potential and water balance.
HCO₃⁻ helps with pH buffering.
Ca²⁺ is essential for nerve/muscle activity and blood clotting; Cl⁻ helps balance pH and ionic concentrations.
Gases: carbon dioxide (CO₂) and oxygen (O₂) are transported in plasma.
Hormones are carried in plasma as part of endocrine communication.
Formed elements overview
Formed elements include white blood cells (WBCs), red blood cells (RBCs), and platelets.
Not all formed elements are complete cells:
White blood cells are complete cells.
Red blood cells are mature cells that lack a nucleus and most organelles to maximize hemoglobin and oxygen-carrying capacity.
Platelets are cell fragments derived from megakaryocytes.
Lifespans and turnover:
Most formed elements survive only a few days in the bloodstream.
Red blood cells can live up to about 3–4 months.
There is constant turnover: new cells are produced as old/damaged cells are removed.
Origin:
All formed elements originate in red bone marrow (hematopoiesis).
In adults, red bone marrow is primarily in the axial skeleton (skull, vertebral column) and proximal ends of the humerus and femur; much of it is replaced by yellow marrow with age.
Red bone marrow contains sinusoidal capillaries with larger openings to allow mature blood cells to enter the bloodstream; most other capillaries do not.
Hematopoietic stem cells and hematopoiesis
Hematopoietic stem cells (HSCs) are also known as hemocytoblasts and give rise to all formed elements.
HSCs reside in red bone marrow and continually produce cells to replace those that die.
The stem cells differentiate into two main pathways:
Lymphoid pathway (lymphoid stem cell → lymphoblast): produces lymphocytes and natural killer (NK) cells. Lymphocytes mature into B cells, T cells, and NK cells; NK cells function in destroying foreign particles.
Myeloid pathway (myeloid stem cell) branches into four lineages:
Monoblast → monocyte (one of the five WBCs).
Myoblast → basophil, neutrophil, eosinophil (these three constitute the granulocytes).
The multipotent stem cell (hemocytoblast) is the common origin for all formed elements; it differentiates into either the lymphoid or myeloid lineage, which then give rise to mature blood cells.
Growth factors and regulation of blood cell production
Erythropoietin (EPO):
Produced by the kidneys and stimulates erythrocyte (RBC) production.
Important link between kidney function and RBC mass.
Thrombopoietin:
Stimulates megakaryocytes to produce platelets.
Cytokines: a broad group of signaling molecules that regulate immune responses and hematopoiesis.
Colony-stimulating factors (CSFs): stimulate growth and differentiation of hematopoietic progenitors, particularly for granulocytes and monocytes.
Interleukins: a subgroup within cytokines that promote cell differentiation and maturation, immunity, and inflammation.
Interleukins (e.g., IL-1, IL-2) help in differentiation and maturation of immune cells and participate in inflammatory responses.
Combined effect: four of the five white blood cell types are produced via common signaling factors (CSFs) from the myeloid pathway; lymphoid-derived cells rely more on lymphoid-specific signals and interleukins for maturation. Overall, CSFs and interleukins coordinate growth, maturation, and activation of blood cells.
Detailed pathways for blood cell lineages
Lymphoid pathway (for lymphocytes):
Hematopoietic stem cell → lymphoid stem cell → lymphoblast → lymphocyte or NK cell.
Lymphocytes eventually differentiate into T lymphocytes, B lymphocytes, and NK cells (specific functions to be covered in immunology).
Myeloid pathway (for other blood cells):
Hematopoietic stem cell → myeloid stem cell → four possible lines:
Monoblast → monocyte (monocyte is a WBC).
Myoblast → basophil, neutrophil, eosinophil (these three are granulocytes).
Proerythroblast → erythrocyte (RBC).
Megakaryoblast → megakaryocytes → platelets (cell fragments that aid clotting).
All of these lines originate from the single multipotent hematopoietic stem cell and diverge via branching differentiation pathways, including intermediate blasts (e.g., lymphoblast, monoblast, myoblast, proerythroblast, megakaryoblast).
Connections and significance
Plasma proteins, especially albumin, maintain blood volume and pressing fluid balance via osmotic pressure, influencing edema and hydration status.
Globulins include transport proteins (alpha and beta) for iron, lipids, and fat-soluble vitamins, as well as antibodies (gamma globulins) critical for immunity.
Fibrinogen is central to the clotting cascade; its presence in plasma is a key step in hemostasis (formation of a fibrin clot).
Electrolytes in plasma are essential for nerve and muscle function, myocardial activity, fluid balance, acid-base balance, and signal transduction.
Oxygen transport and carbon dioxide removal are functions of blood via RBCs and dissolved gases; hormones and nutrients circulate to regulate metabolism and tissue function.
The bone marrow microenvironment (red marrow) is essential for hematopoiesis; specialized sinusoidal capillaries permit mature cells to enter circulation.
Hematopoiesis connects anatomy (axial skeleton; proximal long bones) with physiology (cell production, turnover, and homeostasis).
Health implications: kidney function (via EPO), liver function (protein production), and marrow health all impact circulating blood cell counts and immune competence. Conditions affecting any step (e.g., anemia, thrombocytopenia, leukemias) reflect disruptions in these pathways.
Quick glossary and key terms
Albumin: plasma protein that maintains osmotic pressure; mainly produced by the liver.
Globulins: a group of plasma proteins; alpha/beta globulins transport various substances; gamma globulins are antibodies.
Fibrinogen: clotting factor precursor; converted to fibrin during coagulation.
Hematopoiesis: the formation of blood cellular components.
Hemocytoblast / hematopoietic stem cell: multipotent stem cell giving rise to all formed elements.
Lymphoid vs myeloid pathways: the two major routes of hematopoietic differentiation.