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Human Physiology - Blood

Functions of Blood:

Components of Blood - average adult has about 5 liters (about 5 qts) 

           2 - Plasma = water + dissolved solutes


Red Blood Cells (or erythrocytes):

Erythropoiesis = formation of erythrocytes



White blood cells (or leucocytes or leukocytes):

Granular white blood cells contains numerous granules in the cytoplasm, & their nuclei are lobed. Agranular white blood cells have few or no granules in the cytoplasm & have a large spherical nucleus. Granular white blood cells are produced in the bone marrow, while agranular white blood cells are produced in lymph tissue, e.g., Lymph nodes (specialized dilations of lymphatic tissue which are supported within by a meshwork of connective tissue called reticulin fibers and are populated by dense aggregates of lymphocytes and macrophages).

The primary functions of the various white blood cells are:

Some important characteristics of White Blood Cells (particularly neutrophils):

Platelets (or thrombocytes)


1 - Water - serves as transport medium; carries heat

2 - Proteins

3 - Inorganic constituents (1% of plasma) - e.g., sodium, chloride, potassium, & calcium

4 - Nutrients - glucose, amino acids, lipids & vitamins

5 - Waste products - e.g., nitrogenous wastes like urea

6 - Dissolved gases - oxygen & carbon dioxide

7 - Hormones


Hemostasis - prevention of blood loss from broken vessel

1 - Vascular spasm - vasoconstriction of injured vessel due to contraction of smooth muscle in the wall of the vessel. This 'spasm' may reduce blood flow & blood loss but will not stop blood loss.

2 - Formation of a platelet plug - platelets aggregate at the point where a vessel ruptures. This occurs because platelets are exposed to collagen (a protein found in the connective tissure located just outside the blood vessel). Upon exposure to collagen, platelets release ADP (adenosine diphosphate) & thromboxane. These substances cause the surfaces of nearby platelets to become sticky and, as 'sticky' platelets accumulate, a 'plug' forms.

3 - Blood coagulation (clotting):

Used with permission of Michael W. King, Ph.D / IU School of Medicine

The result of all of this is a clot - formed primarily of fibrin threads (or polymers), but also including blood cells & platelets.

Blood clots in the right places prevent the loss of blood from ruptured vessels, but in the wrong place can cause problems such as a stroke (see below under inappropriate clotting).

Clot retraction:

Over time (with the amount of time depending on the amount of damage), the clot is dissolved and replaced with normal tissue.


Inappropriate clotting:

Excessive bleeding: