Functions of Blood:

1 - Transportation:
• oxygen & carbon dioxide
• nutrients
• waste products (metabolic wastes, excessive water, & ions)
2 - Regulation - hormones & heat (to regulate body temperature)

3 - Protection - clotting mechanism protects against blood loss & leucocytes provide immunity against many disease-causing agents

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

1 - Formed elements:
• Red blood cells (or erythrocytes)
• White blood cells (or leucocytes)
• Platelets (or thrombocytes)

1 - biconcave discs

2 - lack a nucleus & cannot reproduce (average lifespan = about 120 days)

3 - transport hemoglobin (each RBC has about 280 million hemoglobin molecules)

4 - Typical concentration is 4-6 million per cubic mm (or hematocrit [packed cell volume] of about 42% for females & 45% for males)

5 - contain carbonic anhydrase (critical for transport of carbon dioxide)

Erythropoiesis = formation of erythrocytes

• the body must produce about 2.5 million new RBCs every second
• in adults, erythropoiesis occurs mainly in the marrow of the sternum, ribs, vertebral processes, and skull bones
• begins with a cell called a hemocytoblast or stem cell (below)
• rate is regulated by oxygen levels:

• hypoxia (lower than normal oxygen levels) is detected by cells in the kidneys
• kidney cells release the hormone erythropoietin into the blood
• erythropoietin stimulates erythropoiesis by the bone marrow

Source: http://greenfield.fortunecity.com/rattler/46/haemopoiesis.htm

Hemoglobin

• composed of globin (made up of 4 highly folded polypeptide chains) + 4 heme groups (with iron)
• each molecule can carry 4 molecules of oxygen
• called oxyhemoglobin when carrying oxygen & called reduced hemoglobin when not carrying oxygen
• can also combine with carbon dioxide & helps transport carbon dioxide from the tissues to the lungs

White blood cells (or leucocytes or leukocytes):

• have nuclei & do not contain hemoglobin
• typical concentration is 5,000 - 9,000 per cubic millimeter
• types of WBCs:
  • granular white blood cells include:
    • neutrophils (50 - 70% of WBCs)
    • eosinophils (1 - 4%)
    • basophils (less than 1%)
  • agranular (or non-granular) white blood cells include:
    • lymphocytes (25 - 40%)
    • monocytes (2 - 8%)

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:

• Neutrophils - phagocytosis (bacteria & cellular debris); very important in inflammation
• Eosinophils - help break down blood clots & kill parasites
• Basophils - synthesize & store histamine (a substance released during inflammation) & heparin (an anticoagulant); functions(s) remain unclear
• Monocytes - phagocytosis (typically as macrophages in tissues of the liver, spleen, lungs, & lymph nodes)
• Lymphocytes - immune response (including production of antibodies)

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

1 - phagocytic

2 - capable of diapedesis (also called extravasation)

3 - capable of ameboid movement (check out Ameboid Movement - Amoeba)

4 - exhibit chemotaxis (attracted to certain chemicals, such as those released by damaged cells)

Platelets (or thrombocytes)

1 - formed in the bone marrow from cells called megakaryocytes

2 - have no nucleus, but can secrete a variety of substances & can also contract (because they contain actin & myosin)

3 - normal concentration in the blood is about 250,000 per cubic millimeter

4 - remain functional for about 7 - 10 days (after which they are removed from the blood by macrophages in the spleen & liver)

5- play an important role in hemostasis (preventing blood loss)

Plasma:

1 - Water - serves as transport medium; carries heat

2 - Proteins

• Albumins
  • 60-80% of plasma proteins
  • most important in maintenance of osmotic balance
  • produced by liver
• Globulins
  • alpha & beta
    • some are important for transport of materials through the blood (e.g., thyroid hormone & iron)
    • some are clotting factors
    • produced by liver
  • gamma globulins are immunoglobulins (antibodies) produced by lymphocytes
• Fibrinogen
  • important in clotting
  • produced by liver

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):

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:

• "tightening" of clot
• contraction of platelets trapped within clot shrinks fibrin meshwork, pulling edges of damaged vessel closer together

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

Fibrinolysis:

• dissolution of clot
• mechanism = plasminogen (a plasma protein) is activated by many factors & becomes PLASMIN. Plasmin then breaks down fibrin meshwork & phagocytic WBCs remove products of clot dissolution

Inappropriate clotting:

• thrombus - clot formed in an intact vessel, possibly due to:
  • roughened vessel walls (atherosclerosis; see normal & occluded coronary arteries below)
  • slow-moving blood (e.g., in varicose veins) = small quantities of fibrin form & accumulate

• embolus - 'moving' clot

Excessive bleeding:

• Hemophilia
  • genetic 'defect'
  • inability to produce certain factor(s)
• Thrombocytopenia
  • abnormally low platelet count
  • most persons have idiopathic thrombocytopenia (= unknown cause) while in others it's an autoimmune disease.