Functions of Blood:
3 - Protection - clotting mechanism protects against blood loss & leucocytes provide immunity against many disease-causing agents
Components of blood
Components of Blood - average adult has about 5 liters (about 5 qts):
Red blood cell, platelet, and white blood cell
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
Three main classifications of blood cells derive from haematopoietic stem cells (HSCs) (Katsura 2002).
The binding and release of oxygen illustrates the structural differences between oxyhemoglobin and reduced (or deoxy-) hemoglobin. Only one of the four heme groups is shown
Hemoglobin and oxygen transport
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).
How white blood cells are formed
The primary functions of the various white blood cells are:
Eosinophils respond to diverse stimuli, including tissue injury, infections, allografts, allergens, and tumors. Eosinophils can also release a variety of cytokines, chemokines, lipid mediators, and neuromodulators. Eosinophils directly communicate with T cells and mast cells. Eosinophils activate T cells by serving as antigen-presenting cells.
Eosinophils (in green with red nucleus) catapult their mitochondrial DNA out of the cell, forming tangled traps (red) that ensnare foreign bacteria.
(Photo credit: Hans-Uwe Simon, Institute of Pharmacology, University of Bern, Switzerland)
Catapult-like release of mitochondrial DNA by eosinophils -- Although eosinophils are considered useful in defense mechanisms against parasites, their exact function in innate immunity remains unclear. Yousefi et al. (2008) found that eosinophils in the gastrointestinal tract release mitochondrial DNA in a rapid, catapult-like manner—in less than one second. The mitochondrial DNA and proteins released by the eosinophils bind to and kill bacteria. This is a previously undescribed mechanism of eosinophil-mediated innate immune responses that might be crucial for maintaining the intestinal barrier function after inflammation-associated epithelial cell damage, preventing the host from uncontrolled invasion of bacteria.
Some important characteristics of White Blood Cells (particularly neutrophils):
3 - capable of ameboid movement
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)
Platelet adhesion and aggregation
1 - Water - serves as transport medium; carries heat
2 - Proteins
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 (check this Hemostasis animation and this one and this one):
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
vessels, but in the wrong place can cause problems such as a stroke
below under inappropriate clotting).
Over time (with the amount of time depending on the amount of damage), the clot is dissolved and replaced with normal tissue.
Blue arrows = stimulation; red arrows = inhibition. tPA is released by damaged endothelium
Thrombus and embolus
Thrombocytopenia is a condition where platelet counts are lower than normal, potentially leading to mild to serious bleeding. This bleeding can happen inside the body (internal bleeding) or on the skin. A normal platelet count is 150,000 to 450,000 platelets per microliter of blood. A count of less than 150,000 platelets per microliter is lower than normal, but the risk for serious bleeding doesn't occur until the count becomes very low—less than 10,000 or 20,000 platelets per microliter. Milder bleeding sometimes occurs when the count is less than 50,000 platelets per microliter. Several factors can cause a low platelet count, such as:
How long thrombocytopenia lasts depends on its cause. It can range from days to years. The treatment for this condition also depends on its cause and severity. Mild thrombocytopenia most often doesn't need treatment. If the condition is causing serious bleeding, or if you're at risk for serious bleeding, you may need medicines or blood or platelet transfusions. Rarely, the spleen may need to be removed. Thrombocytopenia can be fatal, especially if the bleeding is severe or occurs in the brain. However, the overall outlook is good, especially if the cause of the low platelet count is found and treated (Source: NHLBI).
Introduction to Immunology
Cell Mediated and Humoral Immunity
Understanding the Immune System
Blood Types Tutorial
Yousefi, S., J. A Gold, N. Andina, J. J. Lee, A. M. Kelly, E. Kozlowski, I. Schmid, A. Straumann, J. Reichenbach, G. J. Gleich, and H.-U. Simon. 2008. Catapult-like release of mitochondrial DNA by eosinophils contributes to antibacterial defense. Nature Medicine, published online (10 August 2008).
Lecture Notes 1 - Cell Structure & Metabolism
Lecture Notes 2 - Neurons & the Nervous System I
Lecture Notes 2b - Neurons & the Nervous System II
Lecture Notes 3 - Muscle
Lecture Notes 4b - Blood and Body Defenses II
Lecture Notes 5 - Cardiovascular System
Lecture Notes 6 - Respiratory System
Lymph system graphic used with permission of John Kimball