Cardiovascular System, Blood, and Bone Marrow

Blood vessels are comprised of three layers or tunics: intima, media, and adventitia. In large arteries close to the heart, the tunica media contains high amounts of elastin to buffer the heart’s pulsatile output. Smaller musculararteries distribute blood to organs and capillary beds; their contractions are mediated by both the sympathetic nervous system (SNS) and by humoral factors. Endothelial cells lining the vascular lumen secrete vasoactive substances that regulate relaxation and contraction of the underlying smooth muscle.
Blood vessels are comprised of three layers or tunics: intima, media, and adventitia. In large arteries close to the heart, the tunica media contains high amounts of elastin to buffer the heart’s pulsatile output. Smaller musculararteries distribute blood to organs and capillary beds; their contractions are mediated by both the sympathetic nervous system (SNS) and by humoral factors. Endothelial cells lining the vascular lumen secrete vasoactive substances that regulate relaxation and contraction of the underlying smooth muscle. 


For example, endothelin 1 is a potent vasoconstrictor; nitric oxide is synthesized from L-arginine and induces relaxation of the smooth muscle through a cGMP-dependent mechanism. Prostacyclin, also synthesized by endothelial cells, is a smooth muscle relaxant that functions through a cAMP-dependent mechanism. Prostacyclin inhibits platelet adhesion and prevents intravascular clot formation. Endothelial cells produce molecules that regulate fibrinolysis and thrombogenesis. Endothelial cell-derived factors are stored in intracellular granules and released into the blood stream upon stimulation. There are two contrasting factors, fibrinolytic tissue-type plasminogen activator (tPA) and von Willebrand factor (vWF).

In contrast to tPA, vWF induces coagulation and thrombus formation. Endothelial cells also produce tissue factor, the only nonplasma protein in the clotting cascade, which initiates the common bloods clotting pathway.

E-selectin expression on endothelial cells modulates extravasation of monocytes and neutrophils. Chemokines (chemoattractant cytokines) induce expression of E-selectins on the endothelium under normal conditions and following inflammation. Smooth muscle cells undergo hyperplasia and hypertrophy in hypertension. The heart contains specialized cardiomyocytes that function as impulse-generating and conducting cells regulated by the ANS. The heart also functions as an endocrine organ, releasing atrial natriuretic factor [peptides (ANP or ANF)] in response to increased plasma volume. ANPs reduce plasma volume by 
  1. increasing urinary sodium (natriuretic) and water excretion (diuretic), 
  2. inhibiting aldosterone synthesis and angiotensin II production, and 
  3. inhibiting vasopressin release from the neurohypophysis.

Blood cells include erythrocytes, which are specialized for oxygen transport; lymphocytes that function in cellular and humoral immune responses; neutrophils, which are early responders to acute inflammation; monocytes that are the precursors of tissue macrophages; eosinophils, which respond to parasitic infection and release histaminases to counteract basophils and mast cells; and basophils, which contain histamine and heparin and assist mast cell function. Bone marrow is the site of blood cell development in adults. 

The erythrocyte lineage includes the following stages: proerythroblasts → basophilicerythroblasts → polychromatophilic erythroblasts → orthochromatophilic erythrocytes. The white cell series includes myeloblasts → promyelocytes → myelocytes → metamyelocytes → mature granular leukocytes.

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