BME 419/519 Hernandez 2002 Vascular Biology I - Components of the Vascular System Functions of Vascular system: 1-Active transport of substances through the body (small organisms have mostly passive transport) : Nutrients (glucose, ATP), Oxygen, waste, hormones (chemical signals), host defenses (WBC, platelets, globulins … etc) 2 - Regulation of that flow: Arterial “shunts” to distribute blood flow to organs that need it the most (eg – blood flow to the brain and heart must be maintained at all costs, so in the event of blood loss, vasoconstriction occurs). ‘Basal tone” – tonic contraction of vessel smooth muscle in the absence of external stimulation. 3 – Thermal regulation: Blood flow through skin -> heat radiation -> cooling (quite efficient). By redirecting flow, you can cool down or heat up internal organs. Æ maintenance of homeostasisBME 419/519 Hernandez 2002 Distribution of systematic blood flow Organ Flow Rate at Rest ml/min % of Total Flow Flow Rate with Strenuous Exercise ml/min % of Total Flow Brain 650 13 750 4 Heart 215 4 750 4 Skel. Musc. 1,030 20 12,500 73 Skin 430 9 1,900 11 Kidney 950 20 600 3 Abd. Organs 1,200 24 600 3 Other 545 10 400 2 Total 5,000 100 17,500 100 Components of the Cardiovascular System 1. Heart – obviously the pump that drives the system (more details elsewhere) 2. Blood- main components: a. Erythocytes (Red Blood Cells) – Carry oxygen bound to hemoglobin molecules, Hematocrit” = % of blood volume made up by erythrocytes. b. Leukocytes (White Blood Cels). – bind, engulf and digest debris. c. Plasma – the solvent. Contains mostly water, proteins, dissolved gasses, nutrients, hormones, waste products … d. Platelets. Responsible for clotting (repair of damaged tissue, plug for hemorrhages…) The clotting process consists of a complex cascade of chemical signals. 3. Lymphatic system: endothelial filtration can produce excess water and other materials, mostly protein: “lymph”. Parallel vascular system that returns this fluid into the veinous system at the junctions of subclavian and jugular veins. Flow is passive, powered by muscle contractions and guided by one-way valves. Proteins go from blood to lymph because of osmotic gradient. Need lymphatic system to return them to the blood. 4. Vasculature : arteries (input to tissue) and veins (output from tissue), capillaries (site of exchange)BME 419/519 Hernandez 2002 b - Vascular Components – Macroscopic level Distribution of Blood through vascular system and maintain pressure, pulsatility, etc. -Components (the different types of vessels) 1. Aorta, arteries : very elastic- they store the cardiac stroke volume until it can flow through the system. As you get further from heart, the pressure waves become more smoothed out (they act as a low pass filter because of their compliance) 2. Arterioles: local control of blood flow by regulating the amount of flow to individual organs through arterial sphincters. They are responsible for maintaining the peripheral resistance 3. Capillaries: Are the site of chemical exchanges between blood and tissue (Oxygen, nutrients, ions …). Pulmonary vessels are where oxygen enters the blood stream. 4. Venules: collect capillary blood and direct it into the veins 5. Veins: Take the deoxygenated blood back to the heart. Very compliant, little resistance (passive). Not much muscle tone. The inner walls have pockets that act as one-way valves (not 100%). Return blood flow is often aided by skeletal muscles’s actions (eg- some people faint after standing for a long time, because leg muscles help the blood return to the heart) 6. Lymphatic system: carries interstitial fluid / excess protein into the venous return so that they can enter the vascular system. Similar to veinous network (not identical, though). Flow is driven by oncotic pressure, muscle movement, contractions of lymphatic vessels, and one way valves. Lyphatic system enters veinous system at the junction of sub-clvian and jugular veins.BME 419/519 Hernandez 2002BME 419/519 Hernandez 2002BME 419/519 Hernandez 2002 a. Vascular components: Microscopic level endothelium, connective tissue, VSM. i). Endothelium : lining tissue (endothelial cells) . lots of functions; -Liner for the interior of the blood vessels. Non reactive to RBCs. -Barrier between blood and tissue. Materials penetrate tissues through pores (4 nm diameter), or junctions between endothelial cells, EXCEPT for brain. “brain blood barrier”. -Passive Transport: sum of the following: - Diffusion (J = D*A* dc/dx = P*S*(Cout – Cin)) - hydrostatic pressure (pushes fluid out of capillaries - strongest) - osmotic pressure (keeps fluid in capillaries). (“Starling Hypothesis”: transport can be calculated as the sum of all of the above.) Æ “leftovers’ accumulate in the interstitial space end get taken up by lymphatic system. - Active (as in pinocytosis) -Angiogenesis – generation of new vessels. (also helps produce VSM) - Produce vasoconstriction/dilation factors that signal VSM to act. (Endothelial Derived Contracting Factor, EDRelaxingF) ii)- Connective Tissues Maintain the vessels in place and provide the structure to support the intra-cascular pressure. - Elastin : polypeptide molecules with spring-like properties, give the tissue its eleasticity. “stretchable” - Collagen Fibers: another peptide. Long, Rigid molecules. It’s used all over the body to give structure. Eg – fascia, tendons … iii) – Vascular Smooth Muscle: a. Functions - Regulate the diameter (-> resistance to flow) of the vessel by constriction / dilation. Regulates where blood flow goes. - There can be one or more layers of it around the vessels. - Wraps around the vessels in a helical pattern. (Remember scwann cells?) - Slow reaction to signals relative to skeletal muscle. - Capable of sustaining contractions for extended periods of time. - b. ContractionBME 419/519 Hernandez 2002 - Passive stretching if increased BP - Different from skeletal muscle: no clearly defined sarcomeres, but still uses “sliding filament” mechanism driven by phosphorylation of cross-bridges. (this is activated by the presence of Ca++ ions in the myoplasm.) - Difference from skeletal muscle: no troponin (this is what forms the cross bridges in skeletal muscle. In skeletal muscle, Ca++ ions trigger cross-bridge cycling of