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Download Cardiovascular system: Blood vessels, blood flow, blood pressure
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Cardiovascular system: Blood vessels, blood flow, blood pressure Outline • 1- Physical laws governing blood flow and blood pressure • 2- Overview of vasculature • 3- Arteries • 4. Capillaries and venules • 5. Veins • 6. Lymphatic circulation • 7. Mean arterial pressure and its regulation • 8. Other cardiovascular regulatory processes Vascular system possesses different mechanisms for promoting continuous flow of blood to the capillaries: Elastic recoil smooth m. regulation of diameter valves Muscular arteries Substances causing contraction in vascular smooth muscle Chemical Physiologic role Source Type NE (a ) Baroreceptor reflex Sympathetic neurons Neural Endothelin Paracrine Vascular endothelium Local Serotonin Platelet aggregation, smooth muscle contraction Neurons, digestive tract, platelets Local, neural Substance P Pain, increased capillary permeability Neurons, digestive tract Local, neural Vasopressin Increase blood pressure during hemorrhage Posterior pituitary Hormonal Plasma hormone Hormonal endothelium local Angiotensin II Increase blood pressure Prostacyclin Minimize blood loss from damaged vessels before coagulation Substances that mediate vascular smooth muscle relaxation Chemical Physiologic role Source Type Nitric oxide Paracrine mediator Endothelium Local Atrial natriuretic peptide Reduce blood pressure Atrial myocardium, brain Hormonal Vasoactive intestinal peptide Digestive secretion, relax smooth muscle Neurons Neural, hormonal Histamine Increase blood flow Mast cells Local, systemic Epinephrine (b2) Enhance local blood flow to skeletal muscle, heart, liver Adrenal medulla Hormonal Acetylcholine (muscarinic) Erection of clitoris, penis Parasympathetic neurons neural Bradykinin Increase blood flow via nitric oxide Multiple tissues Local Adenosine Enhance blood flow to match metabolism Hypoxic cells local Even though there are many mechanisms for altering the radius of the vascular system, pressure still drops as blood moves further away from the heart. Why? Resistance = tendency of the vascular system to oppose flow; Flow1= R • Influenced by: length of the tube (L), radius of the tube (r), and viscosity of the blood (h) Poiseuille’s Law R = Lh/r 4 • In a normal human, length of the system is fixed, so blood viscosity (5x that of water) and radius of the blood vessels have the largest effects on resistance Factors promoting total peripheral resistance (TPR) • Total peripheral resistance = TPR -- combined resistance of all vessels -- vasodilation resistance decreases -- vasoconstriction resistance increases Physical laws governing blood flow and blood pressure • Flow of blood through out body = pressure gradient within vessels X resistance to flow - Pressure gradient: aortic pressure – central venous pressure - Resistance: -- vessel radius -- vessel length -- blood viscosity All four tubes have the same driving pressure. Which tube has the greatest flow? The least flow? Why? Biggest has least friction from blood against vessel walls! Outline • 1- Physical laws governing blood flow and blood pressure • 2- Overview of vasculature • 3- Arteries • 4. Capillaries and venules • 5. Veins • 6. Lymphatic circulation • 7. Mean arterial pressure and its regulation • 8. Other cardiovascular regulatory processes Vasculature Structure of vasculature changes in response to different needs Arteries and blood pressure • Arterial walls are able to expand and recoil because of the pressure of elastic fibers in the arterial wall • Systolic pressure: maximum pressure occurring during systole, ventricles contracting • Diastolic pressure: pressure during diastole, ventricles filling Even with a decrease in overall pressure, the pressure in the vessels is not constant. The pressure in the vessels mirrors the pressures generated in the heart – systolic and diastolic pressures. Systolic = ventricles contracting Diastolic = ventricles filling Normal blood pressure = 120/80 High blood pressure = 140/90 Figure 14.8 Outline • 1- Physical laws governing blood flow and blood pressure • 2- Overview of vasculature • 3- Arteries • 4. Capillaries and venules • 5. Veins • 6. Lymphatic circulation • 7. Mean arterial pressure and its regulation • 8. Other cardiovascular regulatory processes Capillaries • Allow exchange of gases, nutrients and wastes between blood and tissues • Overall large surface area and low blood flow • Two main types: - continuous capillaries: narrow space between cells permeable to small or lipid soluble molecules - fenestrated capillaries: large pores between cells large molecules can pass Local control of blood flow in capillaries • Presence of precapillary sphincters on the arteriole and beginning of capillaries • Metarteriole: no sphincter continuous blood flow controls the amount of blood going to neighboring vessels Movement of materials across capillary walls • Small molecules and lipid soluble molecules move by diffusion through the cell membrane • Larger molecules, charged molecules must pass through membrane channels, exocytosis or in between 2 cells • Water movement is controlled by the capillary hydrostatic and osmotic pressures Outline • 1- Physical laws governing blood flow and blood pressure • 2- Overview of vasculature • 3- Arteries • 4. Capillaries and venules • 5. Veins • 6. Lymphatic circulation • 7. Mean arterial pressure and its regulation • 8. Other cardiovascular regulatory processes Veins • Veins are blood volume reservoir • Due to thinness of vessel wall less resistance to stretch = more compliance Factors influencing venous return • 1- Skeletal muscle pump and valves • 2- Respiratory pump • 3- Blood volume • 4- Venomotor tone Outline • 1- Physical laws governing blood flow and blood pressure • 2- Overview of vasculature • 3- Arteries • 4. Capillaries and venules • 5. Veins • 6. Lymphatic circulation • 7. Mean arterial pressure and its regulation • 8. Other cardiovascular regulatory processes Mean arterial pressure and its regulation • Regulation of blood flow in arteries - Intrinsic control - Extrinsic control -- Neural control -- Hormonal control * Control of blood vessel radius * Control of blood volume Mean arterial pressure and its regulation • Regulation of blood flow in arteries - Intrinsic control - Extrinsic control -- Neural control -- Hormonal control * Control of blood vessel radius * Control of blood volume Regulation of blood flow in arteries • It is important to adjust blood flow to organ needs Flow of blood to particular organ can be regulated by varying resistance to flow (or blood vessel diameter) • Vasoconstriction of blood vessel smooth muscle is controlled both by the ANS and at the local level. • Four factors control arterial flow at the organ level: - change in metabolic activity - changes in blood flow - stretch of arterial smooth muscle - local chemical messengers Intrinsic control of local arterial blood flow • Change in metabolic activity – Usually linked to CO2 and O2 levels (↑ CO2 vasodilation ↑ blood flow) intrinsic control • Changes in blood flow - decreased blood flow increased metabolic wastes vasodilation • Stretch of arterial wall = myogenic response - Stretch of arterial wall due to increased pressure reflex constriction • Locally secreted chemicals can promote vasoconstriction or most commonly vasodilation - inflammatory chemicals, (nitric oxide, CO2) Mean arterial pressure and its regulation • Regulation of blood flow in arteries - Intrinsic control - Extrinsic control -- Neural control -- Hormonal control * Control of blood vessel radius * Control of blood volume Extrinsic control of blood pressure • Two ways to control BP: - Neural control - Hormonal control ** Use negative feedback Neural control of BP - 1 • Baroreceptors: carotid and aortic sinuses sense the blood pressure in the aortic arch and internal carotid send signal to the vasomotor center in the medulla oblongata • Other information are sent from the hypothalamus, cortex • Neural control of BP - 2 • The vasomotor center integrates all these information • The vasomotor sends decision to the ANS center: - Both parasympathetic and sympathetic innervate the S/A node can accelerate or slow down the heart rate - The sympathetic NS innervates the myocardium and the smooth muscle of the arteries and veins promotes vasoconstriction Hormonal control of BP • Hormones can control blood vessel radius and blood volume, stroke volume and heart rate • On a normal basis, blood vessel radius and blood volume are the main factors • If there is a critical loss of pressure, then the effects on HR and SV will be noticeable (due to epinephrine kicking in) • Control of blood vessel radius - Epinephrine - Angiotensin II - Vasopressin (?) • Control of blood volume - Anti-diuretic hormone (vasopressin) - Aldosterone • Control of heart rate and stroke volume - Epinephrine Control of blood volume • Anti-diuretic hormone = ADH - - Secreted by the posterior pituitary in response to ↑blood osmolarity (often due to dehydration) Promote water reabsorption by the kidney tubules H2O moves back into the blood less urine formed Control of blood volume • Aldosterone: - - Secretion by the adrenal cortex triggered by angiotensin II Promotes sodium reabsorption by the kidney tubules (Na+ moves back into the blood) H2O follows by osmosis Whereas ADH promotes H2O reabsorption only (in response to dehydration), aldosterone promotes reabsorption of both H2O and salt (in response to ↓ BP)
