--- module: 008-01 language: en chapter: 8 title: "Haemodynamics, Vascular Control, and Blood Pressure" module_title: "Foundations" source_sha256: 72e29cd7bd3ba911683ba9b4674cb09f700c8d3f5f04a1be915a54af7f9d8d7f --- # Haemodynamics and vascular control ## What the circulation must do ### Deliver oxygen, nutrients and heat, clear waste ### Match flow to need at adequate pressure ### Arteries buffer, arterioles set resistance ### Capillaries exchange, veins store and return ### Lymphatics return fluid and macromolecules ## Pressure, flow, and resistance ### Flow equals pressure gradient divided by resistance ### Resistance rises with length and viscosity ### Resistance falls with radius to the fourth power ### Series adds resistance, parallel beds divide it ### Velocity equals flow over total cross-sectional area ### Vast capillary area slows transit for exchange ### Turbulence needs speed, bore and abrupt narrowing ### A silent examination does not exclude disease ## Arterial pressure and pulsation ### Pulse pressure is systolic minus diastolic ### Mean pressure: diastolic plus one third of pulse pressure ### Mean pressure from output and vascular resistance ### Cardiac output equals rate times stroke volume ### Pulse pressure from stroke volume and compliance ### Stiffening raises pulse-wave velocity, widens pulse pressure ### Elastic recoil smooths ejection into steadier flow ## Arterioles and local control ### Calcium activates myosin light-chain kinase ### Relaxation: less calcium, more phosphatase ### Alpha one constricts, beta two dilates muscle beds ### Metabolic dilators #### Low oxygen, high carbon dioxide and hydrogen ions #### Potassium, adenosine and other metabolites ### Myogenic response to stretch stabilises flow ### Reactive hyperaemia after a released occlusion ### Active hyperaemia with rising metabolism ## Endothelial signalling ### Senses shear stress, hormones and inflammation ### Nitric oxide raises cyclic guanosine monophosphate ### Prostacyclin dilates and inhibits platelets ### Endothelin constricts ### Dysfunction loses dilator and antithrombotic cover ## Capillary exchange and lymph ### Diffusion carries most solute #### Lipid-soluble through membranes, water-soluble through clefts #### Rate follows area and gradient, falls with distance ### Hydrostatic pressure filters, oncotic pressure absorbs ### Glycocalyx limits venous-end reabsorption ### Oedema when filtration outruns lymph return #### Raised venous or capillary pressure #### Low albumin or leaky capillaries #### Blocked lymphatics or sodium retention ### Valves, muscle and breathing propel lymph ## Veins and venous return ### Compliant capacitance vessels hold most volume ### Small tone changes shift blood centrally ### Return follows the gradient to the right atrium ### Venoconstriction, muscle pump, breathing and valves ### Standing pools blood below the heart ### Immobility promotes stasis and thrombosis ## Congestion traced back to an organ ### An organ must discharge blood as well as receive it ### Raised venous pressure narrows the gradient ### Congestion favours interstitial accumulation ### High central venous pressure tracks renal decline ### Association is not proof of a single cause ### Ask whether blood cannot arrive, cannot leave, or both ## Pressure control, fast and slow ### Baroreceptors in carotid sinus and aortic arch ### Rising pressure: more vagal, less sympathetic outflow ### Resetting limits long-term baroreflex control ### Chemoreceptors answer low oxygen and acidity ### Long-term control rests on sodium and the kidney ### Renin: low perfusion, sympathetic drive, low macula densa sodium ### Angiotensin two constricts and retains sodium ### Aldosterone trades sodium for potassium ### Antidiuretic hormone holds water and constricts ### Natriuretic peptides favour excretion and dilation ## Hypertension, hypotension and measurement ### Hypertension is multifactorial and usually silent ### Injury to brain, kidney, retina and heart ### Secondary clues: young, abrupt, resistant, hypokalaemic ### Hypotension matters when perfusion fails ### Causes: volume, pump, dilation, obstruction, autonomic failure ### Cuff size, support, rest, position and repeats ### Home and ambulatory monitoring find masked patterns