Hemodynamics & Blood

• Distribution of cardiac output

  • heart rate
  • stroke volume
  • pressure difference
  • resistance to blood flow
• stroke volume is dependent on preload, afterload, and contractility
• pressure difference

  • drives the movement or flow of blood
  • greater the pressure difference, greater the blood flow
• Blood Pressure

  • pressure (force) exerted by the blood on the walls of a vessel
  • hydrostatic pressure
  • blood volume decreases > 10%, BP drops
  • water retention increases BP
  • mostly generated by ventricular contraction
• water makes up 90% of the blood
• there is an inverse relationship between cross-sectional area and velocity when it comes to blood pressure

  • as we get deeper in the system, the vessels get smaller, which impacts resistance & decreases velocity
  • the slowing down of blood in this region allows for the exchange of oxygen and removal of CO2
• blood pressure falls steadily in systemic circulation with distance from left ventricle

  • 35 mmHg entering the capillaries
  • 0 mmHg entering the right atrium
• blood pressure is high at the aorta - highest pressure in our system

  • pressure wave is generated from the stretch and recoil of the aorta
  • pressure fluctuates at the start of systemic circulation
• peak pressure is at systolic pressure
• diastolic pressure = when the ventricles are at the relaxation phase
• when taking a BP measurement, after the cuff is released, the first disappearance of the turbulence sounds = Diastolic pressure
• BP measurement = systolic / diastolic = 120 / 80
• Pulse pressure is the difference between systolic and diastolic pressure
• pulse pressure is an indicator of how well your heart can pump blood
• Mean Arterial Blood Pressure

  • average pressure during entire cardiac cycle
  • system is designed to maintain mean ABP
• Pulse Pressure = systolic BP – diastolic BP
• Mean ABP = Diastolic + 1/3(Pulse Pressure)

  Mean ABP = Cardiac Output x Total Peripheral Resistance

  Mean ABP = HR x SV x TPR
• Pulse Points

  1. common carotid
  2. brachial artery
  3. radial artery
  4. femoral artery
  5. popliteal artery
  6. dorsalis pedis artery
• the dorsals pedis helps indicate the level of perfusion and if there are any issues with pulse pressure
• Factors affecting BP

  • cardiac output (HR, SV)
  • blood volume (blood loss, H2O retention)
  • peripheral vascular resistance
• pressure = flow x resistance
• flow = pressure / resistance
• Vascular Resistance

  • friction between blood and the vessel walls
  • blood vessel radius
  • blood viscosity (thickness)
  • blood vessel length
  • arterioles control BP by changing diameter
  • vasoconstriction and dilation
  • systemic vascular resistance (SVR)
  • total peripheral resistance
• Endothelial wall helps create laminar flow
• blood viscosity is dependent on proteins and RBCs present
• Regulation of BP

  • neural
  • short term
  • baroreceptor reflexes
  • chemoreceptor reflexes
  • Hormonal
  • Short term:
  • released from adrenal medulla
  • epi and norepi
  • Long term:
  • works hours to days to restore ABP
  • renin-angiotensin-aldosterone (RAA) system
  • antidiuretic hormone (ADH)
  • atrial natriuretic peptide (ANP)
• Baroreceptors

  • located on carotid region and arch of aorta
  • sense changes in stretch, alter their signals to the CN to the medulla oblongata, to the spinal cord, which cause stimulation
• Chemoreceptors

  • sense changes in chemical composition
  • O2, CO2, H+ (acidosis)
  • signal to medulla oblongata, spinal cord
• syncope (fainting) when standing quickly = orthostatic hypotension
• Regulation of BP by catecholamines

  • suprarenal
  • increase in sympathetic stimulation
  • increase in epi and norepi
  • increase heart rate and force of contraction
  • vasoconstriction: skin, abdominal organs
  • want the blood to go towards areas that need it and away from places like skin, liver, and spleen
  • vasodilation: cardiac muscle, skeletal muscle
  • ramping up our system for fight or flight
• Angiotensin II both constricts vessels and causes aldosterone secretion
• Regulation of BP by ADH (vasopressin)

  • want to drive the pressure back up
  • antidiuretic hormone (ADH)
  • released from posterior pituitary
  • in response to dehydration/decreased blood volume
  • actions:
  • increased renal water retention
  • systemic vasoconstriction
• Regulation of BP by ANP

  • reduced pressure
  • atrial natriuretic peptide (ANP)
  • released from cells in the right atrium
  • in response to atrial distention
  • too much volume and too much stress, ANP is released
  • actions:
  • increased renal loss of salt and water
  • increased systemic vasodilation
  • reduce systemic vascular resistance
  • decrease blood flow
  • decrease pressure
• What determines oxygen delivery?

  1. blood pressure
  2. systemic vascular resistance
  3. cardiac output
  4. stroke volume
  5. oxygen content
• Blood pressure = CO x SVR
• Systemic vascular resistance = (viscosity x length) / radius ^4
• Cardiac output = SV x HR
• stroke volume = preload, afterload, contractility
• oxygen content = partial pressure of oxygen, [hemoglobin]
• What increased O2 demand?

  • increased basal metabolic rate
  • infection/fever
  • increased work of organs (heart & lungs)
  • agitation or pain

  What about the heart?


  Increased workload (SNS, increased preload / afterload, physical activity, stress. etc)