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Functions of the cardiovascular system (heart, blood vessels and blood)

  • Transport
    • Gases – O2 (lungs to tissue) and CO2 (tissue to lungs)
    • Nutrients (e.g. glucose)
    • Fluid and electrolytes (sodium, potassium etc)
    • Wastes (e.g. urea—blood is filtered in kidneys, which produce urea as a waste)
    • Hormones
    • Defense against infection (leucocytes, antibodies)
    • Delivery of platelets to stop bleeding
    • Thermoregulation (hot->increased bloodflow to skin to get rid of heat)

The Cardiovascular system

  • Right hand side: pumps blood out into the lungs
  • Left hand side: pumps blood into the systemic circulation
  • The heart is 2 pumps, but it has 4 chambers (2 atria and 2 ventricles)
  • The blood comes back from systemic into the right atrium, then into the right ventricle, then into the pulmonary artery, then back from the lungs via the pulmonary vein, into the left atrium, then to the left ventricle, then pumped to the system via the aorta
  • Oxygenated blood comes back to the left side of the heart and is pumped out to the body.
  • In the body, the oxygen is used and relatively deoxygenated blood returns to the right side of the heart and is pumped to the lungs for reoxygenation
  • Deoxygenated blood appears more purple and less bright red than oxygenated blood (Hb turns red)

The Heart

  • Valves make the blood flow in the right direction provided they are working properly:
  • AV valves
    • Tricuspid valve: RA -> RV
    • Mitral valve (or bicuspid valve): LA -> LV
  • Other ones:
    • Pulmonary valve: RV -> pulmonary artery
    • Aortic valve: LV -> aorta
  • Lub dub:
    • Lub = tricuspid and mitral valves shutting
    • Dub = pulmonary and aortic valves shutting

Pressure gradient driving blood flow

  • Pressure gradient
    • Drives bloodflow
    • 85 mm Hg coming from the left ventricle
    • 0 mm Hg returning to right atrium
    • The pressure created by the heart pumping makes the blood flow around
    • Similar picture: right ventricle going to the lungs and returning to the left atrium (except the pressures in the pulmonary circuit are lower – “low pressure circuit”)

Vessel types

  • Arteries – transport blood under high pressure; conduit vessels (major highways)
  • Arterioles – resistance vessels (provide resistance to blood flow); control flow to individual tissue beds (contraction reduces perfusion, dilation increases perfusion); hydraulic filter (stops the blood from being intermittent [like the beat of the heart] and instead very steady [due to damping effect of arterioles])
  • Capillaries – exchange vessels (exchange nutrients/O2/CO2 from/to tissues. Thin walls allow exchange to occur easily)
  • Venules/veins – return blood to heart; capacitance vessels (store volume; reservoir). Haemorrhage – veins will contract to bring more blood back to the heart
  • Different structures of vessels:
    • Aorta and artery have quite a lot of elastic tissue in their walls
    • When the heart pumps, they can “give way” a bit to take the extra volume
    • Arterioles have a LOT of smooth muscles in the walls. They constrict or dilate to control the bloodflow to the distal capillary bed. (Even compared to the aorta or arteries, they have a lot of muscle tissue relative to the amount of elastic tissue present)
    • Capillary wall consists of just a layer of epithelium (endothelium) for easy transfer of molecules

Continuity of blood flow despite intermittency of heart beat

  • Systole = contraction of the ventricles (lumen of ventricle contracts)
  • Diastole = relaxation of the ventricles (lumen of ventricle grows)
  • Arteries are compliant and store blood during ventricular systole.
    • In systole, the aorta gets loaded up with blood
    • In diastole, valve closes, but the aorta can squeeze blood further, and so there is continuous bloodflow
  • During diastole, arteries recoil

Pressure in different vessels

  • In the aorta and large arteries, blood pressure is high and pulses
  • Arterioles
    • the pressure variation decreases (because they are hydraulic filters)
    • the pressure drops considerably (since they are resistance vessels)
  • The rest of the circuit is relatively low pressure
  • In the pulmonary circuit, there is pulsing flow, but the pressure is about 1/7 the pressure in the systemic pressure (the lungs have relatively low resistance compared to the systemic circuit)

Microcirculation

  • Consists of arterioles, capillaries and venules
  • Met artierioles are ones that bypass the capillaries between the arterioles and the venules
  • Precapillary sphincters regulate the flow into the capillaries
    • Contract – prevent blood flowing into the tissue
    • Dilate – allow blood to flow into the tissues)
    • E.g. thermoregulation (cold – contract to prevent heat loss from the surface).
    • When they contract, the flow of blood bypasses the tissue and travels through the met arterioles

Blood volume distribution

  • 5 L of blood total
  • 2/3 is in the venous circulation
  • Donate blood (500mL) or haemorrhage – veins will contract and push more blood back to the heart. This keeps everything going around

Effect of gravity

  • Hydrostatic pressure (due to gravity)
    • Standing up: +90 mm Hg pressure in your feet
    • –ve pressure in the head
    • Hence we need a higher blood pressure in the systemic circulation than the pulmonary circulation, to push blood up to your head

Muscle pump

  • Standing up: 90 mm Hg in the leg venules
  • Slow walking: 20 mm Hg in the leg venules
  • Pushed up by pumping of muscles, and the valves in the veins of your leg
  • You can faint if you stand around for too long (all your blood is in the veins in your legs – wiggle your toes)

Cardiac output

  • Amount of blood pumped out of the heart each minute
  • CO = SV * HR
  • Cardiac output = stroke volume * heart rate
  • Stroke volume = volume pumped each beat
  • CO may be altered by altering SV and/or HR
  • 70 kg man at rest:
    • Average cardiac output = 5L/min
    • Average heart rate = 70 bpm
    • Average stroke volume = 70 mL/beat
    • Note: exercise will increase heart rate

Blood flow

  • Distribution of systemic blood flow (at rest) – total = 5000 mL / min
    • Brain = 650 mL (13%)
    • Heart = 215 mL (4%)
    • Muscle = 1030 (20%)
    • Skin = 430 (9%)
    • Kidney = 950 (20%)
    • Abdominal organs = 1200
    • Other = 525
  • Vessel diameter influences blood flow
    • Controls the blood flow to various organs (note that blood flow to each organ is NOT dependent on the size of the tissue)
    • Narrow diameter = higher
    • Flow = k (radius)^4
    • Resistance of a blood vessel is inversely proportional to the fourth power of the radius of the bloodflow
    • D=1; flow =1
    • D=2; flow = 16
    • D=4; flow = 256
    • Bits in the centre of the blood vessel flow faster than the bits near the wall (see physics from first year).
    • Velocity field is a parabolic dish

Calculating resistance

  • Resistance = impediment to blood flow in a vessel
  • Can’t be measured directly
  • F = ∂P/R
  • R = ∂P/F
  • ∂P = F*R (think of V=IR)
    • F = flow
    • ∂P = change in pressure (voltage)
    • R = resistance
  • Systemic circulation
    • Flow = 100 mL/sec
    • Mean pressure = 100 mmHg
    • Resistance = 100/100 = 1 PRU (peripheral resistance unit)
  • Pulmonary circulation
    • Flow = 100 mL/sec
    • Mean pressure = 14 mm Hg
    • Resistance = 14/100 = 0.14 PRU

Cross sectional area

  • Systemic- the sum (in cm^2) across the WHOLE CIRCULATION – even though each capillary is small, there are many capillaries
    • Aorta = 4.5
    • Arteries = 20
    • Arterioles = 400
    • Capillaries = 4500
    • Venules = 4000
    • Etc (gets smaller going back towards the heart)

Pressure, velocity and cross sectional area in the systemic circulation

  • Cross sectional area (CSA) area gets big in the middle
  • The velocity of blood flow is a mirror image of the CSA (blood flows slowly in capillaries … good because capillaries are exchange vessels)
  • Pressure decreases from arteries to capillaries to veins

Flow vs velocity (Bernoulli equation)

  • Flow = ∂V/∂t
  • Velocity = ∂s/∂t for each particle
  • Flow in = Flow out
  • But volume = A∂s
  • Hence if the CS area of the tube is high, then it moves slower
  • If the CSA of the tube is low, then it moves faster

Measuring blood pressure

  • Sphygmomanometer (blood pressure cuff)
    • Put a cuff in the upper arm
    • Listen in the cubital fossa with a stethoscope
    • Listen for sounds as the blood flow changes
    • Pump the pressure up in the cuff, and gradually let it off:
      • Pump up: pressure in cuff is higher than pressure in the artery, then the artery is squashed and nothing comes through
      • When you let the pressure get a bit lower than the systolic pressure, you’ll hear a sound (turbulence as the blood is whooshing around – you’re not below the diastolic pressure)
      • When you keep letting the pressure down, eventually you can’t hear anything (there is laminar blood flow)
      • 120/80 means first started hearing at 120 (systolic pressure), stopped hearing at 80 (diastolic pressure
  • If you need to monitor it closely (in ICU) you stick a needle in their artery
  • Shape of the arterial pressure wave
    • Mean pressure = Diastolic pressure + (1/3)*(Pulse pressure)
      • Pusle pressure = systolic - diastolic
    • Comes from integration
      • You spend a lot more time closer to diastolic pressure, so you don’t have an arithmetic mean
    • Mean pressure depends on:
      • Cardiac output
      • Peripheral resistance
    • Pulse pressure depends on:
      • Stroke volume
      • Arterial compliance

Baroreceptors

  • 2 sets: aorta and carotid
  • Stretch receptors (high pressure – lots of stretch; low pressure – less stretch).
  • Send pressure information up to the brain
  • Brain then controls heart rate, stroke volume and vessel diameter to keep the blood pressure relatively constant

Basic theory of circulatory function

  • Each different organ/tissue has the amount of bloodflow it needs
  • Sum of all these different bloodflows equals the cardiac output
  • Each individual tissue controls how much blood it gets
  • But overall, the blood pressure is controlled
  • So we control our blood pressure independent of the organs

Blood flow control

  • Acute vs long term
  • Long term involves alterations in the size and number of vessels
  • Acute:
    • Local
      • High metabolism -> blood flow goes up
    • Neural
    • Hormonal
  • Reactive hyperaemia
    • Restrict bloodflow (e.g. blood pressure cuff) then the restricted areas produce a lot of metabolites
    • These metabolites signal your body to increase blood flow to that area
    • Also occurs in exercise (contracting/exercising muscles can actually cut off the blood flow while contracting)
  • Nerves and hormones can also affect blood flow (learn more later on)
  • Control in different tissues
    • Things that use up lots of energy tend to have a lot of local factors
  • In exercise:
    • Skin bloodflow goes up to dissipate heat
    • Muscle bloodflow goes up to exercise
    • Brain stays about the same
    • Abdominal organ bloodflow goes down (don’t need to digest food when running)
  • Neural Control:
    • Sympathetic nervous system
    • Parasympathetic nervous system
  • Humoral control:
    • Vasoconstrictors
      • Adrenaline, Noradrenaline
      • Angiotensin II
      • Vasopressin (ADH)
      • Endothelin
    • Vasodilators
      • Bradykinin
      • Histamine
  • Control in different tissues:
    • Neural regulation predominates
      • Splanchnic
      • Skin
    • Local factors predominate
      • Vital tissues - brain, heart
      • Exercising muscle