Haemodynamics

Question 1

What are haemodynamics?

Answer 1

• The branch of phgy dealing with the forces involved in the circulation of the blood
• the circulation and movement of blood in the body and the forces involved therein

Question 2

What are the five components of haemodynamics?

Answer 2

• volume, flow, pressure, resistance, compliance

Question 3

What is the average blood volume of a person?

Answer 3

• 5 L or about 75 mL/kg
• Number based on the “reference man” of the 1950s which refers to a healthy young man in his 20s living in the 1950s who weighs 70 kg and has not had a meal in a minute and is not exercising but is laying flat on his back

Question 4

What is a unit of blood?

Answer 4

• 450 mL, about 10% of the blood volume, amount given at blood donations

Question 5

What is a cc?

Answer 5

mL = cm3 = cc (cubic cm)

Question 6

Where is the blood?

Answer 6

• 61% is in the veins, so that’s where you wanna get blood, it is why the venous system is referred to as the capacitance vessels (a subset of the arterial system is known as the resistance vessels)
• About 10% in arterioles and capillaries, about 10% in arteries, about 10% in heart and about 10% in the pulmonary circulation

Question 7

What is a stroke volume and what is it in our “reference man” of the 1950s?

Answer 7

The amount of blood that is ejected from the right ventricule and the left ventricule as they are bothe the same and the heart rate is the same for both as well.

70 mL

Question 8

What is cardiac output and what is it in our “reference man” of the 1950s?

Answer 8

The stroke volume x the amount of strokes per minute (heart rate)
Standard heart rate about 60-70 bpm
5 000 mL/min is cardiac output from ventricle and is also the venous return in atrium
Flow in must be equal to the Flow out

Question 9

How do you measure Flow? How do you normalize it?

Answer 9

Flow = V/T (units: mL/min or L/min)
The blood flows in the lumen of the vessel
Normalized flow is mL/min/100g, we divide the flow by the mass of the organ to make a reasonable comparison of flow between different organs of different sizes

Question 10

What is another way to calculate flow?

Answer 10

Flow is also equal to the cross sectional area multiplied by the velocity of the fluid. This gives the volume moved per second in cm3/sec which we can determine using the cross sectional area x velocity because the velocity gives the distance moved in one second which also happens to be the height of the “cylindre” of blood that has moved (refer to image slide 23 CVS)

Question 11

What is a complication of the alternative way of calculating flow?

Answer 11

Velocity is not necessarily the same at all points in a cross section, so we can actually calculate flow as the cross sectional area multiplied by the mean velocity

Question 12

What are the major types of blood vessels?

Answer 12

• Capacitance
• Exchange
• Resistance
• Distribution

Question 13

What are the capacitance vessels?

Answer 13

• Veins and venules
• merging of vessels, venules, to get bigger and bigger until make veins until vena cava, two vena cavas (superior from top of body and inferior from bottom of body) go right into the right atrium to then go into lungs for gaz exchange

Question 14

What are the exchange vessels?

Answer 14

• capillaries which are smaller in size and shorter in length than aorta and other arteries, they have a very thin wall and the concentration gradient helps

Question 15

What are the resistance vessels?

Answer 15

• Arteriole and small artery, shorter and smaller, smaller cross section, more friction, more heat generated, more resistance

Question 16

What are the distribution vessels?

Answer 16

• brings blood to all organs in body
• Aorta and large artery (NOT resistance vessels)
• Large arteries are the arteries that come directly off of the aorta

Question 17

What are the number and dimensions of vessels?

Answer 17

• Most animals have 1 aorta but some can have two aortas, it is the longest, biggest diameter and the thickest wall because you wouldn’t want it to bursts
• From biggest vessels to smallest vessels there are WAY more smallest vessels, smallest diameters, smallest length, and smallest thickness like capillaries are only like two cells thick, endothelial layer and an epithelial layer, for exchange right, but it is more easy to burst and this causes bruises

Question 18

How can we observe branching of vessels?

Answer 18

• Can make an arterial or a venous cast by replacing blood with a different fluid that can solidify and then use enzymes to dissolve the organ just to observe simply where the blood passed
• abdominal aorta separates into two renal arteries which continue to branch into smaller arteries and capillaries, any cell in a kidney is very close to a capillary yay

Question 19

What is the relation between cross sectional area and flow velocity?

Answer 19

Flow = area x mean velocity
- highest total cross sectional area at the level of the capillaries (5000 cm2 total cross sectional area of all capillaries in the body
- THING S that the sum of flow in dif branches must be equal to the flow in aorta unless stabbed and losing blood SO if the flow is the same and the area increases at level of capillaries then the velocity must decrease (cuz hello formula)
- Total flow through any vessel at a specific level of branching is always the same

Question 20

What are the advantage of a branching network?

Answer 20

• Any cell very close to a capillary, big concentration gradient since the distance is smaller and flux/flow can be much bigger
• a high total area of the walls of the capillaries even though shorter and smaller and this allows larger area for exchange to occur => bigger flux/flow by Fick’s law (wall area is the area referred to in Fick’s law)
• a low blood flow velocity in the capillaries since the area increase and helps to give more time to diffuse into cells and waste diffuses out of cells
• a high total cross sectional area so resistance falls and flow can remain constant (not fall too much) making it easier to move even at a low level of pressure, we can get away with having a smaller arterial pressure and a smaller heart

Question 21

What is the formula of pressure?

Answer 21

Pressure = force/area

Question 22

What is the SI unit of pressure and what are practical units of pressure?

Answer 22

• pascal (Pa) = newton/m2
• cm H2O and mmHg bcs 1st instruments to measure pressure used columns of water and mercury
• arterial bp is 120/80 mmHg and central (particular place in body) venous pressure is 5 cm H2O

Question 23

How can we have pressure but no flow? How can we restore flow?

Answer 23

We can squeeze a saline bag with a tube coming out of it but with a stopper so the fluid is enclosed, no place to flow to but there is pressure bcs squeezing saline bag and it is transmitted the same everywhere in the bag and tube. Furthermore since no flow and area is the same and force is pressure x area then this mean the net force is zero so obvi no flow. Pressure has to be the same on both sides so pressure is acting in all directions at all times.To restore flow we can remove the stopper or use a pin to rip the tube and liquid will squirt out with kinetic energy stored where? We’ll work was done on the system to pressurize it (squeezing bag) and pressure energy is stored in the system. Wherever one rips tube the liquid will squirt in that direction further proving that pressure does act in all directions at all times. As the fluid flows out though, we lose pressure energy until there is none and the flow stops.

Question 24

What is the pressure gradient in the tube of saline bag when squeezed? Why?

Answer 24

• Longitudinal pressure gradient if beginning 10 cm H2O, halfway will be five and end will be 0, falls in a linear way as you move down the tube
• difference in pressure but no difference in area so net force however, no acceleration of flow along the tube, all fluid flowing moves at same speed, because actually net force is 0 due to internal fluid friction, viscous losses produce heat which comes from the pressure energy
• real reason for longitudinal pressure gradient because loss of pressure energy due to flow and heat loss

Question 25

What is the pressure like down the systemic vascular tree?

Answer 25

• well the pressure in the arterial system comes from the contraction of ventricules, so really the pressure oscillates up and down with time but for the plot is is best to take the mean pressure over time in the vessel
• pressure falls continuously due to heat loss and branching
• in fact biggest drops in pressure occur from small arteries to arterials and from arterioles to capillaries where lots of branching is occurring though flow must remain the same and this is because resistance is high which allows to keep the flow the same in all categories of vessels

Question 26

All arteries are resistance vessels.

Answer 26

I SWEAR TO GOD WOMAN NOT ALL JUST ARTERIOLES AND SMALL ARTERIES, LARGE ARTERIES ARE PART OF DISTRIBUTION VESSELS

Question 27

Differences between pulmonary and systemic circulation?

Answer 27

Well both pulsatile early on, but then oscillations damp out (we take the mean for early on but then in veins and venues, not much oscillation and “mean pressure” is actually just the real pressure)
Mean pressure is about 100 mmHg in systemic while only about 15 mmHg in pulmonary because the ventricle is smaller in size and mass, in muscle, both eventually drop though because always the heat loss to consider
- systemic pressures are greater than pulmonary pressures always

Question 28

Why do we measure pressure in cm H2O?

Answer 28

• because of hydrostatic pressure
• and the following calculation so volume = area x height = Ah (works when a cylinder is right, not tilted)
  m = mass = density x volume = rhoV=rhoAh
  F = force = mass x acceleration due to gravity = mg = rho Ahg
  Pressure = pressure = force per unit area = F/A = rhogh but then density of water pretty much the same as blood and gravity is always the same everywhere on Earth, so the only variable that really matters for pressure calculation is the height (can change depending on standing, sitting, laying down) => units are cm H2O

Question 29

What is hydrostatic pressure?

Answer 29

The pressure exerted by a fluid at equilibrium (standing still, not moving) at a given point within the fluid, due to the force of gravity so in outer space where gravity is zero, hydrostatic pressure is also zero

Question 30

What is the pressure gradient like with hydrostatic pressure?

Answer 30

If cylinder is 10 cm high
- Top of fluid => 0 cm H2O
- Midpoint fluid => 5 cm H2O
- bottom fluid => 10 cm H2O

Question 31

Difference between weight and mass?

Answer 31

Weight: force downwards with which a mass is pulled down by gravity
Mass: amount of stuff

Question 32

Difficult question: sometimes when people get up too fast, they faint, this is due to hydrostatic pressure. For brain surgery, it is important the the head is tilted in a particular way to maintain good hydrostatic pressure in order to not kill the patient, what is the orientation and why?

Answer 32

• Good lord I do not know

Question 33

What is atmospheric pressure? What is its significance for the CVS?

Answer 33

• Any kind of fluid has a hydrostatic pressure and for the air, atmosphere, it is atmospheric pressure which is of 760 mmHg.
• Technically when measuring the hydrostatic pressure we should account for the atmospheric pressure as well so the pressure at top of fluid is not actually 0 it is atmospheric pressure and at the bottom it is the h cm H2O + the atmospheric pressure. SInce it is present at both points we can arbitrarily pick the top of the fluid as 0 and disregard atmospheric presure at least in the CVS, cannot do that for like the respiratory system
• The pO2 in atm is around 150 mmHg while in arterial blood is about 100 mmHg which works perfectly for respiration (Yay concentration gradient)

Question 34

How did we first measure arterial bp?

Answer 34

• Using a horse that was dying and stuck a glass hollow tube into a neck artery. Blood squirts into the tube because pressure in the tube is atm or “0” and arterial pressure is quite high.
• They can then measure how high it goes when it stops moving (since hydrostatic pressure is pushing down and at some point will be equal to the arterial pressure) and have the pressure in cm H2O since density of blood is pretty much the same as density of water
• The got 280 cm H2O but that is absolutely bonkers to measure so they instead started using Hg which was about 14 x more dense so 1 cm Hg was equivalent to the pressure by 14 cm H2O and then measured it in mm Hg which gave approximately 200 mmHg

Question 35

How does a mercury sphygmomanometer work? Do we still use this tool?

Answer 35

• To measure pressure we can either have a direct method (puncturing an artery) or an indirect method (not puncturing the artery). This is the mercury sphygmomanometer. So the pump pumps air into the bp cuff and air has nowhere to go so no flow just building up of pressure and the bag is connected to a tube that connects to a mercury reservoir. As pressure builds up the Hg moves up column until it no longer can because pressure is equal and this is the arterial bp.
  1mm Hg = 14 mm H2O = 1.4 cm H2O = 0.13 kPa
• No longer use Hg column bcs it’s a neurotoxin, we’ve replaced it by an aneroid gauge.

Question 36

How can we measure central venous pressure or right atrial pressure?

Answer 36

• We use a saline bag, a tube (MANOMETER) and a catheter that goes into the right atrium to measure right atrial pressure (RAP) or, if we move the catheter into the superior vena cava (SVC), the central venous pressure (CVP) (since the SVC is one of the central veins in the chest)
• Dangerous, can harm the patient
• Uses a three way switch all 3 ways are primed and full of fluid. Switch turned in such a way that flow with saline bag is blocked but flow between manometer and catheter is permitted. If saline is high enough in the manometer, high hydrostatic pressure which is higher than the RAP or CVP so the saline flows into the catheter and when it stops it is equal to the RAP or CVP.

Question 37

What is the normal CVP?

Answer 37

About 5 to 10 cm H2O and when concerted to mm Hg we see that it is much smaller than the arterial bp

Question 38

What is perfusion pressure? Do we take atmospheric pressure into account?

Answer 38

• Pressure that controls flow in an organ which obvi depends on both the inlet pressure and the outlet pressure. A perfused organ is an organ that has blood flowing in and out of it
• Formula is perfusion pressure = inlet pressure - outlet pressure or deltaP(difference of pressure) = Pin - Pout
• We can disregard atmospheric pressure that would be added to the inlet and outlet pressure because it cancels out and does not really contribute to flow since what controls flow is the difference in pressure (again just cancels out)

Question 39

More precisely, what is the perfusion pressure? What is this inlet pressure? the outlet pressure?

Answer 39

Perfusion pressure = arterial pressure - venous pressure
deltaP = Pa - Pv
But normally Pa is so much bigger than Pv (average about 100 mm Hg vs 5 mm Hg) so Pv is quite negligeable and we can say that DeltaP is rather equivalent to Pa (in general on average) and Pa provides the pressure to perfuse the organs (main goal of the CVS) so it is the most significant for us

Question 40

What happens if there is no perfusion pressure?

Answer 40

• Well if perfusion pressure is 0, Pin=Pout, deltaP = 0 and flow in organ will be zero. Flow is directly proportional to deltaP.

Question 41

Prove that flow is actually directly proportional to perfusion pressure?

Answer 41

Well if Pin is 100 mm Hg and Pout is 10 mm Hg deltaP is 90 mm Hg and even if we raise it 500 mm Hg and 410 mmHg well deltaP is 90 mm Hg. The flow before and after the overall pressure increase in ml/min will be exactly the same since flow depend on perfusion pressure and this remains the same, plus the properties of the blood and vessel are exactly the same.
Flow = perfusion pressure/resistance

Question 42

What is resitance?

Answer 42

Resistance = perfusion pressure/flow
It is a defined quantity by two other variables, we cannot directly measure it. We must measure perfusion pressure and flow to determine resistance. The units are mm Hg min per mL (so odd).

Question 43

Why do we say that flow is laminar?

Answer 43

A lamina is a plate or a layer. We can think of blood as multiple cylinders within larger cylinders. The cylinder of blood closes to the centre will flow the quickest and falls as it moves closer to the wall. There are infinite plates or layers, it is a continuous process. Therefore, there will be a smooth distribution of change in velocity from the wall to the midline of the vessel so we may instead picture it as a parabolic flow too (refer to slide 47 of CVS). It is the sliding of the laminae over one another that leads to frictional/viscous losses and the generation of heat.

Question 44

What is the main consequence of frictional losses in a viscous flow?

Answer 44

Generation of heat and the fall in pressure down the vessel!

Question 45

What is Poiseuille’s law?

Answer 45

Resistance really depends on three variables: viscosity (v, “nu”) (internal friction which is only valid when laminar flow), length of the vessel and the radius of the vessel. The formula would be:
R = 8piVL/A^2 (so for cylindrical vessel where blood is flowing) => 8VL/pi*r^4

Question 46

How is vessel resistance controlled in the body?

Answer 46

• Through the smooth muscles in the walls of the vessels (all except capillaries)
• State of contraction of the smooth muscle depends on local metabolites (like waste products), innervation and hormones
• The only variable that can really be controlled to control vessel resistance, not length, not viscosity, simply the radius
• R is proportional to 1/r^4, this 4 exponent makes it so that very small changes in the radius cause big changes in the resistance

Question 47

What can be said of the resistance of vessels or organs in series?

Answer 47

Well the perfusion pressure of the individual organ or vessel is deltaP = flow x resistance and you can add the perfusion pressures of both organs or vessels in series since the overall perfusion pressure is the sum of the individual perfusion pressures, it is the flow that is the same for both, and find that total resistance is equal to the sum of the resistance of each one. So, total resistance is always bigger than individual resistance. This is not ideal, because then the heart would have to work quite hard to perfuse everything.
(Do the math you’ll see, or refer to slide 52)

Question 48

What can be said about vessels or organs in parallel?

Answer 48

Well the perfusion pressure of the individual organ or vessel is still deltaP = flow x resistance. About vessels or organs in parallel, it is the perfusion pressure that is equal for both and the flow that changes. By adding the flows knowing that perfusion pressure will stay the same, we see that 1/total resistance is equal to 1/R1 + 1/R2 such that combined R is always smaller than individual Rs. This aligns with the concept that as we continue to branch, overall R goes down so flow can stay high enough to perfuse and do exchanges and we can get away with having a smaller heart that does not need to overwork itself to perfuse all organs.

Question 49

What is the concept of compliance? What vessels are usually associated with this property?

Answer 49

Veins are the capacitance vessels. They contain like 60ish% of all the blood in the body. This is mainly due to their wall. They have a very thin wall to accommodate more blood, bigger lumen. They have a smaller resistance, the pressure does not fall as much to get same-ish flow as arterial system. The perfusion pressure must be higher in the arterial system to get the same flow. In conclusion, the vein is more “stretchy”, it is more compliant. You do not need to increase pressure by a lot for the vein to accommodate the extra volume (not the case for arteries).

Question 50

Why are the walls of the veins thinner?

Answer 50

Arterial vessels have far more layers of smooth muscle and connective tissue.

Question 51

What is compliance mathematically?

Answer 51

Compliance = DeltaV (change in volume)/ DeltaP (change in pressure)
When plotting a P/V plot which is transmural pressure in mmHg over Volume, we see that a small change in volume drastically increases pressure in an artery but only slightly changes pressure in a vein that is more compliant. In such a plot the compliance is 1/slope (if low slope, high compliance, if high slope, low compliance).

Question 52

Other than vessels, what is compliance important for?

Answer 52

Also for the heart chambers that have a certain compliance
As you get older, your heart gets fibrotic, less stretchy, therefore it takes a higher pressure to fill the ventricles with a certain volume. There is a limit to the pressure, so less volume is in the ventricles, the stroke volume decreases, the cardiac output decreases and this is heart failure.