1 Haemodynamics Flashcards
Define and describe haemodynamics
The physical laws governing pressure/flow relationship in blood vessels
- blood will not flow through the vasculature unless it is forced to do so by application of pressure, which is required to overcome
Describe flow resistance and how this resistance is created
- RBCs increase flow resistance by rubbing up against the vessel wall
- RBCs trail through capillaries measuring 2.8 um in diameters (smaller than their size)
- RBC’s readily distort, allowing them to slip through narrow vessels + centre of the vessel
Even so, interactions between RBC and vessel wall create a resistance to flow that the heart perceives as after load
Define afterload
Afterload
- is the pressure that the heart must work against to eject blood dying systole.
- it is proportional to the average arterial pressure
Describe what determines the flow of blood
Flow = (change in pressure)/resistance
Pressure = pressure gradient between arteries and veins
- it is created by the pumping action of the heart
- flow is directly related to pressure difference (Pin - Pout),
- greater the difference the greater the flow
R - resistance
- a meagre of the degree to which the tube (vessel) resists the flow of liquid through it
- flow is inversely related to resistance
Also:
- internal radius of vessel
- length of vessel
- viscosity of blood
Whereas the pressure difference drives the flow, radius, length, and viscosity all contribute to flow resistance and hence total flow
Describe how vessel radius is directly related to flow
Vessel radius is the primary determinant of vascular resistance and hence flow.
- Radius is also a variable because the vascular smooth muscle cells (VSMCs) that make up the walls of small arteries and arterioles contract and relax as a way of controlling flow
As flow is proportional to r^4, a 2-fold change in radius causes. 16-fold change in flow
Describe how vessel length and flow are inversely related
Blood flow through a vessel is inversely related to vessel length
- again reflecting blood’s tendency to interact with the vascular endothelium
Describe how blood viscosity and flow are inversely related
- Blood is a complex fluid whose viscosity varies with flow
- Cells, principally RBC’s have the greatest impact, with viscosity rising at a greater-than-exponential rate with haematocrit
Define and describe Haematocrit
Haematocrit (Hct) measures the percentage of whole blood volume is occupied by RBC’s
- normal values for Hct range between 41-53% for males and 36-46% for females
Describe Systemic Vascular Resistance/Total peripheral resistance (TPR)
SVR = (MAP-CVP)/CO
SVR - systemic vascular resistance
MAP - mean arterial pressure
CVP - central venous pressure
CO - cardiac output
The MAP - CVP represents the pressure difference between the aorta and vena cava
Describe MAP (mean arterial pressure)
MAP
- time-averaged value that recognises the arterial pressure rises and falls in step with the cardiac cycle
MAP = (CO x SVR) + CVP
MAP can also be worked out by taking a BP
MAP= DBP + ([SBP - DBP]/3)
Describe large arteries, it’s roles + functions
This is the arterial system comprising of a network of narrow-bore distribution vessels
- Arteries must carry blood at high pressure, so their walls are thick and lumens (proportionally narrow)
- The walls of the larger arteries (also elastic arteries) contain smooth muscle layers and are rich in elastin fibres
- The muscle layers have.a resting tone, which limits the arterial distensibility and helps maintain the pressure of the blood within
Describe muscular arteries, as well as roles + functions
Most named arteries
- well developed tunica media, lots of smooth muscle
- Prominent internal and external elastic lamina
Describe small arteries and arterioles, their roles + functions
- Walls of the smallest arteries and arterioles are dominated by their smooth muscle layers
- Collectively known as resistance vessels, they act as stopcocks to control blood flow to capillaries
- When a tissue’s demand for O2 and nutrients is high, the vascular smooth muscle cells (VSMCs) relax and flow to the tissues increase
Describe capillaries, their roles, and their functions
They bring blood to within 30um of virtually every cell in the body
- they are designed to keep the blood contained within the vasculature while simultaneously maximising the opportunity for the exchange of material between blood, interstitium and tissues
- Their walls are the thickness of a single endothelium cell + basal lamina
- Depending on tissue type, capillaries permit direct communication between blood and cells via junctional clefts between cells (fenestrations)
Describe veins and venules, along with their roles/functions
Veins and venules are low-pressure conduits that direct blood back to the heart.
- Smaller venues are almost indistinguishable from capillaries, which allows them to participate in fluid + metabolite exchange
- Venules widen and fuse with each other as they progress toward the heart. Large venules contain VSMC’s (vascular smooth muscle cells), but far fewer than those seen in vessels of equivalent size in the arterial system
- The paucity of muscle means that vein walls are thin, making them highly distensible and able to accommodate large volumes of blood
> at rest, around 65% of total blood volume resides in the venous compartment, creating a reservoir that is used to boost ventricular preload and cardiac output (CO) when the need arises
