Haemodynamics Flashcards
What is haemodynamics?
- The study of physical and physiological principles governing the movement of blood through the circulatory system
- Explains the physical laws that govern the flow of blood in the blood vessels
The circulatory system
Function of circulation
How local blood flow is controlled
Function of circulation: supply nutrients to tissues and remove waste products
- Tissue blood flow needs to be maintained by ensuring adquate cardiac output
- Local blood flow is controlled by arterioles which function like a tap
Factors affecting heart rate
Atrial reflex
1. Autonomic innervation
2. Hormones
Heart rate
Factors affecting stroke volume
- Venous return,Filling time,Autonomic innervation,Hormones,Vasodilation or Vasoconstriction
- Preload, Contractility, Afterload
- End-diastolic volume, End-systolic volume
Stroke volume equation
Stroke volume=end diastolic volume-end systolic volume
Cardiac output
Equation
Defintion
The total blood flow out of the heart (litres/min)
- volume of blood pumped from each ventricles per minute
- Cardiac output=heart rate x stroke volume
Blood pressure in vessels
p=f/a
Blood pressure:force that the blood exerts on the surface area of the walls of the blood vessels
- Differences in blood pressure throughout the body keep blood flowing from high-pressure areas to low-pressure areas
Systolic Blood pressure
Diastolic blood pressure
Pulse pressure
Definition
Range
- Pressure during systole (maximum pressure in arteries, 120mmHg)
- Pressure during diastole (minimum pressure in arteries, 80mmHg)
- Difference in pressure between systole and diastole
Mean arterial pressure
Definition
Range
Equation
- Average arterial pressure during the cardiac cycle
- Considered to be the perfusion pressure of tissues and organs
- A MAP of >60mmHg is rquired to sustain organs of a healthy person
MAP=2xDBP+SBP/3
=1/3 SBP+2/3 DBP
=1/3 PP+ 2/3 DBP
2 DBP: 1 SBP (because most time is spent on the DBP)
Why do we need high pressures?
If the blood pressure is high, then the flow through a particular organ can be regulated by relaxing or constricting its input arterioles. (vasoconstriction,vasodilation)
Does high pressure not rupture vessels?
- Blood vessels are designed in such a way that can withstand significantly higher pressures that those generated in normal conditions
- The pressure vessels can withstand is determined by the vessel wall properties and the radius of the vessel
- Vessels damaged by high blood pressure can rupture or leak
Laplace’s Law
What Laplace’s law is
How radius is connected to withstand
- Laplace’s law states that the pressure that an elastic vessel can withstand depends on the wall tension and the radius of the vessel
- The smaller the radius of a vessel,the greater the pressure tha a given wall strength can withstand.
- Thus smaller diameter arterioles only need thin walls to withstand normal arterial pressures
What is an aneurysm?
How aneurysm is caused
What happens in aneurysm
- If an artery wall weakens its radius increases and so the pressure it can withstand decreases
- The wall balloons out and this further reduces the effectiveness of the wall to withstand the pressure
- Eventually an aneurysm may occur and the vessel may rupture
What is blood flow?
Definition
Why blood pressure is different
- Volume of blood that flows through a vessel or an organ over some period of time
- Differences in blood pressure throughout the body keep blood flowing from high-pressures areas, like the arteries, to low-pressure areas, like the veins
- Anlagous to hydraulic circuit
- Flow=volume/time
Ohm’s Law
Equation and what each symbolises in systemic ciruclation
What the equation means
- When a fluid is pumped through a closed system, its flow (Q) is determined by the pressure developed by the pump (P1-P2 or diff in P), and by the resistance to that flow
F=P1-P2/R - The flow equation applies to the systemic circulation
F=CO
P1=Mean arterial pressure (MAP)
P2=Right atrial pressure (RAP)
R=total peripheral resistance (TPR)
Therefore:
CO=MAP/TPR
CO x TPR=MAP
Resistance to flow and its determinants
Vascular resistance: as blood flows, it encounters various factors that resist flow and movement of blood
1. Blood viscosity decrease, resistance decrease
2. Vessel length decrease, resistance decrease
3. Vessel radius decrease, resistance increase
less tension and the radius of the vessels increases → withstand less pressure
Poiseuille’s Law
What each means
Q=(P2−P1)πr^4/8ηl
Q=volume flux
(P2−P1)=change in pressure
r=vessel radius
η=viscosity
l=vessel length
How vessel diameter affects blood flow?
how it can control blood flow
- Flow is proportioinal to the radius^44
- Therefore, small changes in arterial diameter produce large changes in flow
- Radius can change from minute to minute,especially the radius of arteriole
- Vasoconstrict: decrase diameter, increase resistance
- Vasodilate: increase diameter, decrease resistance
Atherosclerosis
What is it
How it affects blood flow, by how much
- Atherosclerotic plaques (fatty deposits) in blood vessels reduce the diameter of the vessel
- This dramatically reduces blood flow through that vessel
- Halving vessel diameter would lead to a 16 times fall in blood flow
Atherosclerosis
What is it
How it affects blood flow, by how much
- Atherosclerotic plaques (fatty deposits) in blood vessels reduce the diameter of the vessel
- This dramatically reduces blood flow through that vessel
- Halving vessel diameter would lead to a 16 times fall in blood flow
Blood viscosity
Definition
How it affects by haematocrit
- Viscosity: quality that liquids have of being thick and sticky
- A measure of a fluid’s resistance to flow caused by internal friction between neighbouring particles as a fluid is moving
- Depends mainly on haematocrit
- Blood flow is inversely proportional to the viscosity of blood (blood becomes thicker, flow reduces0
Clinical conditions increasing viscosity
- Polycythaemia: when a person has too many RBCs their blood becomes move viscous
- Sickle cell disease: malformed RBC that are inflexible can make blood more viscous
- Macrocytosis: If blood cells are big they can make the blood more viscous
Blood vessel length
Definition, how it affects blood flow
- Total blood vessel length is inversely proportional to flow, as longer vessels have higher friction
- Shorter vessles -> less resistance, longer vessels -> more resistance
What is compliance
Definition
how it affects when pressure increase
compliance=v(mL)/p(mmHg)
1. The ability of a blod vessel wall to expand and contract passively with changes in pressure
2. Blood vessels are stretchable tubes
- If the pressure increases, the walls of the tube can actually stretch out a bit to accomodate a larger volume
- How much they stretch out depends on their compliance
compliance: vein>arteries>hardened arteries
How aging affects compliance
- As people age, the elasticity of blood vessels reduce, vessels become less compliant
- In order to pump the same volume of blood, a larger pressure is generated
Compliance during the cardiac cycle
- Because vessels are
