Physiology - Haemodynamics Flashcards
What is systole
contraction of the heart
What is diastole
relaxation of the heart
What is pulse pressure
difference between systole and diastole
- as arteries are elastic pulse pressure normally decreases slightly from the aorta to brachial artery
- systolic - diastolic
What is systolic blood pressure
maximum pressure in arteries
what is diastolic blood pressure
minimum pressure in arteries
what are numbers for systole
At brachial artery systolic pressure normally 120 mmHg (16kPa). Current guidelines are:
>140 mm Hg systolic marginal hypertension.
>160 mm Hg definite intervention threshold
what are the numbers for diastole
At brachial artery systolic pressure normally 80 mmHg (10.7kPa).
>90 mm Hg diastolic marginal hypertension.
>100 mm Hg definite intervention threshold.
How do you calculate mean arterial pressure (MAP)
diastoli + 1/3 pulse pressure
e.g.
So for a systolic of 120 mmHg and a diastolic of 90 mmHg, the MAP is 100 mmHg. (90 + 1/3 of 30)
what is compliance
- stretching
- so if the artery is high in compliance it means it can stretch more
what is compliance caused by
- elastin fibres in the arterial walls
what does compliance reduce?
- the starchiness reduces the work of the heart in pumping the blood as some of the blood is stored in the large arteries by the stretching and increasing their volume
what happens to old people and compliance
- compliance decreases as arteries elastin is replaced by collagen this causes the artery to harden
- this increases the systolic pressure as aorta cannot stretch to accommodate the stroke volume
- it can decrease the cardiac output as the isovolumetric phase of ventricular contraction is longer
What is the windkessel effect
- this is the effect of the compliance of the small elastic arteries
- the walls of the aorta and elastic arteries distend when the blood pressure rises during systole and recoil when the blood pressure falls during diastole
- ## there is therefore a net storage of blood during systole which discharges during diastole
what is hypertension
abnormally high blood pressure
What is hypertension a risk factor for
vascular disease
what is now done to stop hypertension being a risk factor
, Prehypertension is now defined as a classification for cases where a person’s blood pressure is elevated above normal but not to the level requiring medication
define prehypertension
Prehypertension is considered to be blood pressure readings with a systolic pressure from 120 to 139 mm Hg or a diastolic pressure from 80 to 89 mm Hg.
why is hypertension a problem
- person is often unaware anything is wrong - only known when blood pressure is routinely measured
why do we need a blood pressure of 120/80mmHg
- flow through an organ can be regulated by relaxing or constricting the arterioles that input into it
- if the pressure is constant flow is inversely proportional to resistant, a high blood pressure ensures that a local vasodilation is effective in increasing local blood flow
How do you work out local flow
local flow = pressure/local resistance
How do you regulate flow
- by constricting and relaxing the arterioles therefore changing the arterial diameter
What is poseuille’s Law
- this is the idea that the flow of a liquid through a tube depends on the fourth power of the radius of the tube
what is poiseuille’s equation
flow = pie x r^4/8n x dp/L P - pressure R - radius L - length fo tube n - fluid velocity
what does poiseullie’s law effect the most
arterioles - they are smaller therefore only have to constrict a little bit for there to be a reduction in flow
What is cardiac output
- the total blood flow out of the heart
what is the equation to work out cardiac out put
Heart rate x stroke volume
what is the cardiac output typically in an healthy resting man
5L
where can we measure pulse rate from and stroke volume from
Pulse rate - ECG
stroke volume - echocardiography
what can be used to measure cardiac output
Doppler ultrasound
How does the doppler ultrasound measure cardiac output
- the blood velocity through the first part of the atria causes a doppler shift in frequency of returning ultrasound
- shift can be used to calculate flow velocity
- cross section can be measured using ultrasound
- therefore the cardiac output can be obtained
What are the advantages of using a doppler ultrasound to measure cardiac output
- non -invasive
- accurate
- inexpensive
- high levels of reliability and reproducibility
How does a trans oesophageal doppler work
- ultrasound probe is inserted into the oesophagus at the mid-thoracic level
- probe measures the velocity of the blood
- relies on a nomogram based on patient age, height and weight to convert the measured velocity into stroke volume and cardiac output
what are the factors that effect cardiac output
factors affecting the heart rate
- autonomic innervation
- hormones
- fitness level
- age
factors affecting stroke volume
- heart size
- fitness level
- gender
- contractility
- duration of contraction
- preload
- after load resistance
How do you work out stroke volume
EDV- ESV
What are the blood requires to go to the
- brain
- heart
- kidneys
- Your BRAIN needs about 700 ml per minute of blood (14% resting Cardiac Output) stays consent
- Your HEART needs about 200 ml per minute of blood (4% resting C.O.) dependent on level of exercise
- Your KIDNEYS take about 1250 ml/min (25% resting C.O.).
Maintain blood pressure
How much cardiac output does the brain heart and kidneys take up at rest
- These three organs take up about 40-45% C.O at rest.
2-2.3L/MIN
How much cardiac output is available for the rest of the body
2.7-3.0 L/min
where does the rest of the cardiac output go in the body
- depends on how much you have eaten
- when you have eaten little ,little blood goes to the gut but after a meal 1.5L/min goes to the gut in order to be able to digest so only about 1.2-1.5L/min goes to the muscles and skin
- blood Flo to the skin is variable it can increases via A-V shunts or decrease via vasocontriction to give other organs how much blood they need
What happens to cardiac output during exercise
- it increases by nearly 4 times in order to cope with the increased amount of oxygen needed
what mechanisms increase cardiac output during exercise
- either increase heat rate
- increase stroke volume
how does heart rate increase cardiac output
Diastole shortens allowing for the increase in heart rate
why is the stroke volume maintained when her excise increased
- maintained due tot he contraction of the atria - this transfers blood into the ventricles during diastole there is also increased ventricular contractility causing a decreased end systolic (residual) volume
How does oxygen uptake increase during exercise
- oxygen uptake increases more than cardiac output as more oxygen is taken up by the lungs
- resistance is bronchi and trachea decrease as they dilate
- respiration rate and depth increases
- increases the amount of PO2 which causes the pulmonary arterioles to relax
- small increase in pulmonary arterial pressure which improves perfusion of the lungs causing the lungs to become more efficient at taking up oxygen
- almost all blood becomes fully oxygenated
What does the work of the heart depend on
- the viscosity of the blood
- diameter of the arterioles
What does the viscosity depend on
- mainly depends on the haematocrit (the proportion of red blood cells in the blood)
- if haematocrit is high the velocity is high so the heart works harder to pump blood around
- if the haematocrit is low then not enough oxygen is transproted
- the viscosity also depends on the mechanical properties of the erythrocytes such as there deformability e.g in sickle cell disease
What is haematocrit
- proportion of red blood cells in the blood
what is the blood viscosity in capillaries
= viscosity is low - therefore heart has to do less work to push blood through the capillaries
- expected to be large because the red blood cells have a larger diameter than that of the capillary
What happens if blood cells are too large
- they clog up the capillary and oxygen delivery is compromised
- rupture and cause haemolysis
How does vasodilation happen
- erythrocytes interact with polypeptide chain which are in the lumen of the capillary
- erythrocytes move along the endothelium they deflect the chains
- this allows calcium to enter the endothelium
- calcium triggers the formation of nitric oxide which relaxes and dilates the walls and a its as a local anitcoagulant
What does nitric oxide do
- Counteracts excess contraction caused by stimulation of the autonomic nervous system
what happen when blood does not move through a vessel
- clots form
- people with atrial fibrillation have a risk of stroke because clots can form in auricles of the atria where blood is not moving
- if blood doesn’t move in the leg veins you can develop deep vein thrombosis (DVT)
What is polycythemia
- it is a disease state in which the hematocrits increases, can be due to an excessive production of red blood cells or to a decrease in the volume of plasma
How do arterioles sustain arterial pressure with thin walls
- use Laplace’s Law
what does Laplace’s law state
Laplace’s law states that the pressure that an elastic vessel can withstand depends on the tension produced in the walls by their elasticity divided by the radius (diameter) of the vessel
describe how to work out Laplace’s Law
In a CYLINDER (eg a blood vessel) the pressure withstood is proportional to T/R
In a SPHERE (eg an alveolus) the pressure withstood is
proportional to T/2R
P(withstood) = K(T/R)
- T/R - tension in the wall divided by the radius
the smaller the radius….
of a vessel the greater the pressure that a given wall strength can withstand
- therefore ether small diameter arterioles only need thin walls to withstand normal arterial pressures
describe why capillaries can withstand pressures
Because capillaries have such a small diameter, they can withstand fluid pressures of >20 mm Hg using only the small tension generated by the basement membrane.
Why don’t capillaries have any smooth muscle
- muscle would impede the exchange of fluids and gases
What is an atheroma
- a fatty deposit inside the artery
- because of piseuille law even a small atheroma can decrease blood flow and cause hypoxic states in tissue
what is a common site for an aneurysm
- aorta
- at points before the blood vessels branch as pulsatile flow of blood occurs here
- cerebral arteries - more convoluted that arteries and have twists which cause extra stress on the walls
what happens if an artery wall gets a tear
- radius increases therefore the balancing pressure that the elastic tissue generates is less
- the wall balloons out and this reduces the effectiveness of the wall to withstand the pressure and can cause an aneurysm
What are the factors that lead to aneurysm occurring
- atheroma - narrows the lumen of the artery and if tuber lent flow happens this can cause high pressure in the artery causing the walls to break
- if there is an inflammation reaction to the atheroma this can cause the destruction of the elastin fibres in the artery wall and weakens it so it can stretch more when there is a blood pressure spike
- hypertension
- artery disease
How can you discover an aneurysm
- during MRI or angiography
- rupture causing subarchanoid brain haemorrhage
- present with symptoms of mass effect on neural structures
How do you treat cerebral aneurysms
- clip or coil
- coiling - fine wire is pushed into the swollen artery and forms a coil, the blood clots around the coil and takes the pressure off the wall and forms a stable structure
