Session 5 Flashcards
Define velocity
Rate of movement of fluid particles alone the tube. Relationship between velocity and flow determines blood pressure.
Define flow
Volume of fluid passing a given point per unit time (e.g. ml/min)
Describe the relationship between velocity and cross sectional area for a given flow
Velocity is inversely proportional to cross sectional area.
Therefore vessels such as aorta have a high velocity whereas capillaries have a low velocity (capillaries have a large csa)
What is laminar flow?
When there is a uniform gradient of velocity from the middle to the edge of the vessel
What is turbulent flow?
Break down of velocity gradient. Fluid tumbles over, flow resistance increases greatly.
What does the mean velocity depend on?
Viscosity of fluid
Radius of tube
If flow is fixed then what is the relationship between the resistance and pressure?
Higher the resistance, the greater the pressure change from one end of the vessel to the other.
If the pressure is fixed then what is the relationship between the resistance and the flow?
The higher the resistance, the lower the flow.
Which of the two have a higher resistance; arteries or arterioles?
Arteries because the pressure drop in the arteries is small whereas the pressure drop in the arterioles is larger.
Why is the resistance in capillaries low?
Although they have high resistance individually there are multiple capillaries connected in parallel therefore overall their resistance is low.
Is the resistance in veins and venules high or low?
Low because the pressure drop is low.
Is pressure within arteries high or low?
High resistance of arterioles means that there is a greater total peripheral resistance and therefore the pressure in the arteries is high.
Define transmural pressure
Transmural pressure is the difference in pressure between two sides of a wall.
What does the distensibility of blood vessels allow?
Allows changes in blood pressure to not disrupt the flow dramatically however if the pressure falls too much then the walls will eventually collapse.
How is capacitance possible?
As vessels widen with increased pressure, more blood flows transiently in than out and vice versa. Distensible vessels therefore store blood.
Which of the vessels are most distensible?
Veins
What is the composition of blood in the vessels?
Two main components - cells and plasma. Blood cells congregate in the middle of the flow, so apparent viscosity increases and cells go round faster than the plasma.
What are the resistance vessels?
The blood vessels, including small arteries, arterioles, and metarterioles that form the major part of the total peripheral resistance to blood flow.
Describe the flow of the blood by the heart
Pulsatile flow i.e. the heart ejects blood intermittently
Describe the pressure changes in the arteries in systole and diastole
In systole - arterial pressure rises to a max
In diastole - arterial pressure falls to a minimum
The pressure goes up faster than it goes down (systole occupies 2/3 of the time)
What are the values of arterial pressure during systole and diastole?
Systole - 120mmHg
Diastole - 80mmHg
What are the factors affecting systolic pressure?
Contraction force of the heart
Total peripheral resistance
Stretchiness (compliance) of the arteries
What are the factors affecting diastolic pressure?
Systolic pressure
Total peripheral resistance
Define pulse pressure and give a value for it
Difference between systolic and diastolic pressure (approx. 40 mmHg)
Define vasomotor tone
Tonic contraction of smooth muscle. Constant contraction –> tonic contraction.
Vasoconstriction/vasodilatation
What are the factors affecting smooth muscle contraction?
Vasomotor tone mostly produced by sympathetic brach of NS
Tone is antagonised by vasodilator factors.
Actual resistance is determined by a balance between the two
What is reactive hyperaemia?
Circulation to an area (e.g. arm) is cut off for a minute or two. When blood flow is restored there’s an enormous increase for a short while.
Give examples of vasodilator metabolites
H+, K+, adenosine.
H+ produced due to a decrease in oxygen
K+ and adenosine leak from cells due to compromised metabolic activity
What happens to metabolites when there is no blood flow?
What happens when the flow is returned?
They accumulate, causing the arterioles to dilate maximally. Thus when flow is returned, resistance is very low and therefore the flow is high. The high flow washes the metabolites away therefore causing constriction of the arterioles.
Which organ is good particularly good at autoregulation?
Kidney
What is the relationship between total peripheral resistance (TPR) and the body’s need for blood?
TPR is inversely proportional 1/body’s need for blood
What is the central venous pressure?
Pressure in RA (or great veins). Fills the heart during diastole.
What does the central venous pressure depend on?
Return of blood from the body
Pumping of the heart
Gravity and ‘muscle pumping’
What is autoregulation?
If supply pressure changes, blood flow will decrease therefore metabolite concentration increases. This alters the resistance of arterioles so blood flow returns to an appropriate level for metabolism.
Define venous pressure
The pressure exerted on the walls of the veins by the circulating blood
