Lecture 27. Circulation Flashcards
What system is the cardiovascular system?
Dual circulation
What do elastic arteries do?
Smooth pulsatile flow
What do muscular arteries?
Control resistance to flow
What are veins?
Capacitance vessels holding a reservoir of blood
What are capillaries?
Exchange vessels
What is the common blood vessel structure (from outwards to inwards)?
Tunica adventitia (externa - principally collagen)
Tunica media (elastic tissue and smooth muscle)
Tunica intima (interna - endothelium)
Lumen
What do elastic arteries store?
Energy
What is the tunica intima?
Single layer
When is the energy stored in the wall of elastic arteries released?
In diastole, maintaining the blood flow at this time and smoothing it
What. is the Windkessel effect?
As blood is pumped into the aorta and major arteries, they stretch
Thus in systole, more blood flows in than out
The walls of the aorta and elastic arteries recoil in diastole, maintaining blood flow
When does aortic pressure rise to a maximum and when does it fall to a minimum?
Rises to a maximum during systole - the systolic pressure
Falls to a minimum during diastole - the diastolic pressure
What does blood flow depend on?
Blood vessel radius
What determines resistance?
Length of blood vessels
Viscosity of blood
Radius of blood vessels
What are the sites of controlled resistance to flow?
Small arteries and arterioles
What is Ohms Law?
Q = (P₁ – P₂)/R
Where Q = flow and P₁ – P₂ = pressure difference between the two ends and R is the resistance of the vessel
What is blood flow normally?
Laminar - The flow is considered essentially as layers of fluid that move over each other. Flow is fastest at the centre and slowest – essentially stationary – at the outside
What is the equation for laminar flow?
R = (8ηl)/πr⁴
Where R = resistance, η = viscosity, l = length, r = radius and π = π
What would a reduction in radius by 1/2 change in resistance?
Resistance increases 16 fold
When does flow become turbulent?
If velocity is high enough, the layers (laminae) of laminar flow break up and flow becomes disordered
What do sphincters control?
Access to the microcirculation
What are capillaries designed to optimise?
Diffusion - their structure minimises resistance to diffusion into and from the interstitium
What is the permeability of lipid soluble molecules?
Lipid soluble molecules, which include O₂ and CO₂, diffuse easily through capillary cell membranes
What is the permeability of hydrophilic molecules?
Travel through pores, via a paracellular route
What is the permeability of molecules >60kd?
Not transferred and many plasma proteins are retained in the circulation – important in the equilibrium between plasma and the e.c.f.
What are the two compartments of extracellular fluid called?
Plasma and intracellular fluid
What determines equilibrium between the plasma and the interstitial fluid?
Hydrostatic pressure and oncotic pressure
What determines the filtration pressure?
Filtraction pressure - hydrostatic pressure - oncotic pressure
What takes up excess fluid and returns it to the circulation?
Lymphocytes
What enhances venous return?
Muscle pump
