CVS 5 - Microcirculation Flashcards
What equation connects blood flow, pressure difference and resistance?
Flow = pressure difference/resistance (like in an electrical circuit where pressure difference is like potential difference (V))
How can Poiseuille’s law be simplified?
R proportional to 1/r^4
Because viscosity and vessel length constants.
What factors affect vascular resistance?
Vessel radius, vessel length, blood viscosity
What are the major resistance vessels?
Arterioles
Why can change in blood pressure be substituted by MAP?
The arterial blood pressure is usually MAP and the blood pressure in the veins is usually around 0 mm Hg so the change in blood pressure through a capillary bed is usually around MAP.
PA - PB = MAP - 0 = MAP
So R determines blood flow to an organ.
What is the normal state of vascular smooth muscle?
They are normally in a state of partial vascular constriction - vascular tone
For what 2 functions is the radii of arterioles adjusted?
- Match blood flow to the metabolic needs of specific tissues - intrinsic control (=local)
- Help regulate arterial BP - extrinsic control
Describe how vessel radius responds to the chemical environment (intrinsic control)
When tissues are highly metabolically active they will use up a lot of oxygen and ATP. The increase uptake of oxygen is detected by the tissues, which sends a message to the arteriolar smooth muscle to dilate. This is active hyperaemia.
There is also a physical response to cold and to STRETCH - like an increase in BP, this expands blood vessel and will induce vasoconstriction -> myogenic vasoconstriction
Describe how vessel radius responds to the physical environment.
When there is a decrease in blood temperature, the vascular smooth muscle will constrict so that less blood reaches the surface and so less heat is radiated away.
How can the flow rate, pressure difference and resistance equation be applied to the entire circulation?
Flow rate is cardiac output, pressure difference is mean arterial blood pressure and resistance is total peripheral resistance. MABP = CO x TPR
What are the two pathways controlling arterial blood pressure?
Neural and hormonal pathways
Where is the centre that regulates arterial blood pressure found?
In the medulla - cardiovascular control centre
Describe the neuronal control of arterial blood pressure.
The brain controls arterial blood pressure via ADRENORECEPTORS which respond to NA and A:
Alpha - CONSTRICTION
Beta - Dilation
The sympathetic nervous system can also increase production of catecholamines (noradrenaline and adrenaline) from the adrenal medulla, which binds to increase heart rate and blood pressure
Describe the hormonal control of arterial blood pressure.
Main hormones involved in blood pressure: Angiotensin II Vasopressin These are both potent vasoconstrictors Also adrenaline
What is capillary exchange?
Delivery of metabolic substrates to the cells of an organism
What tissues have a high capillary density?
Skeletal muscle, myocardium, brain, lungs
What are the three main types of capillary and how do they differ?
CONTINUOUS - small water filled gap junctions that allow the passage of electrolytes and small molecules (most substances move through endothelial cells) - MOST COMMON
FENESTRATED - slightly bigger gaps allowing slightly larger molecules to pass through
DISCONTINUOUS - large holes in the capillary
How is the blood brain barrier different to other capillaries?
You do NOT have water-filled gap junctions but instead you have TIGHT gap junctions. So access of substances to the brain is tightly regulated. Only can access brain if goes through endothelial cell.
What is the name given to hydrostatic pressure and plasma osmotic pressure (= oncotic pressure - osmotic pressure exerted by protein in plasma)?
Starling’s Forces
“balance between the hydrostatic pressure of the blood in the capillaries and the osmotic attraction of the blood for the surrounding fluids”
Overall, more fluid leaves the capillaries (ultrafiltration) than is absorbed (reabsorption).
Describe some characteristics of the lymphatic system.
Consists of blind-ended lymphatic capillaries
Valves - prevent backflow
All but the right upper quadrant of the body drains via the thoracic duct into the left subclavian vein
The right upper quadrant drains into the left subclavian vein
