Resistance flow and blood pressure Flashcards
What drives blood flow?
- The fundamental purpose of the CVS is to generate a pressure gradient (via the pumping of the heart) to drive blood flow through the blood vessels
- The heart generates a mean pressure of ~100mmHg in the aorta, which drops down to ~2-5mmHg in the vena cava
- Therefore, there is a pressure gradient for blood to flow throughout the entire circulation
- Note: at each point in the circulation there is a pressure gradient, where the pressure to the heart is higher
- The pressure gradient is caused by energy loss due to frictional forces b/w the blood and the wall of the vessel → this is called resistance
idea of hydrostatic pressure = test tubes diagram
- As blood flows, pressure falls because of loss of energy (due to friction)
- A fluid exerts a pressure on the walls of its container called the hydrostatic pressure
- In a static fluid, the pressure will be the same along the length of the tube
- However, when the fluid begins to flow, pressure falls with distance because energy is lost due to friction b/w the fluid and the walls of the tube
- This friction is known as resistance to flow
What are the determinants of resistance?
- Once a pressure is generated, the amount of flow is dependent upon the resistance, which opposes the flow → the higher the resistance the lower the flow
- This r/s is known as Ohm’s Law
- The factors that determined flow are given Poiseuille’s Law
OHM’S LAW: PRESSURE = QR (where Q is the flow and R is the resistance to flow)
POISEUILLE’S LAW: demonstrates the main factor affecting resistance is radius R = 1/r^4
The radius is the major determinant of resistance and blood flow
- Our pressure is generated by the pumping of the heart and it pumps the blood into the circulation (first into aorta then flow through the other vessels) → what determine how much flow = pressure but also resistance which is opposing the flow → the main determinant of the flow + that impacts on resistance is the radius of the vessels → and in particular it is the radius of the arterioles that really impacts on the total resistance to flow
Regulation of vascular resistance - factors affecting arteriolar radius
Local factors:
- myogenic activity, metabolites, histamine, heat/cold
Autonomic nerves:
- sympathetic vasoconstrictor nerves (nearly all beds)
- parasympathetic vasodilator nerves (very few beds)
Circulating hormones:
- adrenaline, noradrenaline, vasopressin, angiotensin II
*The local factors and in particular the metabolites are the most important factors for controlling arteriole radius + therefore controlling blood flow to tissues
LOCAL FACTORS
- metabolites cause arteriolar dilation (a negative feedback system)
Increase in muscle metabolic activity (in specific region) –> increase in consumption of O2 and production of CO2 and other metabolites –> decrease in local O2 concentration and increase in concentration of CO2 and other metabolites in extracellular fluid –> relaxation of smooth muscle causing vasodilation of arterioles in local region –> increase in local blood flow –> increase in rate of supply of O2 and removal of CO2 –> O2 and CO2 levels in extracellular fluid return to normal
Extrinsic factors: (i) autonomic nervous system
- The autonomic nervous system has two divisions:
- the sympathetic +
- parasympathetic nervous systems
- Sympathetic “fright and flight” - sets the body for action
- Parasympathetic “rest and digest” - sets the body for quiescence
- The heart is controlled by both the sympathetic and parasympathetic
- Blood vessels are controlled largely by sympathetic
- Mostly constrictor - Parasympathetic vasodilator nerves in specialist regions
- Heart, salivary glands and GI tract, erectile tissue (penis, clitoris)
Sympathetic control of blood vessels
- Sympathetic nerves innervate all types of blood vessels, except capillaries
- They are called vasomotor nerves
- The density of innervation of sympathetic vasomotor nerves varies according to the type of vessel (greatest in arterioles, least in arteries) and…
- The region (e.g. dense in arterioles in skin, skeletal muscle, kidney etc., but much less in brain and heart)
- Most sympathetic vasomotor nerves are vasoconstrictor - cause constriction of blood vessels that they innervate
- The neurotransmitter is usually noradrenaline. The primary receptor is alpha-1
- Sympathetic vasoconstrictor nerves are tonically active - decreasing activity causes dilation of blood vessels (vasodilation)
- Their activity is altered reflexly
- Sympathetics play a very important role in the reflex control of blood pressure (e.g. baroreceptor reflex) rather than control blood flow
Extrinsic factors: (ii) circulating vasoactive hormones
Adrenaline (Ad) and noradrenaline (Nad)
- released from adrenal medulla = more adrenaline than non-adrenaline (ratio of 4:1)
- In response to sympathetic stimulation
- act on α and β adrenergic receptors
- Nad acts mainly on α receptors to cause widespread peripheral vasoconstriction
- Ad acts mainly on β receptors cause vasodilation, mainly in skeletal muscle
Vasopressin (antidiuretic hormone, ADH),
- is produced in the hypothalamus and stored in the pituitary gland.
- Released in response to low blood volume (e.g. haemorrhage) and other stimuli
- Primary role as an antidiuretic effect, but also potent vasoconstrictor
Angiotensin
- produced from the plasma protein angiotensinogen in response to low blood volume/pressure
- It is the most potent natural vasoconstrictor known
Histamine
- vasodilator and increases capillary permeability
- Released from mast cells in allergic reactions
- Causes the flare (redness) and wheal (swelling) in local allergic reactions
Redistribution of cardiac output during exercise
Reasons for blood flow changes:
- Heart and skeletal muscle: mainly local metabolic factors (↑ metabolic activity) causing vasodilation.
- Kidney and abdominal viscera: mainly increase in sympathetic vasoconstrictor activity.
- Skin**: mainly decrease in sympathetic vasoconstrictor effects, plus effects of bradykinin in sweat. Both cause vasodilation, as part of thermoregulatory reflexes.
- Brain: no change, because cerebral blood flow is mainly affected by local metabolic activity (i.e. neural activity), which does not change as a result of exercise.
Blood Pressure control following haemorrhage
Mechanism that regulated BP in the short term
- i.e. sec or mins
- is the BARORECEPTOR REFLEX
- located in the walls of the aorta + carotid artery
- STRETCH receptors
- BP increases = arteries stretched = baroreceptor activity increases
- is tonically active = can detect increase + decrease in pressure
- BR reflex increase HR when BP drops to get back to normal
Mechanism that regulated BP in the long term
- controlled by changing blood volume
- regulated by renal function + production of urine
- = called pressure diuresis
- When BP increases, there is a dramatic increase in urine output (VICE VERSA)
- with the vice versa (decrease in both) = helps to conserve water + w/ drinking helps to increase blood volume
