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Arterial Blood Pressure
Arterial blood pressure
“Blood pressure” generally refers to arterial blood pressure
Definition - ‘B.P.’ is the lateral
pressure exerted by the column of blood on the walls of the arteries’ It is not steady Fluctuates during
the Cardiac Cycle
Arterial blood pressure
During ventricular systole B.P is higher than that during diastole
Systolic B.P -the peak pressure during the ventricular systole
Diastolic B.P -the minimum pressure during ventricular diastole
Arterial blood pressure
Pulse pressure - the difference between systolic and diastolic B.P
Mean arterial blood pressure – It is the average pressure throughout the cardiac
cycle
PulsePressure
Arterial blood pressure
Mean B.P –
Normally diastole is longer than systoleThus mean arterial BP is not the arithmetic mean of systolic and diastolic BP
Mean BP = Diastolic B.P + 1 of pulse pressure
Mean BP = SBP + (2xDBP)
Clinically important BP is SBP & DBP and not mean BP
3
3
Arterial blood pressure
Arterial blood pressurePressure – depends onthe amount
of bloodAmount ofBloodEntering – cardiac output
Amount ofBlood leaving – determined by resistanceto flow
Determines mainlysystolic BP
Determines mainlydiastolic BP
Arterial blood pressure
A concept:
B.P. = Cardiac output x Total peripheral resistance
BP = CO x TPRThis equation holds true for actual values of
the above parameters
But, clinically neither the CO nor the TPR is measured routinely.
Total peripheral resistance
total peripheral resistance (TPR) refers to the resistance to blood flow offered by all of the systemic vasculature excluding the pulmonary vasculature.
R = 8Lv
r4
R = resistance
L = length of the blood vesselv = viscosity of bloodr = radius of the blood vessel
Total peripheral resistance
TPR is therefore determined by those factors that
influence vascular resistance in individual vascular beds.
The main site of vascular resistance within the circulation is at the ARTERIOLAR level.
TPR is primarily determined by changes in blood vessel diameters
vasodilataion – increase in vessel diametervasoconstriction – decrease in vessel diameter
Changes in blood viscosity will also affect TPR
Control of arterioles
The blood vessels are controlled by
1. Central mechanisms Neural mechanisms Circulating hormones
2. Local mechanisms Local factors
Tissue factors Endothelial factors
Myogenic factors
Control of arterioles
Arteriolar tone Vascular tone refers to the degree of constriction High
arteriolar tone) / dilatation (low arteriolar tone) of an arteriole
Control of arteriolar tone – central mechanisms
Central mechanisms are –
Neural and endocrine
Regulate a systemic factor Blood pressure Body temperature
Control of arteriolar tone – central mechanisms
1. Neural control
Mostly sympathetics – cause vasoconstriction in most arterioles via alpha 1 receptors.Beta 2 receptors cause vasodilatation e.g. in skeletal muscle
Parasympathetics – in some organs only e.g. external genitalia
Control of arteriolar tone – central mechanisms
2. Hormonal control Adrenalin binds to the beta 2 adrenoceptors to cause
vasodilation in some organs Angiotensin II Antidiuretic hormone Atrial natriuretic peptide
Control of arteriolar tone – Local mechanisms
Local mechanisms are useful for modifying blood flow in a limited area (e.g. one organ or one area of the body)
They are dependent on 1. Metabolic activities of that area2. Pressure within the blood vessels in that area3. Vascular endothelium
Control of arteriolar tone – Local mechanisms
1. Metabolic mechanisms of vasodilation
There is a close coupling between the metabolic activity and blood flow in most organs of the body
an increase in tissue metabolism, as occurs during muscle contraction, will lead to an increase in blood flow (active hyperaemia)
Control of arteriolar tone – Local mechanisms
Local factors causing vasodilation
Hypoxia Adenosine K+
CO2 excess – hypercapnia H+
Lactic acid Inorganic phosphates
Control of arteriolar tone – Local mechanisms
2. Myogenic MechanismsAutoregulation Autoregulation – the intrinsic ability of an organ to
maintain a constant blood flow despite changes in perfusion pressure.
for example, if the pressure within the arterioles of an organ is decreased blood flow will initially fall this results in dilatation of the arterioles due to the
reduction in stretch blood flow then returns towards normal levels over the
next few minutes this autoregulatory response occurs independently of
any neural or humoral influences and therefore is intrinsic to the organ
Control of arteriolar tone – Local mechanisms
3. Other factors influencing vascular tone – some increase and some decrease arteriolar tone.
Endothelial factors prostacyclin - vasodilatation nitric oxide - vasodilatation endothelin - vasoconstriction
Local hormones/chemical substances thrombaxane, other arachidonic acid metabolites histamine Bradykinin
These factors interact with each other through many mechanisms and help fine-tune the vascular tone
Normal BP
Physiological variations in BPAgeGenetic – racial differencesGenderBody buildSleepPostureRespirationExerciseEmotion and stressPain
Normal BP
Because of the multitude of factors affecting blood pressure it is impossible to determine the normal blood pressure for any individual
In general, blood pressure is said to be normal if at rest –
systolic is 130 mmHg or lessANDdiastolic is 80 mmHg or less
A BP of more than 140/90 mmHg is generally considered to be high
Regulation of BP
The primary goal of the cardiovascular system is the maintainance adequate tissue perfusion
Blood pressure is one major factor that affects tissue perfusion
In order to maintain adequate tissue perfusion the body regulates blood pressure (through the regulation of cardiac output and arteriolar diameter)
Regulation of BP
The blood pressure –
Mostly maintained constant
Increased when necessary (e.g. exercise)
Even allowed to fall a little if appropriate (e.g. sleep)
Maintenance of BP
Short term – minutes to hours, neuro-humoral
Long term – hours to days, renal and endocrine
Maintenance of a constant BP
Short term – neuro-humoral
Mostly neural mechanisms. Hormones (adrenaline) also contribute
This regulation is quick acting (seconds – minutes)
A negative feedback mechanism
Maintenance of a constant BP
Arterial baroreceptors are located in the carotid sinus (at
bifurcation of external and internal carotids)
in the aortic arch
The sinus nerve, a branch of the glossopharyngeal nerve (IX cranial nerve), innervates the carotid sinus. The aortic arch baroreceptors are innervated by the vagus nerve The baroreceptors are tonically active
Arterial baroreceptors
Arterial baroreceptors are sensitive to stretching of the walls of the vessels in which the nerve endings lie.
Stretching occurs when arterial pressure increases
Arterial baroreceptors
The baroreceptors send impulses to the vasomotor centre and cardiac centres (cardioacceleratory & cardioinhibitory) in the medulla oblongata
Impulses along the vagusand glossopharyngeal nerves inhibit the medullary centres
The normal activity of the centres is to activate the sympathetics and inhibit parasympathetics
Increased baroreceptor activityreduce sympathetic activityand enhance parasympathetics
Arterial baroreceptors
A decrease in arterial pressure results in decreased baroreceptor firing
The CVS centers respond by increasing sympathetic outflow and decreasing parasympathetic outflow
TPR
Arterial baroreceptors
Influences on medullary centres
Baroreceptors
Peripheralchemoreceptors
Pain receptors
Temperaturecentre
Proprioceptors
Intracranialpressure
O2 and CO2
partial pressures
Respiratory centre
Temperature receptors
Cerebralcortex
Baroreceptors are only one of many factors influencing the medullary centers
Local short term regulation
Some factors affect the heart directly and influence
•Heart rate
•Force of contraction
Long term – hours to days, renal & endocrine
through maintaining blood volume
Renin angiotensin aldosterone mechanism Other renal mechanisms Vasopressin Atrial natriuretic peptide Thirst
Regulation – Long term
Secretion of renin –From the Juxta-glomerular apparatus (in kidney)
Major stimuli - sympathetic stimulation (as does occur in arterial
hypotension) renal artery hypotension decreased sodium delivery to the distal tubules
Renin angiotensin aldosterone mechanism
Renin angiotensin aldosterone mechanism
Renin
Angiotensinogen
Angiotensin I
Angiotensin II
Increased aldosterone secretion
Vasoconstriction
Thirst
Increased ADH secretion
Angiotensin converting enzyme
Kidneys also regulate blood volume by adjusting the excretion of water and sodium into the urine through mechanisms other than renin-angiotensin-aldosterone pathway
Other renal mechanisms
Vasopressin (ADH)
ADH
Angiotensin II Hyperosmolality Decreased blood volume
Absorption of waterIn kidney
Vasoconstriction Thirst
Atrial natriuretic peptide
Atrial Natriuretic Peptide
Decreased renin secretion
Increased excretion of Sodium and water
Atrial distension
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