1. Arterial Blood Pressure

2. 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

3. 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

4. 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
Pulse Pressure

5. Arterial blood pressure
Mean B.P –
Normally diastole is longer than systole
Thus 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

6. Arterial blood pressure
Pressure – depends on the amount of blood
Amount of
Blood leaving – determined
by resistance to flow
Amount of
Blood
Entering – cardiac output
Determines mainly systolic BP
Determines mainly diastolic BP

7. Arterial blood pressure
A concept:
B.P. = Cardiac output x Total peripheral resistance
BP = CO x TPR
This equation holds true for actual values of the above parameters
But, clinically neither the CO nor the TPR is measured routinely.

8. 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
R = resistance
L = length of the blood vessel
v = viscosity of blood
r = radius of the blood vessel r4

9. 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 diameter vasoconstriction – decrease in vessel diameter
Changes in blood viscosity will also affect TPR

10. 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

11. Control of arterioles Arteriolar tone
Vascular tone refers to the degree of constriction High arteriolar tone) / dilatation (low arteriolar tone) of an arteriole

12. Control of arteriolar tone – central mechanisms
Central mechanisms are –
Neural and endocrine
Regulate a systemic factor
Blood pressure
Body temperature

13. Control of arteriolar tone – central mechanisms
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

14. 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

15. 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
Metabolic activities of that area
Pressure within the blood vessels in that area
Vascular endothelium

16. Control of arteriolar tone – Local mechanisms
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)

17. Control of arteriolar tone – Local mechanisms
Local factors causing vasodilation
Hypoxia
Adenosine K+
CO2 excess – hypercapnia H+
Lactic acid
Inorganic phosphates

18. Control of arteriolar tone – Local mechanisms
2. Myogenic Mechanisms
Autoregulation
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

19. 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

20. Normal BP Physiological variations in BP Age
Genetic – racial differences
Gender
Body build
Sleep
Posture
Respiration
Exercise
Emotion and stress
Pain

21. 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 less
AND
diastolic is 80 mmHg or less
A BP of more than 140/90 mmHg is generally considered to be high

22. 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)

23. 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)

24. Maintenance of a constant BP
Maintenance of BP
Short term – minutes to hours, neuro-humoral
Long term – hours to days, renal and endocrine

25. 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

26. Arterial baroreceptors
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

27. 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

28. Arterial baroreceptors
The baroreceptors send impulses to the vasomotor centre and cardiac centres (cardioacceleratory & cardioinhibitory) in the medulla oblongata
Impulses along the vagus and glossopharyngeal nerves inhibit the medullary centres
The normal activity of the centres is to activate the sympathetics and inhibit parasympathetics
Increased baroreceptor activity reduce sympathetic activity and enhance parasympathetics

29. 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

30. Influences on medullary centres
Baroreceptors are only one of many factors influencing the
medullary centers
Baroreceptors
Temperature centre
Peripheral chemoreceptors
Cerebral cortex
Pain receptors
O2 and CO2 partial pressures
Proprioceptors
Intracranial pressure
Respiratory centre
Temperature receptors

31. Local short term regulation
Some factors affect the heart directly and influence
Heart rate
Force of contraction

32. Regulation – Long term Long term – hours to days, renal & endocrine
through maintaining blood volume
Renin angiotensin aldosterone mechanism
Other renal mechanisms
Vasopressin
Atrial natriuretic peptide
Thirst

33. Renin angiotensin aldosterone mechanism
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

34. Renin angiotensin aldosterone mechanism
Angiotensinogen
Renin
Angiotensin I
Angiotensin
converting
enzyme
Increased
aldosterone secretion
Vasoconstriction
Angiotensin II
Thirst
Increased ADH
secretion

35. Other renal mechanisms
Kidneys also regulate blood volume by adjusting the excretion of water and sodium into the urine through mechanisms other than renin-angiotensin-aldosterone pathway

36. Vasopressin (ADH) ADH Angiotensin II Hyperosmolality Decreased blood
volume
ADH
Vasoconstriction
Absorption of water
In kidney
Thirst

37. Atrial natriuretic peptide
Atrial distension
Atrial Natriuretic Peptide
Increased excretion of
Sodium and water
Decreased renin
secretion