Cardiovascular PhysiologyCardiovascular Physiology

Dr. Abdulhalim Serafi, MB ChB,MSc,PhD,FESCDr. Abdulhalim Serafi, MB ChB,MSc,PhD,FESC

Assistant Professor & Consultant CardiologistAssistant Professor & Consultant Cardiologist

Faculty of Medicine and Medical SciencesFaculty of Medicine and Medical Sciences

Umm Al-Qura UniversityUmm Al-Qura University

Makkah Al-MukarramahMakkah Al-Mukarramah

Saudi ArabiaSaudi Arabia

Part IIPart II

CARDIOVASCULAR CARDIOVASCULAR PHYSIOLOGYPHYSIOLOGY

Outline:

- Systolic BP and diastolic BP.

- Pulse pressure and mean ABP.

- Physiological variations of ABP.

- Factors that maintain normal ABP:: Cardiac output (co): Peripheral resistance (PR): Arterial elasticity: Blood volume

Further Reading:• Guyton: Textbook of Medical Physiology• Ganong: Review of Medical Physiology

LECTURE II:LECTURE II: ARTERIAL BLOOD PRESSURE (ABP)

ARTERIAL BLOOD PRESSURE (ABP) ARTERIAL BLOOD PRESSURE (ABP)

- Arterial Blood Pressure (ABP) is the lateral pressure exerted by the blood on the arterial walls. It oscillates during each cardiac cycle between a maximum called systolic BP and a minimum called diastolic BP.

• Systolic BP:- It is the maximal pressure exerted by the blood on the

arterial walls during ventricular systole (ejection of blood from the left ventricle into the aorta).

- It ranges between 95 and 145 mm Hg with an average of about 120 mm Hg in adults i.e. normal systolic BP=120 ± 25. Mm Hg.

• Diastolic BP:- It is the minimal pressure exerted by the blood on the

arterial walls during ventricular diastole (just before ventricular systole and ejection of blood)

• Pulse pressure:- It is the difference between systolic blood pressure and

diastolic blood pressure e.g. PP=120-80 = 40 mm Hg.

• Mean Arterial BP:- It is the average pressure in the arteries throughout

cardiac cycle.- It is calculated by the following formula:

Mean arterial BP = Diastolic BP + 1/3 Pulse Pressure e.g. = 80 + 1/3 x 40 = about

93 mm Hg.

IMPORTANCE OF ABP IMPORTANCE OF ABP • Normal mean ABP provides the force that drives blood to

the tissues I.e. ensures steady blood flow (perfusion) to the tissues.

• Normal diastolic blood pressure is needed for filling of the coronary arteries with blood which occurs mainly during ventricular diastole.

PHYSIOLOGICAL VARIATIONS IN ABP:

(1) AGE:- At birth, the ABP is about 80/40- At the age of 120 years, the ABP is about 120/ 80- The ABP tends to increase with age e.g. it becomes

about 150/90 at the age of 60 years.

(2) SEX:- The ABP is slightly higher in adult males than adult

females- After menopause (stoppage of menstrual cycle in females which occurs usually at the of 45 years), the ABP may be the same in females and males, but sometimes it becomes higher in females, probably due to hormonal change.

(3) Body Built:- The ABP of obese persons is usually higher than that of persons with normal body weight.- Also, obese persons have higher tendency to develop hypertension.

(4) Exercise:- the ABP (systolic BP) increases during muscular exercise (due to increase of cardiac output) but it drops to pre-exercise level during recovery.

- the diastolic BP may remain unchanged or it id slightly decreased due to vasodilatation of the arterioles of the active skeletal muscles.

(5) Emotions:- ABP (mainly systolic) increases during emotions due to sympathetic over activity.

(6) Meals:- ABP rises slightly after meals.

(7) Sleep:- ABP decreases slightly during sleep.

- When sleep is accompanied with dreams (nightmares) of emotional or motor activity, the ABP may increase to high levels.

CONTROL OF ABPCONTROL OF ABP

Factors that determine and maintain ABPFactors that determine and maintain ABP

1. Cardiac output.2. Peripheral resistance3. Arterial elasticity4. Blood volume

- ABP is related to the cardiac output and peripheral resistance according to the following equation:

ABP = Cardiac output x Peripheral resistance

1) Cardiac Output (COP)- Normal COP is essential for normal ABP.- ABP is directly proportional to COP (provided that all

other factors affecting ABP remain constant).- Change in COP affects systolic BP more than diastolic BP.

COP = Heart rate x Stroke volume

• Effect of change in stroke volume:

- Changes in SV affect systolic BP more than diastolic BP. Thus, SV systolic BP & SV systolic BP.

- The diastolic BP shows little or no change.

• Effect of change in heart rate:

- Changes in HR affect diastolic more than systolic BP i.e. heart rate of diastolic BP & HR of diastolic BP.

- Systolic BP shows little or no change.

2) Peripheral resistance (PR)- PR is the resistance which blood meets during its

passage in the peripheral arterioles and to a smaller extent in the blood capillaries.

- ABP is directly proportional to PR. Changes in the peripheral resistance affect diastolic BP more than systolic BP.

- The PR depends mainly on 2 factors: (diameter of arterioles and blood viscosity)

a) Diameter of arterioles:- The PR is inversely proportional to the diameter of

arterioles i.e.

• Vasoconstriction of arterioles of their diameter PR ABP (manly diastolic)

• Vasodilatation of arterioles of their diameter PR ABP (mainly diastolic)

b) Blood Viscosity:

- PR is directly proportional to the blood viscosity.- Blood viscosity depends on 2 factors HV (haematocrit value) Plasma proteins

Blood viscosity as in polycythaemia PR ABP blood viscosity as in severe anemia or hypoproteinaemia PR ABP

Factors that determine the peripheral resistanceFactors that determine the peripheral resistance

The peripheral resistance (PR) is essential for maintenance of the arterial B.P. (particularly the diastolic B.P.). It occurs mainly in the arterioles and is determined by the following factors:

1. Radius (r) of the blood vessel: The PR varies inversely with the fourth power of the radius (i.e. PR

I/r4).2. Blood viscosity (V): This is determined mainly by the

haematocrit value and to a lesser extent by the

plasma proteins. The PR varies directly with the blood viscosity (i.e. PR V).

3. Length (L) of the blood vessel: The PR varies directly with the length of the blood vessel (i.e. PR L)

However, the radius is the most important factor that determines the PR because both V and L are normally

constant.

Effect of peripheral resistance Effect of peripheral resistance

(PR) on ABP(PR) on ABP

PR = resistance which the blood meets during its passage in the peripheral arterioles and to a smaller extent in the blood capillaries.

Changes in PR affect diastolic BP more than systolic BP

PR depends mainly on 2 factors:

1) Diameter of arterioles:

- VC of arterioles PR ABP

- VD of arterioles PR ABP

2) Blood viscosity

- Blood viscosity PR ABP

- Blood viscosity PR ABP

Regulation of the diameter of the arteriolesRegulation of the diameter of the arterioles

(=Nervous & chemical regulation)(=Nervous & chemical regulation)

Nervous Regulation

- The diameter of arteriole is under the control of the vaso-motor center (VMC) present in the medulla oblongata and the VC sympathetic tone (from VMC) to the arterioles)

- The activity of the VMC is modified by impulses from the arterial baroreceptors, chemoreceptors, atrial receptors and other receptors e.g. in skeletal and skin

Chemical Regulation

- The arterioles show VD or VC by some chemical or hormonal agents.

- Vasoconstrictor substance e.g. noradrenalin, vasopressin (ADH) & angiotensin II

- Vasodilator substances e.g. metabolites, acetylcholine, histamine and bradykinine.

- The diameter of arterioles is regulated by 2 mechanisms (nervous and chemical):

Nervous regulation:

- The diameter of arterioles is under the control of the vasomotor center (VMC) which is present in the medulla oblongata.

- The VMC discharges continuous vasoconstrictor impulses through sympathetic nerve fibers to the various arterioles to keep them in a state of moderate or partial vasoconstriction. This is called vasomotor tone and it is important to maintain normal ABP.

- The activity of the VMC (:: sympathetic vasomotor tone) is modified impulses from the arterial baroreceptors, the peripheral chemoreceptors, atrial receptors and other receptors e.g. in skeletal muscles and skin.

vasomotor tone more constriction of the arterioles PR ABP

vasomotor tone less constriction of the arterioles VD PR ABP

Chemical regulation:

- The arterioles show VC or VD by some chemical or

hormonal agents:

Vasoconstrictor substance: e.g. noradrenalin,

vasopressin (ADH) angiotensin II.

Vasoconstrictor substance: e.g. metabolites,

acetylcholine, histamine and bradykinine.

3. Elasticity of the arterial walls:

The elasticity of the wall of the aorta and its large branches buffers excessive changes in ABP during each cardiac cycle;

During ventricular systole

The aorta distends and its capacity is increased to accommodate for the ejected blood pressure from the left ventricle. This prevents the systolic BP from reaching high level.

During ventricular diastole

The aorta recoils and its capacity is decreased. This prevents the diastolic BP from reaching a low level. Therefore, elastic recoil of the arterial wall is important for maintenance of normal diastolic BP.

The elastic recoil of the arterial wall is also useful in pushing the blood towards the tissues during diastole.

In arterioscleroses

The arterial elasticity is decreased systolic BP diastolic BP.

Normal diastolic BP is produced and maintained by:

1. Elastic recoil of aorta and large arteries.

2. Normal degree of peripheral resistance.

Normal diastolic BP is important for:

1. Filling of the coronary arteries which occurs mainly during ventricular diastole.

2. Maintenance of blood flow to the tissues during ventricular diastole

4. Blood volume:

Normal blood volume (5-6 liters with normal vascular capacity) is important for normal ABP (systolic and diastolic)

Slight or moderate change in the blood volume almost no

effect on ABP due to some compensatory reaction in the

CVS that restore normal BP e.g.

Moderate of blood volume baroreceptors reflex

heart rate and vasoconstriction of arterioles ABP

towards normal.

Marked or excessive decrease of blood volume e.g. after

severe haemorrhage decrease of ABP (hypotension as

hemorrhagic shock). This occurs because the

compensatory reactions are not sufficient to restore normal

BP.

Decrease of the circulatory capacity (with normal blood

volume) ABP e.g. injection of noradrenalin

generalized vasoconstriction of the circulatory capacity

ABP.

Increase of the circulatory capacity (with normal blood

volume) ABP e.g. injection of histamine (in

experimental animals) generalized vasodilatation

circulatory capacity ABP (=histamine shock).

Regulation (control) of the blood volume includes the

following:

Red cell volume is kept constant by a balance between

the rate of destruction & regeneration of RBCs

Plasma volume is kept constant by:

Rapid mechanism = interchange of fluid between plasma and the tissue fluid.

Slow mechanism = balance between water loss and water gain.

Water excretion in urine is controlled by 2 hormones.Antidiuretic hormone (ADH) secreted by the posterior

pituitary.Aldosterone secreted by the supra renal cortex.

These hormones regulate the blood volume through their effects on the kidney (urine formation). Long-term

regulation of the ABP.

Part IIPart II

CARDIOVASCULAR CARDIOVASCULAR PHYSIOLOGYPHYSIOLOGY

LECTURE III:LECTURE III: REGULATION OF ARTERIAL BPOutline:

- Short-term regulation of the ABP.: Nervous mechanisms e.g. baroreceptors

reflexes.: Hormonal mechanisms e.g. epinephrine –

nor epinephrine.: Capillary fluid shift mechanism.

- Long-term regulation of the ABP.: Renal-body fluid – pressure control mechanism

- Hypertension (definition, types & causes).

Further Reading:• Guyton: Textbook of Medical Physiology• Ganong: Review of Medical Physiology

CONTROL OF ABPCONTROL OF ABP

- The normal mean ABP ensures a steady blood flow or perfusion to the tissues, particularly the vital organs e.g. brain and heart. Regulation of mean ABP includes:

I) Short-term regulation.

II) Long-term regulation.

I-Short-term regulation of the ABP:

- This includes the following rapidly-acting pressure control mechanisms:

1) Nervous mechanisms for rapid control of ABP e.g.:

a) Baroreceptors reflexes (=arterial baroreceptors reflex mechanisms):

= Feed-back control system that prevents sudden or

of ABP e.g.

ABP reflex of heart rate + reflex vasodilatation of arterioles ABP.

ABP reflex of heart rate + reflex vasodilatation of arterioles ABP.

b) Chemoreceptor reflexes:

ABP (as in haemorrhage) blood flow to the peripheral chemoreceptors hypoxia of these receptors ( PO2) reflex of heart rate + reflex vasoconstriction of the arterioles ABP.

c) Atrial reflexes:

blood volume venous return (VR) stretch of the right atrium stimulation of stretch (volume) receptors reflex venodilation venous capacity venous return.

N.B.(1)

ARTERIAL BAROREFLEXES:

An acute rise on arterial blood pressure produces reflex:

(i) Decrease in heart rate.

(ii) Peripheral vasodilatation (specially the blood vessels of the skeletal muscles).

A fall in ABP produces the opposite reflex effects, i.e.

increase in the heart rate and vasoconstriction.

The reflex pathway:

Receptors : Baroreceptors in the carotid sinus & aortic arch

Afferents : The carotid sinus branch of the glassopharyngeal nerve, and the

aortic branch of the vagus nerve

Center : The medullar cardiovascular centers

Efferent : The vagal and sympathetic cardiac nerves and sympathetic nerves to the blood vessels.

Effected organs : The heart and blood vessels.

These reflexes correct a rise in pressure by decreasing the cardiac pumping (decrease in heart rate) and the peripheral resistance (vasodilatation). The opposite effects occur if the ABP falls. In this way, these reflexes maintain the normal level of ABP

The arterial baroreflexes are very rapid. They start in less

than one second. They are quite powerful reflexes. They

play a very important role to prevents reflexes correct a

rise in pressure by decreasing the cardiac pumping

(decrease in heart rate) and the peripheral resistance

(vasodilatation). The opposite effects occur if the ABP falls.

In this way, these reflexes maintain the normal level of

ABP.

N.B. (2)

THE CNS ISCHEMIC RESPONSE:

Ischemia of the CNS produces powerful generalized

vasoconstriction and elevation of ABP.

This response is very important in cases of severe

hypotension, at blood pressures below 60 mm Hg.

Hypotension reduces the blood flow to the brain

(ischemia). In the medulla, ischemia produces local

hypoxia, hypercapnia and acidosis which are strong

stimulants of the medullar vasoconstrictor center.

The severe vasoconstriction elevates the ABP toward normal to improve the blood flow to the brain.

2) Hormonal mechanisms for rapid control of ABP e.g:

a) Epinephrine-nor epinephrine mechanism:

- ABP a baroreceptors reflex sympathetic

stimulation secretion of epinephrine and nor

epinephrine from the suprarenal medulla.

- Epinephrine causes HR & nor epinephrine causes

strong VC ABP.

b) Vasopressin mechanism:

- blood volume & ABP stimulation of stretch

receptors in the right atrium and in aortic arch and

carotid sinus reflex of vasopressin (ADH) from

the posterior pituitary. Vasopressin urine vol. &

vasoconstriction. ABP.

c) Renin-angiotensin-aldosterone mechanism:

ABP renal ischemia release of a chemical substance called rennin from the juxtaglomerular cells (JGC) of the kidney.

Renin acts as a proteolytic enzyme which acts on a plasma 2 – globulin formed by the liver called angiotensinogen and converts it into decapeptide called angiotensin I (AI).

Angiotensin I (AI) is converted into octapeptide called angiotensin II (AII) by angiotensin converting enzyme (ACE) mainly in the lungs.

Angiotensin II (AII) ABP by two mechanisms:

1) AII is a powerful vasoconstrictor generalized

vasoconstriction peripheral resistance ABP.

2) A II stimulates the secretion of Aldosterone

hormone from the suprarenal cortex. Aldosterone

causes Na & H2O reabsorption from renal tubules

salt and water retention blood volume

ABP.

3) Capillary fluid shift mechanism for rapid control of

ABP:

- The interstitial fluid acts as a reservoir for the plasma i.e for regulation of blood volume,

and in turn, regulation of ABP.

- ABP filtration (=fluid shift) from the capillaries to the interstitial fluid (tissue

fluid) ABP.

- ABP filtration & reabsorption (fluid shift) from the interstitial fluid into the capillaries blood volume ABP.

II) Long-term regulation of the ABP:

This is a slowly-acting pressure control mechanism called “renal-body fluid-pressure control mechanism”.

It includes the hormonal mechanism which acts through the kidney

1) Regulation of extra-cellular fluid (blood volume with its effects on ABP)

2) Excretion of water and electrolytes in urine.

Two hormones are involved in this mechanism:

1) Anti diuretic hormone (ADH) which is secreted from the posterior pituitary and it helps H2O reabsorption from the renal tubules blood volume.

2) Aldosterone hormone which is secreted from the suprarenal cortex and it helps Na+ and H2O reabsorption from the renal tubules blood volume

Summary

REGULATION OF THE ABP

Short-term regulation Long-term regulation

= Rapidly-acting pressure = Slowly-acting pressure

control mechanism. control mechanism.

Includes: Includes:

a) Nervous mechanisms e.g.: Hormonal mechanism which

- baroreceptors reflexes. acts through the kidney

- Chemoreceptor reflexes. Regulation of extra cellular

- Atrial reflexes. fluid (blood volume with

effects on ABP).

b) Hormonal mechanisms e.g.:

- Epinephrine nor Excretion of water &

epinephrine secretion electrolytes in urine

- ADH (vasopressin) Hormones which affect

secretion urine formation (volume)

- Renin-angiostensin - ADH (vasopressin).

mechanism - Aldosterone

c) Capillary fluid shift Long-term regulation=renal

mechanisms: - body fluid-pressure

- Shift of fluid from capillaries control mechanism.

into the interstitial fluid.

- Shift of fluid from the interstitial

fluid into the blood capillaries.

HYPERTENSION- Hypertension is diagnosed when the arterial BP exceeds

150/95 (in adults 20-50 years) on repeated estimation. It

may be one of the following types:

1- Primary (essential) hypertension: This occurs as a result

of arteriolar vasoconstriction. The exact cause is

unknown.

2- Secondary hypertension: This occurs as a result of

another disease e.g. secondary to a tumor in the supra-

renal gland secretion of cortisol, Aldosterone &

adrenaline.

3- Malignant hypertension: This is a severe type

characterized by marked elevation of ABP without obvious

causes.

EXPERIMENTAL HYPERTENSION- Hypertension is a sustained elevation of ABP.

Hypertension can be produced in experimental animals by

the following methods:

1- Renal ischemia (interference with renal blood flow).

2- Denervation of vaso-sensory area (aortic arch and carotid

sinus).

3- Experimental neurosis by efferent stimuli such as loud

noises.

4- Chronic injection of supra-renal cortical hormones.

Experimental hypertension is of great value in study in the

possible causes of hypertension in man and the effect of

drugs on hypertension.

Hypertension is a chronic state of

elevated arterial blood pressure:

The upper limits of normal ABP in different age groups

Age (years)Age (years)

Upper limits of normal Upper limits of normal ABP (mmHg)ABP (mmHg)

1010 – – 2020

2020 – – 5050

Over 50Over 50

140140 / /9090

150150 / /9595

160160 / /100100

Any levels higher than these limits are considered as hypertension

Renal ischemia and ABP:

Unilateral renal ischemia temporary hypertension because the healthy kidney (which is not ischaemic) produces angiotensinase enzyme which inactivates angiotensin II

Bilateral renal ischemia permanent hypertension.

N.BN.B::1) Unilateral constriction of renal artery temporary and

not permanent hypertension because the healthy kidney (not ischemic) produces an enzyme called angiotensinase which inactivates angiotensin II.

2) ABP, blood volume, body sodium release of rennin activation of rennin – angiotensin system.