Very important

Extra information

* Guyton corners, anything that is colored with grey is EXTRA explanation

 Coronary Circulation
Objectives :

• Define autoregulation and its mechanism (metabolic & myogenic).

• Identify the vasodilator metabolites.

• List vasodilator and vasoconstrictor agents and those secreted by the

endothelium.

• Identify vessels supplying the heart (coronaries).

• Understand and list factors affecting coronary blood flow (cardiac cycle,
autoregulation, chemical factors & neural factors.

* We recommend studying anatomy lecture (Blood supply of the heart) first.

2 Contact us : Physiology435@gmail.com
 Coronary Circulation

It is important ! Why?
One third of all deaths in the world result from coronary artery disease.
Also it’s most likely to be seen in elderly patients.
Consists of :
1. Arterial supply. (supply of O2 to the cardiac muscle.)
2. Venous drainage. (removal of wastes and CO2)
3. Lymphatic drainage. (wastes removal)

- Anterior descending > left side of the heart

- Posterior descending > right side of the heart
3
 1-Arterial Supply

Cardiac muscle is supplied by two coronary arteries:

A. Right coronary artery (RCA.)

B. Left coronary artery (LCA.)

*both arise from the coronary sinuses just superior to the

aortic valve cusps at the aortic root.

- Coronary arteries deliver oxygenated blood to the cardiac muscle.

The aortic valve has three cusps:

A. left coronary (LC) cusp.
B. Right coronary (RC) cusp.
C. Posterior non-coronary (NC) cusp.

*There may be variations in the number, shape & location

of coronary ostia or origins of the coronary arteries,
most of which are of no clinical significance. Right coronary cusp gives rise to right coronary artery.
Left coronary cusp gives rise to left coronary artery.

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 1-Arterial Supply- EXTRA

Guyton corner : Figure 21-3 shows the heart and its

coronary blood supply. Note that the main coronary arteries lie on
the surface of the heart and smaller arteries then penetrate from
the surface into the cardiac muscle mass. It is almost entirely
through these arteries that the heart receives its nutritive blood
supply. Only the inner 1/10 millimeter of the endocardial surface
can obtain significant nutrition directly from the blood inside the
cardiac chambers, so this source of muscle nutrition is minuscule.
The left coronary artery supplies mainly the anterior and left
lateral portions of the left ventricle, whereas the right coronary
artery supplies most of the right ventricle, as well as the posterior
part of the left ventricle in 80 to 90 percent of people.
Most of the coronary venous blood flow from the left ventricular
muscle returns to the right atrium of the heart by way of the
coronary sinus, which is about 75 percent of the total coronary
blood flow. On the other hand, most of the coronary venous blood
from the right ventricular muscle returns through small anterior
cardiac veins that flow directly into the right atrium, not by way of
the coronary sinus. A very small amount of coronary venous blood
also flows back into the heart through very minute thebesian
veins, which empty directly into all chambers of the heart. Page
262

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 Coronary arteries

Left coronary artery (LCA) Right coronary artery (RCA)

Size larger smaller

origin* Left posterior aortic sinus Anterior aortic sinus

By anastomosing with the right By anastomosing with the left coronary

Termination coronary artery artery

• Left Anterior Descending (LAD) • Posterior Descending Branch

( = Anterior interventricular) ( = posterior interventricular)
Branches • Marginal artery • Marginal artery
• Circumflex artery (CX)

*Regarding the origin of coronary arteries, we are not quite sure about the points written in this physiology lecture
“not mentioned in Guyton” So, we prefer you cover this from anatomy (read more from the box below).

• SNELL Corner :
The aortic valve guards the aortic orifice and is precisely similar in
structure to the pulmonary valve. One cusp is situated on the
anterior wall (right cusp), and two cusps are located on the
posterior wall (left and posterior cusps). Behind each cusp, the
aortic wall bulges to form an aortic sinus. The anterior aortic sinus
gives origin to the right coronary artery, and the left posterior sinus
gives origin to the left coronary artery.

6 “Page 143 from Clinical Anatomy By Systems by Richard S. SNELL”

 Coronary arteries

RCA LCA

Left coronary artery branches :

1) Left anterior ascending “LAD”
2) Left marginal artery “LMA”
3) Circumflex artery “CX”
Right coronary artery branches :
1) Right marginal artery “RMA”
2) Posterior descending artery (posterior
7 interventricular artery) “PIv”
 Summary - Branches of the coronary arteries

8
 variations

Variations in coronary anatomy are often seen in association with structural forms of congenital heart disease like Fallot's tetralogy,
transposition of the great vessels, Taussig-Bing heart (double-outlet right ventricle), or common arterial trunk. Importantly, coronary
artery anomalies are a cause of sudden death in young athletes even in the absence of additional heart abnormalitiesPrior knowledge of
such variants and anomalies is necessary for planning various interventional procedures
9
 Areas of Distribution of Coronary Arteries

Left Circumflex Artery

Supplies the left
atrium and left
ventricles

Left Circumflex
Artery
Supplies the left
atrium and left
Left Anterior Descending ventricles
Artery
Supplies the right Right Coronary
ventricles ,left ventricles Artery
and interventricular Left Marginal Artery Supplies the right
Right Coronary
septum Supplies left atrium and right
Artery
Supplies the right ventricles ventricles
atrium and right Left Marginal Artery
ventricles Supplies left
ventricles

Posterior Interventricular
Artery
Supplies the right and left
Right Marginal Artery ventricles and interventricular
Supplies the right septum
Right Marginal Artery ventricles and the apex
Supplies the right
ventricles and the
apex
 10
 Areas of Distribution of LCA & RCA

Areas of distribution of Left Areas of distribution of Right

Coronary Artery Coronary Artery
• Left Coronary Artery (LCA) supplies: • Right atrium.

- Anterior & apical parts of the heart. • Ventricles:

- Greater part of Right ventricle, except the
- Anterior 2/3rd of the inter ventricular (IV) area adjoining the anterior inter ventricular
septum. groove.
- Inferior part of Lt ventricle adjoining the
• Circumflex (CX) branch supplies: posterior inter ventricular groove

- Lateral & posterior surfaces of the heart. • posterior 1/3rd of the inter
ventricular septum.

• The conducting system of the heart,

except :

- A part of the Lt branch of AV- Bundle.

- The SA- node is supplied by the LCA in
40% of cases.
 11
 Collateral Circulation

Cardiac Anastomosis:
The two coronary arteries anastomose in the myocardium
Extra cardiac anastomosis:
the two coronary arteries anastomose with:

1. Vasa vasorum of the aorta.

2. Vasa vasorum of pulmonary arteries.
3. Internal thoracic arteries.
4. The bronchial arteries.
5. Phrenic arteries

Extra cardiac channels open up in emergencies when the coronary arteries

are blocked.

• Collateral circulation = anastomosis

• What’s the benefit of the extra anastomosis?

It just open in emergency, when the coronary arteries have poor supply of the O2 and we need more O2 it’ll open

• It’s found originally in the heart, but it won’t be activated unless there’s emergency
 12
 2-Venous Drainage of The Heart

Coronary sinus, which lies in the

• Venous drainage brings deoxygenated cardiac
posterior part of the atrioventricular
blood back to the heart. groove & is a continuation of the great
cardiac vein.

• Cardiac venous drainage occur through: Anterior, middle & small cardiac
veins.

• Most of the venous blood return to the heart Venae Cordis Minimae
into the right atrium through the coronary (smallest cardiac veins.)
sinus via the cardiac veins.

• 5- 10% drains directly into heart chambers, right

atrium & right ventricle, by the anterior cardiac
vein & by the small veins that open directly into
the heart chambers.

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 3-Lymphatic Drainage of The Heart

Lymphatics of the heart

accompany the two coronary
arteries & form two trunks

The left trunk, ends into the

The right trunk, ends in the
tracheo-bronchial lymph nodes at
brachiocephalic node.
the bifurcation of the trachea.

 14
 Coronary Dominance

Coronary dominance depends on which artery (or arteries) gives rise to the posterior
descending artery (PDA), which runs along the posterior side of the heart & supplies
the AV- node.

Right dominant
Guyton corner :
A person can be: Co-dominant P 246, 12TH
Left dominant edition
The left coronary artery
supplies mainly the
anterior and left lateral
Clinical importance portions of the left
ventricle, whereas the
right coronary artery
In right or balances In left dominance, a supplies most of the
right ventricle, as well as
dominance, a block in block in left the posterior part of the
right coronary artery coronary artery left ventricle in 80 to 90
at least spares part affect the entire Lt percent of people.
(2/3) of the septum & ventricle & IV
left ventricle. septum.

 15
 Coronary Dominance

Coronary Dominance is recognized by the presence of septal perforating branches arise

from:

Balanced or co-dominance
is found in 7-10% of
The right coronary population where the The circumflex branch of
artery is dominant, posterior inter ventricular the left coronary artery,
in 80–85% cases. artery is formed by both in 8-10% cases.
right coronary &
circumflex branch of the
left coronary arteries.

Coronary Dominance
Right
dominant
left
dominant
Co-dominant
 16
 Coronary Blood Flow (CBF)

At rest
- CBF is about 225-250
mL/min (5% of cardiac
output.)

- Heart extracts 60-70% of

CBF increases during
O2, due to presence of
more mitochondria which exercise or work output.
generate energy for
contraction by aerobic
metabolism (other tissue
extract only 25%.)

• Guyton corner :
During strenuous exercise, the heart in the young adult increases its cardiac output fourfold to
sevenfold, and it pumps this blood against a higher than normal arterial pressure. Consequently, the
work output of the heart under severe conditions may increase sixfold to ninefold. At the same time, the
coronary blood flow increases threefold to fourfold to supply the extra nutrients needed by the heart.
This increase is not as much as the increase in workload, which means that the ratio of energy
expenditure by the heart to coronary blood flow increases. Thus, the "efficiency" of cardiac utilization of
 17 energy increases to make up for the relative deficiency of coronary blood supply.
 Phasic changes in CBF during systole & diastole

During Systole During Diastole

Blood flows to the

subendocardial portion of
• Coronary arteries the left ventricle occurs
are compressed. only during diastole, and
• Blood flow to the is not there during
left ventricle is systole. Therefore, this
reduced. region (subendocardial) is
prone to ischemic
damage & it is the most
Notes: common site of
- Less flow of blood in the systole, and more in the diastole. myocardial infarction.
- The blood reaches the subendocardial portion only during diastole.

Guyton corner: Figure shows the changes in blood flow through the nutrient capillaries of the left ventricular coronary system in
ml/min in the human heart during systole and diastole, as extrapolated from studies in experimental animals. Note from this diagram
that the coronary capillary blood flow in the left ventricle muscle falls to a low value during systole, which is opposite to flow in vascular
beds elsewhere in the body. The reason for this is strong compression of the left ventricular muscle around the intramuscular vessels
during systolic contraction.
During diastole, the cardiac muscle relaxes and no longer obstructs blood flow through the left ventricular muscle capillaries, so blood
flows rapidly during all of diastole.

 18
 Factors Affecting CBF

Factors affecting coronary blood flow

Pressure in the aorta

Chemical factors

Neural factors

 19
 Effect of Pressure Gradient of Aorta
& Different Chambers of the Heart

• CBF to the right side is

not much affected during
systole.

• Pressure difference
between the aorta & right
ventricle is greater during
systole than during
diastole, therefore, more
blood flow to right
ventricles occurs during
systole.

 20
 Factors Affecting Coronary Blood Flow

Chemical factors causing Coronary vasodilatation (Increased coronary blood flow):

• Lack of oxygen.
• Increased local concentration of Co2.
• Increased local concentration of H+ ion.
• Increased local concentration of k + ion.
• Increased local concentration of Lactate, Prostaglandin, Adenosine, Adenine nucleotides.

Neural Factors Affecting Coronary Blood Flow:

• Sympathetic stimulation.
• Parasympathetic stimulation.

 21
 Effect of Sympathetic Stimulation on
Coronary Blood Flow

Coronary arteries have:

• Alpha Adrenergic receptors, which mediate vasoconstriction (Epicardial.)

• Beta Adrenergic receptors, which mediate vasodilatation (Intramuscular.)

Sympathetic stimulation can either be Indirect or Direct.

Indirect effect of sympathetic stimulation:

Sympathetic stimulation in intact body will lead to release of adrenaline & nor
adrenaline, which in turn increase heart rate & force of contraction.
Vasodilator metabolites will be released leading to coronary vasodilatation.

Direct effect of sympathetic stimulation:

Experimentally, injection of noradrenalin after blocking of the Beta adrenergic

receptors in un anesthetized animals elicits coronary vasoconstriction.

 22
 Benefits of the indirect effect of Nor adrenergic discharge

What are the benefits of the indirect effect of Nor adrenergic discharge?

• The answer is : to preserve circulation of the heart while the flow to other
organs is compromised.

When systemic blood pressure gets low.

As a Reflex nor adrenergic discharge will increase.

Coronary Blood Flow will increase secondary to metabolic changes in the myocardium.

 23
 Effect of Parasympathetic Stimulation on
Coronary Blood Flow

• Vagal stimulation (Parasympathetic) causes coronary

vasodilatation.

• However, parasympathetic distribution is not great.

• There is more sympathetic innervation of coronary

vessels.

 24
 Coronary Blood Flow

Effect of Tachycardia on Coronary Blood Flow

The diastolic Coronary

Tachycardia period will be Blood Flow is
shortened reduced

Control of Coronary Blood Flow

• Coronary Blood Flow shows considerable auto regulation.

• Local muscle metabolism is the primary controller:

1. Oxygen demand is a major factor in local coronary blood flow regulation.

• Nervous control of Coronary Blood Flow:

1. Direct effects of nervous stimuli on the coronary vasculature.
2. Sympathetic greater effects than parasympathetic.

 25
 Extra “just read it, you will get the idea”

• Guyton corner :
Nervous Control of Coronary Blood Flow
Stimulation of the autonomic nerves to the heart can affect coronary blood flow both directly and indirectly. The direct effects result from
action of the nervous transmitter substances acetylcholine from the vagus nerves and norepinephrine from the sympathetic nerves on the
coronary vessels. The indirect effects result from secondary changes in coronary blood flow caused by increased or decreased activity of the
heart.
The indirect effects, which are mostly opposite to the direct effects, play a far more important role in normal control of coronary blood flow.
Thus, sympathetic stimulation, which releases norepinephrine from the sympathetic nerves and epinephrine as well as norepinephrine from the
adrenal medullae, increases both heart rate and heart contractility and increases the rate of metabolism of the heart. In turn, the increased
metabolism of the heart sets off local blood flow regulatory mechanisms for dilating the coronary vessels, and the blood flow increases
approximately in proportion to the metabolic needs of the heart muscle. In contrast, vagal stimulation, with its release of acetylcholine, slows
the heart and has a slight depressive effect on heart contractility. These effects decrease cardiac oxygen consumption and, therefore, indirectly
constrict the coronary arteries
Direct Effects of Nervous Stimuli on the Coronary Vasculature.
The distribution of parasympathetic (vagal) nerve fibers to the ventricular coronary system is not very great. However, the acetylcholine
released by parasympathetic stimulation has a direct effect to dilate the coronary arteries.
Much more extensive sympathetic innervation of the coronary vessels occurs. In Chapter 61, we see that the sympathetic transmitter
substances norepinephrine and epinephrine can have either vascular constrictor or vascular dilator effects, depending on the presence or
absence of constrictor or dilator receptors in the blood vessel walls. The constrictor receptors are called alpha receptors and the dilator
receptors are called beta receptors. Both alpha and beta receptors exist in the coronary vessels. In general, the epicardial coronary vessels have
a preponderance of alpha receptors, whereas the intramuscular arteries may have a preponderance of beta receptors. Therefore, sympathetic
stimulation can, at least theoretically, cause slight overall coronary constriction or dilation, but usually constriction. In some people, the alpha
vasoconstrictor effects seem to be disproportionately severe, and these people can have vasospastic myocardial ischemia during periods of
excess sympathetic drive, often with resultant anginal pain.
Metabolic factors, especially myocardial oxygen consumption, are the major controllers of myocardial blood flow. Whenever the direct effects
of nervous stimulation reduce coronary blood flow, the metabolic control of coronary flow usually overrides the direct coronary nervous effects
within seconds.

 26
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