Circulation of blood through a particular organ or a region of the body is called regional circulation.

Blood flow through a region is proportional to the local activity.

Different Regional Circulations

• Coronary circulation
• Cerebral circulation
• Splanchnic circulation
• Capillary circulation
• Circulation through skeletal muscle
• Cutaneous circulation
• Renal circulation
• Pulmonary circulation

Coronary circulation is the circulation of blood through blood vessels of heart muscle (myocardium).

Blood flowing through chambers of heart does not nourish the myocardium. Only blood in coronary blood vessels supplies adequate oxygen and nutrients to myocardium.

Distribution of Coronary Blood Vessels

Coronary Arteries

Heart muscle is supplied by two coronary arteries, namely right and left coronary arteries, which are the first branches of aorta.

Coronary arteries encircle the heart like a crown; hence the name coronary arteries (Latin word corona = crown).

Branches of Coronary Arteries

Coronary arteries divide and subdivide into smaller branches, which run all along the surface of the heart.

Smaller arterial branches are called epicardial arteries.

Epicardial arteries give rise to further smaller branches known as intramural or final arteries.

Intramural arteries run at right angles through heart muscle near the inner aspect of wall of the heart.

Venous Drainage

Venous drainage from heart muscle is by three types of vessels.

1. Coronary Sinus

Coronary sinus is the larger vein draining about 75% of coronary flow.

It drains blood from left side of the heart and opens into right atrium near the tricuspid valve.

2. Anterior Coronary Veins

Anterior coronary veins drain blood from right side of heart and open directly into right atrium.

3. Thebesian Veins

Thebesian veins drain deoxygenated blood from myocardium directly into the concerned chamber of heart.

Physiological Shunt

Physiological shunt is a diverted route through which venous blood is mixed with arterial blood.

Deoxygenated blood flowing from thebesian veins into cardiac chambers makes this part of normal physiological shunt.

Another component of physiological shunt is the drainage of deoxygenated blood from bronchial circulation into pulmonary vein without oxygenated blood flow.

Normal Coronary Blood Flow

Normal blood flow through coronary circulation is about 200 mL/min.

It forms about 4% of cardiac output.

It is about 65 to 70 mL/min/100 g of cardiac muscle.

Phasic Changes in Coronary Blood Flow

Blood flow through coronary arteries is not constant. It decreases during systole and increases during diastole.

During systole, the coronary blood vessels are compressed and blood flow is reduced.

During diastole, compression is released and blood vessels are distended, so the blood flow is increased.

Like any other organ, heart also has the capacity to regulate its own blood flow by autoregulation.

Coronary blood flow is not affected much when mean arterial pressure varies between 60 and 150 mm Hg.

Factors Regulating Coronary Blood Flow

1. Need for oxygen
2. Metabolic factors
3. Coronary perfusion pressure
4. Nervous factors

1. Need for Oxygen

Amount of blood passing through coronary circulation is proportional to the consumption of oxygen by cardiac muscle.

Even in resting condition, a large amount of oxygen, i.e. 70 to 80%, is consumed by heart muscle than in any other tissues.

In conditions associated with increased cardiac activity, the need for oxygen increases enormously.

This leads to coronary vasodilation and increase in blood flow to heart.

2. Metabolic Factors

Metabolic products increase coronary blood flow by causing coronary vasodilation under hypoxic conditions.

Increase in blood flow due to vasodilator effects of metabolites is called reactive hyperemia.

Metabolic Products Which Increase Coronary Blood Flow

• Adenosine
• Potassium
• Hydrogen
• Carbon dioxide
• Adenosine phosphate compounds

3. Coronary Perfusion Pressure

Coronary perfusion pressure is the balance between mean arterial pressure in aorta and right atrial pressure.

Since right atrial pressure is low, mean arterial pressure becomes the major factor that maintains coronary blood flow.

4. Nervous Factors

Coronary blood vessels are innervated both by parasympathetic and sympathetic divisions of autonomic nervous system.

These influence the coronary blood flow only indirectly by acting on musculature of heart.

During sympathetic stimulation, the rate and force of contraction of heart are increased, resulting in liberation of metabolites.

Metabolites dilate the blood vessels and increase blood flow.

During parasympathetic stimulation, cardiac functions are inhibited and the liberation of metabolites is less.

Coronary blood flow decreases.

Salient Features of Coronary Circulation

1. It is very short and very rapid.
2. Blood flow occurs mainly during diastolic phase.
3. There is no effective anastomoses between coronary vessels.
4. It is a rich circulation with 4% of cardiac output flowing through it.
5. Its regulation is mainly by metabolites and not neural factors.
6. Capillary permeability is high which makes the cardiac lymph rich in protein.
7. Coronary vessels are highly susceptible to degeneration and atherosclerosis.

Microcirculation and Capillary Circulation

Microcirculation is the flow of blood through the minute blood vessels such as arterioles, capillaries and venules.

Capillary circulation forms the major part of microcirculation.

Capillaries are formed by a single layer of endothelial cells which are wrapped around by pericytes.

Salient Features of Capillaries

1. Capillaries arise from arterioles and form the area for exchange of materials between blood and tissues.
2. Capillaries outnumber the other blood vessels. About 10 billion capillaries are present in the body.
3. Each capillary lies in very close proximity to the cells of tissues at a distance of about 20 to 30 mm. This enables easy and rapid exchange of substances between blood and tissues through interstitial fluid.

Pattern of Capillary System

Capillaries are disposed between arterioles and venules.

From the arterioles, the meta-arterioles take origin.

From meta-arterioles, two types of capillaries arise:

1. Preferential channels
2. True capillaries

1. Preferential Channels or Continuous Capillaries

After arising from the meta-arterioles, the preferential channels form a network and finally join the venules.

2. True Capillaries

After arising from meta-arterioles, true capillaries also form a network and join the venules.

Smooth muscle fibers encircle the beginning of true capillaries forming a sphincter called precapillary sphincter.

This sphincter controls the blood flow through true capillaries.

Anatomical and Physiological Shunts

Anatomical Shunt: Arteriovenous Shunt

Anatomical shunt is the direct link between arterioles and venules. It is also called arteriovenous shunt.

Flow of blood through the capillaries where exchange of nutrients, gases and other substances takes place is called nutritional flow.

Flow of blood through anatomical shunt is called non-nutritional flow.

Non-nutritional blood flow occurs in many tissues of the body particularly during conditions when metabolic activities are low.

Physiological Shunt

Physiological shunt is a link between arterial and venous side of circulation provided by meta-arteriole.

Many tissues of the body such as muscles do not have anatomical shunts. However, the arterioles in these tissues act as the physiological shunt between arterial and venous sides of circulation.

Non-nutritional blood flow occurs through physiological shunt under resting conditions.

Shunt in Capillaries vs Shunt in Heart

Physiological shunt in capillaries is different from physiological shunt in heart.

In capillaries, the shunt diverts blood flow towards deoxygenated blood.

But in heart, the shunt directs deoxygenated blood towards oxygenated blood.

Peculiarities of Capillary Blood Flow

1. Blood does not pass through capillary system continuously. It is because of alternate constriction and dilation of meta-arterioles and alternate opening and closure of precapillary sphincters.
2. Direction of blood flow through capillaries is not fixed as in the case of other blood vessels. Blood may flow in opposite direction in two adjacent capillaries.
3. In capillaries, blood flows as a single pile or single row of blood cells. In other blood vessels, the blood flows in either axial stream containing mainly blood cells or peripheral stream containing plasma.
4. Under resting conditions, most of the capillaries lie in collapsed state. Only during activity, all the capillaries open up and increase the vascularity.
5. Amount of blood flowing through the capillary system throughout the body is very low. It is only about 150 mL/min.
6. Velocity of blood flow is least in capillaries. It is only about 0.5 to 1 mm/sec. It facilitates exchange of substances between the capillaries and tissues.

Functions of Capillaries

Most important function of capillaries is the exchange of substances between blood and tissues.

• Oxygen, nutrients and other essential substances enter the tissues from capillary blood.
• Carbon dioxide, metabolites and other unwanted substances are removed from the tissues by capillary blood.

Exchange of materials across capillary endothelium occurs primarily by diffusion.

It also occurs by means of filtration and pinocytosis.

Normal Blood Flow to Liver

Liver receives maximum amount of blood as compared to any other organ in the body since most of the metabolic activities are carried out in liver.

Blood flow to liver is 1,500 mL/min, which forms 30% of cardiac output.

It is about 100 mL/100 g tissue per minute.

Normally, about 1,100 mL of blood flows through portal vein and remaining 400 mL of blood flows through hepatic artery.

However, portal vein carries only about 25% of oxygen to liver. It is because it carries blood which has already passed through blood vessels of GI tract where oxygen might have been used.

Hepatic artery transports 75% of oxygen to the liver.

Skeletal muscle is a unique organ because amount of blood flowing through it can vary over a large range.

Blood Flow to Skeletal Muscles

During resting condition, blood flow to skeletal muscle is about 4 to 7 mL/100 g/min.

During exercise, it increases to about 100 mL/100 g/min.

Muscular Contraction and Blood Flow

During contraction of muscle, blood vessels are compressed and the blood flow decreases.

During relaxation of muscle, the compression of blood vessels is relieved and the blood flow increases.

In severe muscular exercise, blood flow increases in between the muscle contractions.

Architecture of Cutaneous Blood Vessels

1. Arterioles arising from smaller arteries reach the dermis of skin.
2. After taking origin, arterioles turn horizontally and give rise to meta-arterioles.
3. From meta-arterioles, hairpin-shaped capillary loops arise. Arterial limb of the loop ascends vertically and then forms a venous limb, which descends down.
4. After reaching the base of dermis, few venous limbs of neighboring papillae unite to form the collecting venule.
5. Collecting venules anastomose with one another to form the subpapillary venous plexus. Subpapillary plexus runs horizontally and drains into the deeper veins.

Functions of Cutaneous Circulation

Cutaneous blood flow performs two functions:

1. Supply of nutrition to skin.
2. Loss of heat from the body and regulation of body temperature.

Normal Blood Flow to Skin

Under normal conditions, blood flow to skin is about 250 mL/sq. m/min.

When body temperature increases, cutaneous blood flow increases to 2,800 mL/sq. m/min because of cutaneous vasodilation.

Applied Physiology: Vascular Responses of Skin to Mechanical Stimuli

Vascular responses of skin are reactions developed in cutaneous blood vessels when some mechanical stimuli are applied over the surface of skin.

Vascular responses of skin are of two types:

1. White reaction
2. Lewis triple response

1. White Reaction

White reaction is the response of blood vessels in skin to mechanical stimulus.

When the surface of skin is stroked lightly with a pointed object, a pale line appears within 20 seconds.

This line takes the path of stroke. This response in skin is known as white reaction.

Maximum intensity of the line is obtained in 1 minute and it fades away after 5 minutes.

White reaction is due to the constriction of cutaneous capillaries.

Capillaries constrict because of local stimulation of capillary wall and exertion of tension upon capillary wall.

No nervous factor is involved in this process.

2. Lewis Triple Response

Lewis triple response is a vascular response of skin that includes three consecutive reactions of blood vessels of skin to a mechanical stimulus.

It was discovered by Lewis Sir Thomas in 1927.

He noticed that the vascular reactions of skin to various injuries occur in three stages and named these reactions as triple response.

Triple Response Has Three Reactions

1. Red reaction
2. Flare
3. Wheal

1. Red Reaction

Red reaction is appearance of a red line when a pointed instrument is drawn firmly over the surface of skin.

This reaction occurs over the line of stroke.

Red reaction appears within 15 seconds after the stroke. It obtains maximum intensity at the end of 1 minute and disappears later gradually.

Red reaction is because of dilation of capillaries due to mechanical stimulus.

This reaction is purely a local response. It occurs due to release of histamine-like substance from tissues damaged by the stimulus. Lewis called it H substance.

Red reaction does not depend upon nervous factors. It occurs even after sectioning or degeneration of nerves of skin.

2. Flare

If the stroke is applied with little more force or if the stroke is repeated on the same line, red reaction spreads around the line of stroke.

It spreads for about 10 cm from the line of the stroke, depending upon the force applied.

The area is called flare or spreading flush.

Flare appears within 30 seconds after appearance of red line. It also disappears later.

It is due to dilation of arterioles.

It depends upon nervous mechanism and is due to axon reflex.

Axon Reflex or Antidromic Reflex

It is a process by which impulses are conducted in a direction opposite to the normal direction.

Normally, impulses produced by a cutaneous pain receptor pass through sensory nerve fibers towards the nerve cell body in posterior root ganglion.

Some of the impulses pass through the other branches of the same fiber in opposite direction and reach the blood vessels supplied by these branches.

Impulses now dilate the blood vessels. This is called the antidromic or axon reflex.

Nerve fibers transmitting the impulses in opposite direction are called antidromic vasodilator fibers.

Flare occurs if the main trunk of nerves is intact. It does not occur when the nerves degenerate.

3. Wheal

When intensity of stimulus is severe, surface of skin on the line of stroke is interrupted.

A small elevation or swelling is seen in the surrounding area up to a height of 2 mm.

It is called wheal or local edema.

Wheal appears within 3 minutes after the stimulus and it replaces the red line.

Maximum height is obtained within 5 minutes and it disappears after several hours.

Wheal appears due to leakage of fluid from capillaries since permeability of capillary membrane is increased.

Wheal does not depend upon nervous mechanism.

Dermographism and Tattoo

Dermographism or skin writing is a procedure to emboss letters or designs over skin in the same manner by which the wheal is produced.

The marks done by dermographism disappear within 30 minutes to some days.

Dermographism is different from tattoo.

Tattoo is the permanent marking of letters or designs over the skin with tattoo ink.

Ingredients of tattoo ink include pigments, binders and additives.

Adverse effects of tattoo in some cases are allergic reactions, bacterial or viral infections.

Tattoo ink also may burn during MRI scanning.

Cerebral circulation means flow of blood through the blood vessels of brain.

Importance of Cerebral Circulation

Brain tissues need adequate blood supply continuously.

Stoppage of blood flow for 5 seconds leads to unconsciousness and stoppage for 5 minutes leads to irreparable damage to the brain cells.

Cerebral Blood Vessels

Brain receives blood from basilar artery and internal carotid artery.

Branches from both the arteries form the circle of Willis.

Venous drainage from brain tissues is by sinuses, which open into internal jugular vein.

Normal Cerebral Blood Flow

Normally, brain receives about 750 to 800 mL of blood per minute.

It is about 15 to 16% of total cardiac output and about 50 to 55 mL/100 g of brain tissue per minute.

Regulation of Cerebral Blood Flow

Cerebral circulation is regulated by three factors:

1. Autoregulation
2. Chemical factors
3. Neural factors

1. Autoregulation

Like any other vital organ, brain also regulates its own blood flow by means of autoregulation.

However, the autoregulation in brain has got its limitations.

2. Chemical Factors

Chemical factors which increase cerebral blood flow are:

1. Decreased oxygen tension
2. Increased carbon dioxide tension
3. Increased hydrogen ion concentration

3. Nervous Factors

Cerebral blood vessels are supplied by sympathetic vasoconstrictor fibers.

But these fibers do not regulate cerebral blood flow under normal conditions.

In pathological conditions like hypertension, sympathetic nerves cause constriction of cerebral blood vessels, leading to reduction in blood flow.

It prevents cerebral vascular hemorrhage and cerebral stroke.

Splanchnic or visceral circulation constitutes three portions:

1. Mesenteric circulation supplying blood to GI tract.
2. Splenic circulation supplying blood to spleen.
3. Hepatic circulation supplying blood to liver.

Unique feature of splanchnic circulation is that, blood from mesenteric bed and spleen forms a major amount of blood flowing to liver.

Blood flows to liver from GI tract and spleen through portal system.

I. Mesenteric Circulation

Distribution of Blood Flow in Mesenteric Circulation

II. Splenic Circulation

Importance of Splenic Circulation

Spleen is the main reservoir for blood.

Due to dilation of blood vessels, a large amount of blood is stored in spleen.

Constriction of blood vessels by sympathetic stimulation releases blood into circulation.

Storage of Blood in Spleen

Two structures are involved in spleen in storage of blood, namely:

1. Splenic venous sinuses
2. Splenic pulp

Small arteries open directly into the venous sinuses.

When spleen expands, venous sinuses swell and large quantity of blood is stored.

Capillaries of splenic pulp are highly permeable.

So, most of the blood cells pass through capillary membrane and are stored in the pulp.

III. Hepatic Circulation

Hepatic Blood Vessels

Liver receives blood from two sources:

1. Hepatic artery from aorta.
2. Portal vein from mesenteric and splenic vascular bed.

More details are given in the relevant chapter on liver circulation.

Coronary Artery Disease

Coronary artery disease (CAD) or coronary heart disease is a heart disease caused by inadequate blood supply to cardiac muscle due to occlusion of coronary artery.

Coronary Occlusion

Coronary occlusion means partial or complete obstruction of coronary artery.

Occlusion occurs because of atherosclerosis, a condition associated with deposition of cholesterol and other substances on the wall of artery.

In due course, this part of the arterial wall becomes fibrotic and it is called an atherosclerotic plaque.

This plaque is made up of cholesterol, calcium and other substances from blood.

Because of atherosclerotic plaque, the lumen of coronary artery becomes narrow. In severe conditions, the artery is completely occluded.

Development of atherosclerotic plaque is common in coronary arteries near their origin from aorta.

This plaque activates platelets, resulting in thrombosis and blood clot is called thrombus.

When three-fourth of the lumen of coronary artery is obstructed either by atherosclerotic plaque or thrombus, blood flow to myocardium is reduced.

It results in ischemia of myocardium.

Coronary thrombosis is associated with spasm of coronary artery.

Smaller blood vessels are occluded by the thrombus or part of atherosclerotic plaque detached from coronary artery.

This thrombus or part of the plaque is called embolus.

Myocardial Ischemia

Ischemia means inadequate blood supply to any organ or area of body due to obstruction of blood vessels.

Myocardial ischemia is the inadequate blood supply to myocardium of heart caused by occlusion of coronary arteries.

Lack of adequate blood supply results in hypoxia.

Blood flow is usually restored if a small quantum of myocardium is affected by ischemia due to obstruction of smaller blood vessels.

It is because of rapid development of coronary collateral arteries.

Necrosis

Necrosis is the death of cells or tissues by injury or disease in a localized area.

When coronary occlusion is severe involving larger blood vessels, severe ischemia develops and it leads to necrosis of myocardium.

Necrosis is irreversible.

Myocardial Infarction: Heart Attack

Myocardial infarction is the necrosis caused by insufficient blood flow of myocardium due to embolism, thrombus or vascular spasm.

It is also called heart attack.

In myocardial infarction, death occurs rapidly due to ventricular fibrillation.

Risk Factors of Coronary Artery Disease / Heart Attack

1. Hypertension
2. High level of cholesterol, triglyceride or low-density lipoprotein in blood
3. Diabetes mellitus
4. Overweight and obesity
5. Lack of physical activity
6. Smoking
7. High stress
8. Family history

Common Symptoms of Myocardial Infarction

1. Cardiac pain
2. Nausea
3. Vomiting
4. Palpitations
5. Difficulty in breathing
6. Extreme weakness
7. Sweating
8. Anxiety

Cardiac Pain: Angina Pectoris

Cardiac pain is the chest pain caused by myocardial ischemia.

It is also called angina pectoris.

It is the common manifestation of coronary artery disease.

Pain starts beneath the sternum and radiates to the surface of left arm and left shoulder.

Cardiac pain is a referred pain since it is felt over the body area away from heart.

It is because heart and left arm are derived from the same dermatomal segment in embryo.

Applied Physiology: Varicose Veins

Varicose vein is the vein that becomes irregularly swollen, twisted or tortuous and enlarged.

Superficial veins of the leg are mostly affected.

Causes of Varicose Vein

1. Permanent dilation of veins due to incompetence of the valves of the veins or absence of muscular activity for long periods.
2. Thrombophlebitis: inflammation of vein associated with formation of thrombus.

Applied Physiology: Stroke

Definition

Stroke is the sudden death of neurons in localized area of brain due to inadequate blood supply.

It is characterized by reversible or irreversible paralysis with other symptoms.

Stroke is also called cardiovascular accident (CVA) or brain attack.

Types of Stroke

1. Ischemic stroke: Occurs due to interruption of blood flow to a part of brain by thrombus or atherosclerotic embolus.
2. Hemorrhagic stroke: Develops due to leakage or rupture of an aneurysm in brain and spilling of blood into surrounding area.

An aneurysm means bulging or ballooning of an area of a blood vessel caused by weakness of vessel wall.

Causes of Stroke

1. Heart disease
2. Hypertension
3. High cholesterol in blood
4. High blood sugar (diabetes mellitus)
5. Heavy smoking
6. Heavy alcohol consumption

Symptoms of Stroke

Symptoms of stroke depend upon the area of brain that is damaged.

1. Weakness
2. Numbness or paralysis particularly on one side of the body
3. Impairment of speech
4. Emotional disturbances
5. Loss of coordination
6. Loss of memory
7. Dizziness
8. Loss of consciousness
9. Coma or death