The Heart

What is the Human Heart?• It is an electro-mechanical muscular pump

receiving blood from all parts of the body and pumps blood back to them.

• It measures about 12 cm in length and 8.5 cm width• Weighs about 300 grams in men and 250 grams in

women• Beats about 100,000 times per day• Beats 2.5 billion times in an average 70 yr. lifetime• Pumps about 9000 litres of blood each day• Circulates blood completely 1000 times each day

Where is the heart situated?

• It is situated in the thorax (chest cage) to the left side of the chest

• The lungs are its neighbours

• It is guarded by the ribs.

What Does the Heart Do?

Its function is to receive deoxygenated [‘impure’] blood through veins

Send it to the lungs for oxygenation [‘purification’]

Collect oxygenated [‘purified’] blood from lungs

Pump oxygenated [‘purified] blood into the rest of the body through the arteries

How Does the Heart Look From the Outside?

What roles do various parts of the heart play?

• The heart has two veins flowing into the right atrium carrying deoxygenated (‘impure’) blood from various parts of the body called:

1. Superior Vena Cava (SVC)2. Inferior Vena Cava (IVC) SVC drains head, neck & upper limbs IVC drains blood from the thoracic & abdominal

organs and the lower limbs• SVC and IVC drain into the right atrium (RA)• There is a valve allowing unidirectional flow between

the RA & RV called tricuspid valve (TV)

SV

CIVC

RA

TV

What roles do various parts of the heart play?

• Blood then enters the right ventricle (RV) from the RA across TV

• The RV pumps blood to the lungs via pulmonary artery (PA).

• The main pulmonary artery (MPA) or the pulmonary trunk divides into two i.e. Left pulmonary artery (LPA) and right pulmonary artery (RPA) thereby carrying blood to the left and right lungs respectively.

• There is a unidirectional valve between RV and PA known as the pulmonary valve (PV)

SV

C

RA

IVC

TV

RV

PV

MPA

LPARPA

What roles do various parts of the heart play?

• Blood is then oxygenated (‘purified’) in the lungs.• Flow of blood from lungs back to the heart is

through four pulmonary veins (PV) known as left upper (LAPV), left lower (LLPV), right upper (RUPV) and right lower (RLPV) pulmonary veins respectively

• Blood from the PV flows into the left atrium (LA)• There is a unidirectional valve separating LA

from left ventricle (LV) called the mitral valve (MV)

SV

C

RA

IVC

TV

RV

PV

MPA

LPARPA

RUPV

RLPV

LUPV

LLPVLA

MV

What roles do various parts of the heart play?

• Blood enters LV through MV• Oxygenated (‘pure’) blood is pumped out of the LV to the

rest of the body into the main artery called aorta (AO).• There exists an unidirectional valve between LV and AO

known as the Aortic valve (AV)• The atria are separated from each-other by a muscular

wall known as inter-atrial septum (IAS)• The ventricles are separated from each-other by a

muscular wall known as inter-ventricular septum (IVS)S

VC

RA

IVC

TV

RV

PV

MPA

LPARPA

RUPV

RLPV

LUPV

LLPVLA

MVAV

LV

IVSAO

AO

• The Left Ventricle & the arterial system is a high pressure – high resistance circuit.

• The right sided chambers and lungs are a low pressure – low resistance.

How does the Inside of the Heart Look?

How Does the heart pump?• It is first electrically activated. • The impulse forming point of the

heart is known as the Sino-atrial (SA) node

• The impulse of activation then passes through the atria

• Then on to the Atrio-ventricular (AV) node

• The impulse is delayed at the AV node for a few hundredths of a second (nodal delay) and then passes to the bundle of His

• The bundle of His divides into left & right branches, activating the corresponding ventricles.

• The muscle fibres of the ventricles are supplied by Purkinje fibres.

How can heart’s electrical activity be captured on paper?

• The graphic representation of heart’s electrical activity is known as the electrocardiogram (ECG / EKG)

What happens to the heart after it is electrically activated?

• Electrical activation triggers mechanical action i.e. contraction. In other words it is the electrical activation that activate the muscle to contract in an orderly & rhythmic manner.

• Thereby enabling reception & distribution of blood

Slides that follow will explain heart’s mechanical action

Blood Flows Into the Heart…• Form the veins and

then on to SVC and IVC

• Enters RA

• Tricuspid Valve opens

• Blood then enters the Right Ventricle Atria Contract

svcIVC

RA

TV

RV

Then….Into the Lungs

• Exits the heart through the Pulmonary Valve

• Ventricles Contract

• Enters Pulmonary Trunk

• Then Into Lungs

PV

MPA

RPA LPA

Blood Returns to Heart

• From Lungs enters Pulmonary veins.

• Enters Heart in Left Atrium

• Mitral valve opens• Blood flows into Left Ventricle• Atria contract

PV

PV

PV

PVLA

MV

LV

Then …Pumped Back Into the Body

• Ventricles contract• Blood flows through

Aortic Valve.• Blood enters Ascending

Aorta (coronary arteries i.e. arteries supplying the heart are at the aortic root)

• Aortic Arch (arteries supplying head and upper limbs are situated here)

• Descending Aorta (thoracic, abdominal organs & lower limbs are thus supplied)

LV

AV

Asc.Ao

AO Arch

Des. AO

The Normal Heart and Regional Circulation

Anterior Cutaway View

Pulmonary Valve

Superior Vena Cava

Aorta

Left Pulmonary Artery

Right Pulmonary Artery

Right Pulmonary Veins

Inferior Vena Cava

Tricuspid Valve

Septum

Lungs

Left Atrium

Left Pulmonary

Veins

Mitral Valve

AorticValve

Blood Supply to the Heart• Heart is supplied by coronary arteries.• They have their origin just above the aortic root. [Aortic

root is the area of the aorta just above the aortic valve i.e. the point of attachment of aorta to the heart]

• There are two of coronary arteries i.e. the left and right• The left main coronary artery (LMCA) bifurcates into two

i.e. the left anterior descending (LAD) and left circumflex (LCx)

• The right coronary artery (RCA) continues as such and gives off its branches.

• Majority of the venous drainage of the heart is from the corresponding coronary veins which drain into the coronary sinus which empties into RA

Paths of Circulation

Paths of Circulation

Pulmonary Circuit• Consists of vessels

that carry blood from the heart to the lungs and back to the heart

Systemic Circuit• Carries blood from

the heart to all other parts of the body and back again

• Includes coronary circulation (supplies the heart)

Pulmonary CircuitRight Ventricle

Pulmonary Trunk

Right and Left Pulmonary Arteries

Capillary Networks associated with walls of alveoli (air sacs in lungs where gas exchange occurs between air and blood)

Pulmonary Venules

Pulmonary veins – 2 from each lung

Left atrium

Systemic Circuit

Left Atrium (oxygenated blood)

To Left Ventricle

Aorta and its branches that lead to all body tissues

Accompanying system of veins returns blood to Right Atrium

The Normal Heart and Regional Circulation

Anterior Cutaway View

Pulmonary Valve

Superior Vena Cava

Aorta

Left Pulmonary Artery

Right Pulmonary Artery

Right Pulmonary Veins

Inferior Vena Cava

Tricuspid Valve

Septum

Lungs

Left Atrium

Left Pulmonary

Veins

Mitral Valve

AorticValve

Blood Vessels

Blood Vessels

• They are a closed circuit of tubes that carry blood from heart to cells and back

• They include: arteries, arterioles, capillaries, venules, and veins

• 100,000 Km of blood vessels in the body (2 ½ times around the world)

Arteries

• Strong elastic vessels adapted for carrying blood away from the heart under high pressure

• Subdivide into progressively thinner tubes and give rise to arterioles (finer branched tubes that connect artery to capillary)

• Walls of artery consist of 3 layers– Arterioles have 3 layers similar to arteries, but

thin as they approach capillaries.

Internal Structure of Arteries

Artery

Tunica externa(adventitia)

Tunica media

Externalelasticlamina

Tunica intima(endothelium)

Elastic membrane

Courtesy Servier

Tunica Intima (Interna)

• Innermost layer• Composed of endothelium (squamous

epithelium) and collagenous and elastic connective tissue

• Helps prevent blood clotting by providing a smooth surface that allows blood cells and platelets to flow through without being damaged.

• Secretes chemical substances that inhibit platelet aggregation (thereby preventing establishment of blood clots)

• May help regulate blood flow by secreting substances that dilate or constrict blood vessels.

Tunica Media

• Middle layer

• Bulk of arterial wall

• Comprises of

- Smooth muscle fibers encircling it

- Thick layer of elastic connective tissue

Tunica Adventitia (Externa)

• Outer layer

• Connective tissue with irregularly organized elastic and collagen fibers

• Attaches the artery to surrounding tissues.

Nerve Supply to Blood Vessels

• Sympathetic branches of the autonomic nervous system use vasomotor fibers to innervate smooth muscle in artery and arteriole walls

• Vasoconstriction: vasomotor impulses stimulate contraction which reduces diameter of vessel

• Vasodilation: vasomotor impulses are inhibited, muscle fibers relax and diameter of vessel increases.

• Changes in diameter of vessels greatly influence blood flow and pressure.

Capillaries

• Smallest-diameter blood vessels• Connect the smallest arterioles and smallest

venules• Composed of extensions of the inner linings of

arterioles in that their walls are endothelium• Thin walls form semipermeable layer through

which substances in blood are exchanged with tissue fluid surrounding body cells

Capillaries

• Osmotic Pressure: force due to salts etc. in the blood that brings water into vessel.

• Blood Pressure: force exerted by the heart in contractions.

• BP changes as the arteries get further away from heart.

• OP remains the same.• Difference between BP & OP determines

materials movement in or out of vessel.

Capillaries

Arterialend

Venous end

BP 30

Osmotic21

Osmotic21

Diff: 9Outward

BP15

Diff: 6Inward

Capillaries

• At arterial end: nutrients, oxygen etc. move from vessel to cells.

• At venous end: waste, carbon dioxide etc. move from cells to vessel.

• But not all fluid moves into the vessels.

• A third vessel found along vein, Lymphatic Vessel.

Exchanges in capillaries

• Gases, nutrients, and metabolic by-products are exchanged by diffusion, filtration and osmosis

• Diffusion– Substances diffuse down concentration

gradient.– Plasma proteins remain in blood. Too large

to diffuse through membrane pores and also are not soluble in the lipid portion of capillary cell membranes.

Venules and Veins

• Venules: microscopic vessels that continue from capillaries and merge to from veins.

• Veins: carry blood back to the atria following pathways that roughly parallel arteries.

Veins Have Valves arteries dont!

Vein

Valve

Tunica externa(adventitia)

Tunica media

Tunica intima(endothelium)

Elastic membrane

Courtesy Servier

Venous Valves

• Found in many veins, especially in lower limbs• Project inward from the lining• Composed of 2 leaflets that close if blood begins

to back up in a vein.• Aid in returning blood to the heart (open if flow is

toward the heart)

Veins as Blood Reservoirs

• In a hemorrhage accompanied by a drop in arterial pressure, sympathetic nerve impulses reflexively stimulate venous constriction of the walls.

• This helps maintain blood pressure by returning more blood to the heart.

• Normal flow can be maintained when even as much as 25% volume is lost.