Size and Location
• About the size of the fist and weighs less
than a pound
• Enclosed within the mediastinum, medial
cavity of the thorax
• Rests on superior surface of diaphragm
• Anterior to vertebral column and
posterior to sternum
• Lungs flank laterally and partially
• ~ 2/3 of it lies to left
• Broad, flat base or posterior surface
• Heart enclosed in double-wall sac
called the pericardium
• Loosely fitting superficial part of sac
is the fibrous pericardium
• The pericardium:
– Protects heart
– Anchors it to surrounding surfaces
– Prevents overfilling of the heart with
• Deep to fibrous pericardium is the
serous pericardium, which is thin
and slippery and composed of 2
– The two layers have a film of serous
fluid between them
– Allows the hear to work in a relatively
• Inflammation of the pericardium,
pericarditis, hinders production of
serous fluid and roughens the
• Creates a creaking sound that can be
hear with a stethoscope and pain
deep to the chest.
• If persists, can cause adhesions and
impede heart activity
Layers of Heart
- visceral layer of the serous pericardium
- often infiltrated with fat
- composed mainly of cardiac muscle
- forms bulk of heart
- layer that contracts
- glistening white sheet of endothelium
- lines heart chambers and covers valves
The heart has 4 chambers:
2 superior atria and
2 inferior ventricles
The internal partition that
divides the heart is the
interatrial septum when it
separates the atria and the
when it separates the
Atria: The Receiving
• The atrium has two basic parts:
– A smooth-walled posterior part
– An anterior part with ridged walls
• The interatrial septum has a shallow
depression called the fossa ovalis.
This marks the spot where the
foramen ovale existed in the fetal
• Receiving chambers for blood
returning to the heart from the
• Contract minimally to push blood
into the ventricles; therefore, they
are relatively small and thin.
• Blood enters the right atrium
through 3 veins:
– Superior vena cava – returns blood
from body regions superior to the
– Inferior vena cava – returns blood from
body areas below the diaphragm, and
– Coronary sinus – collects blood from the
• Four pulmonary veins enter the left
atrium, which make up most of the
• The pulmonary veins transport
blood from the lungs back to the
• These vessels are best seen in a
posterior view of the heart.
Ventricles: The Discharging
• Make up most of the heart
• Right ventricle forms heart anterior
• Left ventricle forms inferior surface
• When contracted, blood is propelled out of
the heart into circulation.
– Right ventricle pumps blood into pulmonary
trunk to the lungs for gas exchange
– Left ventricle pumps blood into the aorta to
the body’s systems
• Heart is two side-by-side pumps
• Each side serves two different
– Pulmonary Circuit
– Systemic Circuit
Pulmonary Circuit Pump
• Right side
• Blood returns from body, which is oxygen-
poor and carbon dioxide-rich and enters
the right atrium.
• Then, it passes into the right ventricle,
which pumps it to the lungs via the
• In the lungs, blood unloads the carbon
dioxide and pucks up oxygen.
• Freshly oxygenated blood is carried to the
left side of heart.
Systemic Circuit Pump
• Left side of heart
• Freshly oxygenated blood leaves lungs to
return to left atrium and passes into left
ventricle, which pumps into the aorta.
• Blood is transported via smaller arteries
to body tissues, where gases and nutrients
• Blood loaded with carbon dioxide and
oxygen depleted, returns through the
systemic veins to right side of heart,
where enters venae cavae.
• Although equal volumes are
pumped, 2 ventricles have unequal
– Pulmonary circuit is short and low-
– Systemic circuit is very long and high-
•Encounters 5x’s as much friction
•Walls are 3x’s as thick
•Cavity is nearly circular
• Feeds the heart and is the shortest
circulation of body
• Actively delivers blood when heart
is relaxed, but are ineffective when
ventricles are contracting because:
– They’re compressed by contracting
– The entrances are partly blocked by
flaps of valves.
• Myocardial cells are weakened by
temporary lack of oxygen, but don’t
• Complete blockage of a coronary
artery leads to tissue death and a
myocardial infarction, or heart
attack or coronary.
• Cardiac muscle is amitotic, which is
replaced by noncontractile scar
Blood flows through the heart in one
direction: from atria to ventricles.
One way traffic is enforced by heart
Valves open and close in response to
changes in blood pressure.
• Located at each atrial-ventricular
junction, preventing backflow into
the atria when the ventricles are
• Right AV valve, the tricuspid valve,
has 3 flaps. Left AV valve, the
bicuspid valve, has 2 flaps.
• Attached to the valve flaps are tiny
white collagen cords called chordae
tendineae, or “heart strings,” anchor
flaps to heart walls.
• When heart is relaxed, AV flaps hang limply
into ventricular chambers below; blood
flows into atria and through open AV valves
• When ventricles contract, blood is
compressed into chambers, intraventricular
pressure rises, forcing blood upwards
against valve flaps.
• Valve flap edges meet, closing valves.
• Chordae tendineae serve as guidewires to
anchor flaps in place.
Semilunar (SL) Valves
• Aortic and pulmonary SL valves guard
bases of large arteries that exit the
• When ventricles are contracting and
intraventricular pressure rises, the SL
valves are forced open and flaps flatten
against the arterial walls and blood rushes
• When ventricles relax, blood flows back
toward the heart and closes the valves.
• Heart writhes in the chest when it
• Forces blood out of chambers when
it contracts and fills with blood
when it relaxes.
• Two terms are used to refer to heart
– Systole contraction
– Diastole relaxation
Cardiac Cycle (con’t)
• Includes all events associated with
the flow of blood through the heart
during one heartbeat.
• Marked by a succession of pressure
and blood volume change in heart.
• Lasts about 0.8 seconds
– Atrial systole 0.1 s
– Ventricular systole 0.3 s
– Total heart relaxation 0.4 s
Cardiac Cycle (con’t)
• Two important points:
1. Blood flow through the hear tis
controlled by pressure changes, and
2. Blood flow along a pressure gradient is
always from higher pressure to lower
pressure through any available
• Two distinguishable sound during
cardiac cycle can be heard.
• Often described as “lub-dup”, which
is associated with the closing of the
• Pause between lub-dup is the
• An abnormal or unusual heart sound
• Caused by obstruction along blood
• Fairly common in young children
• Indicate valve problems (or possible
hole in heart)
• If a valve is incomplete, a swishing
sound can be heard.
Cardiac Output (CO)
• Amount of blood pumped out be
each ventricle in 1 min.
• Highly variable and increases in
response to demands
• Difference in resting and maximal
CO is called the cardiac reserve.
– Nonathletes’ reserve is 4-5x’s normal CO
– Athletes’ reserve can be 7x’s the normal
Regulation of Heart Rate
• With a healthy cardiovascular system the
amount of blood pumped (stroke volume) is
• When blood volume drops sharply or the
heart is weakened, stroke volume declines
and the heart maintains the CO by beating
• Also, the nervous system can affect heart
rate. Fear, anxiety, stress, etc. causes an
increase in norepinephrine, which causes the
pacemaker to fire more rapidly and the
heart beats faster.