CHAPTER 20The Heart

I. Heart

A. Functions

1. Generates the pressure to propel blood through blood vessels.

2. Separates oxygenated and deoxygenated blood

3. Regulates body’s blood supply.

B. Heart position

1. w/in the mediastinum (medial cavity of thorax)

2. Apex (bottom point) rests on the superior surface of the diaphragm, points toward the left hip.

3. Base (posterior) points towards the right shoulder.

4. Medial to the lungs, anterior to the esophagus and vertebrae, and posterior to the sternum.

C. Pericardium- layer that encloses the heart, made up of :

1. fibrous pericardium

-outermost layer

- a collagenous structure

-protects and anchors the heart and prevents it from distending (swelling up)

2. serous pericardium: layer under fibrous pericardium, two layered membrane

a. Parietal serous pericardium –outer layer of serous pericardium, meets fibrous pericardium

b. Visceral serous pericardium (epicardium)

-inner layer of serous pericardium, layer that covers external surface of the heart

c. Parietal and visceral layers are continuous with one another where the great vessels leave the heart.

d. Pericardial cavity is the space btwn the parietal and visceral layers

-contains serous fluid, which reduces friction.

D. Heart wall

1. Divided into 3 layers.

2. Epicardium

a. Most superficial, is also called visceral serous pericardium.

b. Composed of simple squamous epithelium overlaying thin loose CT.

3. Myocardium

a. Middle layer

b. Primarily cardiac muscle, but also contains blood vessels, nerves, and CT.

c. connective tissue forms a dense network known as the fibrous skeleton

-supports the heart valves

-acts as origin/insertion for the cardiac muscle cells

-helps direct the spread of electrical activity within the heart

4. Endocardium

a. Inner layer of heart wall

b. Consists of endothelium (simple squamous epithelium) resting on a layer of thin CT.

c. Lines the heart chambers; its folds create the heart valves.

E. Heart Chambers

1. 2 superior atria (atrium) and 2 inferior ventricles.

2. Thin interatrial septum divides the 2 atria

3. Thick interventricular septum divides the 2 ventricles.

F. Heart consists of 2 pumps connected in series.

1. Each pump sends blood to a different circuit.

2. Pulmonary circuit runs btwn the heart and the lungs.

3. Systemic circuit runs btwn the heart and the rest of the body tissues.

4. Right side of the heart

- receives de-oxygenated blood

-from the systemic circuit

- pumps it through the pulmonary circuit

5. Left side of the heart

-receives oxygen rich blood

- from the pulmonary circuit

-pumps it thru the systemic circuit.

G. Atria

1. Heart’s receiving chambers

2. Small and thinly muscled- atrial contraction propels a small amount of blood to the ventricles.

F. Right atrium

1. Receives deoxygenated blood from the systemic circuit via three vessels:

  a. Superior vena cava carries blood from arms, head, and upper torso

b. Inferior vena cava carries blood from the legs, abdomen, and pelvis

c. Coronary sinus carries blood from the coronary circulation – (nourishes the heart)

2. Sends blood to right ventricle via the tricuspid valve

G. Left atrium

1. Receives oxygen rich blood from the pulmonary circuit via four pulmonary veins.

2. Sends blood to left ventricle through mitral (bicuspid) orifice, via the mitral (bicuspid) valve.

H. Fossa ovalis- shallow depression in adult heart

1. Remnant of the foramen ovale, a hole in the fetal atrial septum.

a. Fetal blood flowed through the hole from right atrium to left atrium

b. this bypasses the pulmonary circuit (fetal lungs are neither developed nor oxygenated).

I. Auricle

1. pouches connected to atria (left & right)

2. function as reservoirs for blood

3. means “ear”

J. Ventricles

1. Large, muscular chamber, thick musculature, actual pump

2. contain muscular ridges- trabeculae carneae and muscular bulges- papillary muscles

K. Right Ventricle

1. Discharges blood into the pulmonary trunk, the first vessel of the pulmonary circuit.

2. Separated from the pulmonary trunk by the pulmonary semilunar valve

L . Left ventricle

1. Discharges blood into the aorta, the first vessel of the systemic circuit.

2. Separated from the aorta by the aortic semilunar valve.

 

3. More muscular than right ventricle

a. because the LV pumps blood a farther distance and against greater pressure (note – RV and LV pump the same volume of blood per beat).

M. Superficial anatomy of the heart

1. coronary sulcus: groove between atria & ventricles

2. anterior interventricular sulcus & posterior ventricular sulcus: shallower depressions, mark boundaries between left and right ventricles

3. connective tissue at these sulci generally contain quite a lot of fat

4. sulci also contain arteries & veins that carry blood to & from the cardiac muscle

II. Circuits

A. Systemic circuit

1. blood flows from:

left ventricle aorta systemic arteries

systemic capillaries systemic veins

venae cavae right atrium

1. systemic differs from the pulmonary in that it’s

-longer

-much larger blood volume in circuit

- resistance to blood movement is greater

- systemic arteries (away from heart): oxygen rich

- systemic veins (toward heart): oxygen poor

B. Pulmonary circuit

1. blood flows from:

Right ventricle pulmonary trunk pulmonary arteries pulmonary capillaries

pulmonary veins left atrium

1. pulmonary circuit has

- pulmonary arteries (away from heart, but toward the lungs!): low oxygen

- pulmonary veins (toward heart, from the lungs): oxygen rich blood

C. Coronary circuit

1. blood vessels supply/drain the 3 layers of the heart

2. needed as the heart requires large amounts of oxygen and nutrients, and little to no oxygen & nutrients can diffuse through the thick myocardium

III. The Heart Valves

A. Form & Functions

1. ensure 1 way blood flow within the heart

2. two atrioventricular valves: separate atria from ventricles

3. two semilunar valves: separate ventricles from their great vessels

B. Atrioventicular Valves

1. flap of endothelium

2. tricuspid prevents backflow from RV to RA

3. mitral valve prevents backflow from LV to LA

4. AV valve flaps attached to “heartstrings”: chordae tendinae

5. chordae tendinae attached to papillary muscles in ventricle walls

6. blood can pass through valves to go from atria to ventricle (papillary muscles relaxed)

7. blood attempts to flow back into atria, pushing valve flaps closed (toward atria)

8. chord. tend. tighten as papillary muscles contract, preventing valves from opening into the atrium= efficient flow of blood since prevents regurgitation (backflow)

C. semilunar valves

1. prevent backflow from pulmonary trunk and aorta (arteries) into right and left ventricles

2. don’t require muscular braces since arterial walls don’t contract; also 3 cusps support each other (like a tripod)

3. aortic sinuses- sacs adjacent to cusps in aortic valve, prevent cusps from sticking to aorta wall when valve opens; coronary arteries originate here

IV. Cardiac Muscle

A. cardiac skeleton

1. bulk of the heart wall

2. involuntary muscle

3. 2 types of cardiac muscle cells

a. contractile cells: 99% are this type,

generate force involved in pumping

b. striated, short & branched

4. autorythmic cells

a. 1% are this type

b. depolarize to set rate of contraction

B. Intercalated discs

1. link cardiac muscles together mechanically

and electrically

2. contain gap junctions & desmosomes

a. gap junctions are protein channels where ions can flow btwn cells, create electrical connection btwn cells and allow depolarization wave to spread,

allows heart to function as single unit

b. desmosomes are protein filaments that connect muscle cells, preventing them from separating during contraction

C. Fibrous skeleton of heart

1. dense irregular connective tissue

2. provides origins & insertions for cardiac contractile cells

3. supports the valves

4. separates atria from ventricles (physically & electrically)

V. Conducting System -controls heart rate

A. cardiac muscles contract on their own, initiated by the conducting system

1. heart contracts in a series

2. atria contract, then ventricles contract

3. two types of muscle cells involved

a. contractile cells- produce contractions

b. specialize cells called the conducting system- control & coordinate heartbeat

B. Each heart beat begins with electrical impulsegenerated at the SA node (pacemaker)- sets pace for contraction

1. the SA (sinoatrial) node is located in the wall of the RA

2. the AV (atrioventricular) node is between the atria & ventricles

3. conducting cells interconnect the 2 nodes, and distribute the electrical impulse to contractile muscle cells

4. conducting cells in the ventricles include: AV bundle and bundle branches

5. conducting cells called Purkinje fibers wind through the ventricles, sending impulse to moderator band & papillary muscles

6. spread of electrical impulse:

SA node

Impulse travels to atrial cells

wave travels to AV node

Atria contract (RA first)

wave travels down AVbundle & bundle branches

wave travels to ventricles through Purkinje fibers

ventricular contractile cells

contract

VII. Cardiodynamics

A. The movement and force generated by cardiac contractions

1. End-diastolic volume (EDV)

2. End-systolic volume (ESV)

3. Stroke volume (SV)

a. SV = EDV – ESV

4. Ejection fraction

a. The percentage of EDV represented by SV

5. Cardiac output (CO)

a. The volume pumped by left ventricle in 1 minute

B. Cardiac Output

1. CO = HR (Heart rate) X SV (Stroke volume), indication of blood flow through peripheral, provides useful indication of ventricle efficiency over time

a. CO = cardiac output (mL/min)

b. HR = heart rate (beats/min)

c. SV = stroke volume (mL/beat)

d. example: If heart rate is 75 bpm & stroke volume is 80 mL/beat,

CO = 75bpm x 80mL/beat= 6000mL/min (6L/minute)

2. Factors Affecting Cardiac Output- body adjusts output so tissues receive circulatory supply under a variety of conditions

a. Adjusted by changes in heart rate or stroke volume

i. Heart rate-Adjusted by autonomic nervous system or hormones

ii. Stroke volume -Adjusted by changing EDV (how full ventricles are when starting) or ESV (how much blood remains in ventricle after it contracts); stroke volume peaks when EDV is highest and ESV is lowest

C. Factors Affecting the Heart Rate

1. Autonomic innervation of the heart through the cardiac plexus

2. Vagus nerves (N X): carry parasympathetic preganglionic fibers to small ganglia in cardiac plexus)

4. Cardiac centers of medulla oblongata contain area of cardiac control

–cardioacceleratory center controls sympathetic neurons (increases heart rate)

–cardioinhibitory center controls parasympathetic neurons (slows heart rate)

D. Cardiac reflexes

1. Cardiac centers monitor:

a. blood pressure (baroreceptors)

b. arterial oxygen and carbon dioxide levels (chemoreceptors)

c. Cardiac centers adjust cardiac activity, for example: decline in oxygen concentrations indicate that the heart must work harder, cardiac centers then call for increased cardiac

activity

E. Autonomic tone

1. Dual innervation (from parasympathetic & sympathetic divisions) maintain resting tone by releasing neurotransmitters ACh (acetylcholine) and NE (norepinephrine)

a. resting heart rate w/o autonomic innervation is around 80-100bpm, while resting heart rate w/ normal innervation is around 70-80 bmp.

2. Fine adjustments meet needs of other systems

a. parasympathetic activity decreases heart rate, sympathetic activity increases heart rate

F. Effects on the SA Node

1. Sympathetic and parasympathetic divisions alter heart rate by changing the ionic permeability of cells in the conducting system

2. Sympathetic and parasympathetic stimulation-

a. Greatest at SA node (heart rate)- change in rate of depolarization or length of repolarization will change the heart rate

3. Rate of spontaneous depolarization depends on

a. Resting membrane potential

b. Rate of depolarization: Ach opens K+ channels, slowing rate of depolarization (heart rate drops), while NE opens sodium-calcium ions, increasing rate of depolarization (heart rate increases)

G. Atrial Reflex (Bainbridge reflex)

1. Adjusts heart rate in response to venous return

2. Stretch receptors in right atrium

a. Trigger increase in heart rate through increased sympathetic activity

H. Hormonal Effects on Heart Rate

1. Increase heart rate (by sympathetic stimulation of SA node)

a. Epinephrine (E)

b. Norepinephrine (NE)

c. Thyroid hormone

I. Factors Affecting the Stroke Volume

1. The EDV (amount of blood a ventricle contains at the end of diastole), affected by

a. Filling time- duration of ventricular diastole, dependent on heart rate

b. Venous return - rate of blood flow during ventricular diastole, changes in relation to blood volume, peripheral circulation patterns, skeletal muscle activity, & other factors.

2. Preload- The degree of ventricular stretching during ventricular diastole

a. Directly proportional to EDV, the greater the EDV, greater the preload

b. Affects ability of muscle cells to produce tension- allows for more efficient & forceful contractions

3. The EDV and Stroke Volume

a. At rest

-EDV is low

-Myocardium stretches less

-Stroke volume is low

b. With exercise

-EDV increases

-Myocardium stretches more

-Stroke volume increases

4. The Frank–Starling Principle

a. As EDV increases, stroke volume increases

5. Physical Limits

a. Ventricular expansion is limited from over stretching past the optimal length by

-Myocardial connective tissue

-The cardiac (fibrous) skeleton

-The pericardial sac

J. End-Systolic Volume (ESV)- the amount of blood that remains in the ventricle at the end of ventricular systole is the ESV

1. Three Factors That Affect ESV

a. Preload- Ventricular stretching during diastole

b. Contractility- Force produced during contraction, at a given preload

c. Afterload- Tension the ventricle produces to open the semilunar valve and eject blood

2. Contractility

a. Is affected by: Autonomic activity & hormones

b. Effects of autonomic activity on contractility

i. Sympathetic stimulation

-NE released by postganglionic fibers of cardiac nerves

-Epinephrine and NE released by suprarenal (adrenal) medullae

-Causes ventricles to contract with more force

-Increases ejection fraction and decreases ESV

-Increases ejection fraction and decreases ESV

ii. Parasympathetic activity

-Acetylcholine released by vagus nerves

-Reduces force of cardiac contractions

c. Hormones - Many hormones affect heart contraction

i. Pharmaceutical drugs mimic hormone actions

- Stimulate or block beta receptors (beta blockers used to treat high blood pressure)

3. Afterload

a. Is increased by any factor that restricts arterial blood flow

b. As afterload increases, stroke volume decreases

K. Heart Rate Control Factors

1. Autonomic nervous system

a. Sympathetic and parasympathetic

2. Circulating hormones

a. Venous return and stretch receptors

3. Cardiac Reserve

a. The difference between resting and maximal cardiac output

L. The Heart and Cardiovascular System

1. Cardiovascular regulation- Ensures adequate circulation to body tissues

2. Cardiovascular centers- Control heart and peripheral blood vessels

3. Cardiovascular system responds to- Changing activity patterns &circulatory emergencies