Heart

the heart connects to the blood vessels that transport blood between the heart and all body tissues.

arteries: blood away from the heart

• transport blood high in oxygen (except pulmonary arteries)

veins: blood toward the heart

• transport blood low in oxygen (except for pulmonary veins)

capillaries: serve as the site of exchange, either between the blood and the alveoli of the lungs or between the blood and the systemic cells

functions of the heart/characteristics:

• heart’s anatomy ensures unidirectional flow of blood through it. backflow of blood is prevented by valves within the heart.

• it acts like two side-by-side pumps that work at the same rate and pump the same volume of blood; one directs blood to lungs, other to body tissues.

• establishes and maintains blood pressure through alternate cycles of heart wall contraction and relaxation

→ Blood pressure (BP) force of blood pushing against inside walls of vessels

two types of circulations!

pulmonary circulation: transports deoxygenated blood from the right side of the heart through blood vessels to the capillaries in the lungs for the pickup of oxygen and the release of carbon dioxide, then BACK through blood vessels to the left side of the heart.

systemic circulation: transports oxygenated blood from left side of the heart through blood vessels to systemic capillaries such as those in the liver, skin, muscle, and brain.

• nutrients, respiratory gases, and wastes are exchanged with these systemic cells before blood is returned is returned to the right side of the heart.

—> basic blood flow of the heart:

• right side of the heart → lungs → left side of heart → systemic capillaries of body → back to right side of heart

position in the heart is at left side of the body midline posterior to the sternum in the mediastinum. it is slightly rotated that its right side (border) is more anterior, formed by right atrium and ventricle. the left side (border) is more posterior, formed by left atrium and ventricle.

• posterosuperior surface of heart = base; formed by left atrium!

• superior border is formed by great arterial trunks (ascending aorta and pulmonary trunk) and the superior vena cava.

• inferior end = apex; projects slightly anteroinferiorly toward left side of body, inferior border is formed by right ventricle

everything is contained within the pericardium! also called pericardial sac, serous lining, or fibrous sac. it restricts heart movements so it doesn’t bounce and prevents the heart filling with blood.

two parts of pericardium!

• fibrous pericardium: the outer portion and is a tough dense connective tissue

  • attached inferiorly to the diaphragm and superiorly to base of great vessels

• serous pericardium: inner portion and is a thin double-layered serous membrane

  • two layers of serous pericardium: parietal layer (lines inner surface of fibrous pericardium) and visceral layer (also called epicardium and covers outside of heart).

pericardial cavity: narrow space between parietal and visceral layers of serous pericardium.

• both layers secrete lubricating serous fluid into pericardial cavity to decrease friction when heart beats and layers rub against each other

• it is a potential space with a thin lining of serous fluid

heart wall structure:

weighs 250-350 grams

epicardium - external

• known as visceral layer of serous pericardium; composed of a serous membrane and areolar connective tissue

myocardium - middle

• composed of cardiac muscle tissue and is thickest of three heart wall layers. deep to epicardium and superficial to endocardium; contraction of cardiac muscle composing myocardium generates force necessary to pump blood

endocardium - internal

• composed of simple squamous epithelium and underlying layer of areolar connective tissue; continuous with epithelial layer (endothelium) which lines blood vessels

external heart anatomy:

four hollow chambers: two smaller atria and two larger ventricles

• left and right atria are superior chamber, anterior part is a wrinkled extension = auricle

  • receives all blood RETURNING to heart

    → right atrium receives blood from systemic circulation

    → left atrium receives blood from pulmonary circulation

  • blood that enters an atrium is passed to ventricle on same side of heart

• right and left ventricles are inferior chamber

  • two large arteries, pulmonary trunk, and aorta, exit heart at superior border

  • pulmonary trunk transports blood from right ventricle into pulmonary circulation, where aorta transports blood from left ventricle into systemic circulation.

    → both ventricles pump same volume of blood per minute

atria are separated from ventricles externally by deep coronary sulcus (atrioventricular sulcus) and it extends around surface of heart

→ interventricular sulcus = groove between ventricles that extends inferiorly from coronary sulcus toward heart apex and delineates superficial boundary between right ventricle and left ventricle.

• anterior interventricular sulcus = anterior side of heart; inferior interventricular sulcus = inferior side of heart

chambers and valves

valve cusps are tapering projection of cardiac valve; also called flaps/leaflets

• when flaps of valves are closed, makes lubb-dubb sound.

  • lubb sound = atrioventricular (AV) valves closing

  • dubb sound = semilunar valves closing

Fibrous skeleton: formed by dense regular CT and located between the atria and ventricles

fibrous skeleton functions:

• provides structural support and boundary between atria and ventricles

• forms supportive fibrous rings to anchor heart valves

• provides rigid framework for attachment of cardiac muscle tissue

• acts as electrical insulator to prevent muscle impulses in atria passing to ventricles, so that the ventricles don’t contract same time as the atria

cardiac muscle cells are spiral bundles around heart chambers attached to fibrous skeleton. when the atrias contract, they compress the wall of chambers inward to move blood into ventricles; when the ventricles contract, it is similar to wringing a mop that begins at apex of heart and compresses superiorly, moving blood into great arteries

right atrium:

• receives deoxygenated venous blood from systemic circulation and heart muscle itself

  3 vessels empty into the right atria:

  • superior vena cava: drains blood from head, neck, upper limbs, and superior region of trunk

  • inferior vena cava: drains blood from lower limbs and trunk

  • coronary sinus: drains blood from heart wall

right atrium has an interatrial septum that forms a thin wall between the right and left atria! the posterior atrial wall is smooth, and the auricle and anterior wall exhibit muscular ridges = pectinate muscles

• structural differences occur because the 2 walls formed from separate structures during embryonic development

• inspection of interatrial septum reveals an oval depression: fossa ovalis

  → it occupies former location of fetal foramen ovale, which shunted blood from right atrium to left atrium during fetal life

right atrium also has a right atrioventricular opening! it separates the right atrium from the right ventricle.

• it is covered by tricuspid (right atrioventricular) valve

• blood flows from right atrium, through right AV opening when valve is open into the right ventricle.

• the right atrioventricular valve closes when the right ventricle contracts; it prevents backflow of blood into the right atrium

right ventricle:

receives blood from right atrium

• interventricular septum forms thick wall between left and right ventricles

• the internal wall surface of each ventricle displays large, smooth, irregular, muscular ridges = trabeculae carnae

  → three, cone-shaped muscular projections = papillary muscles: they anchor numerus thin strands of collagen fibers called chordae tendinae (attaches to lower surface of cusps of right atrioventricular valve and prevents the valve from everting and flipping into the atrium when right ventricle is contracting.

the septomarginal trabecula (moderator band): muscle bundle of fibers that connect base of anterior papillary muscle to interventricular septum

• at superior end, right ventricle narrows to smooth-walled, conical region = conus arteriosus

  → beyond is pulmonary semilunar valve which separates right ventricle and pulmonary trunk

• pulmonary trunk divides into right and left pulmonary arteries! it transports deoxygenated blood to the lungs

semilunar valves are located within walls of both ventricles immediately before the connections of right and left ventricles to the aorta and pulmonary trunk.

• each valve is composed of 3, half-moon-shaped pocketlike semilunar cusps.

  → as blood is pumped into arterial trunks, it pushes against cusps forcing it open

  → when ventricular contraction ceases, blood is prevented from flowing back into ventricles from the trunk

  → as cusps fill with blood, they expand and meet at artery center, blocking backflow of blood

Left atrium:

oxygenated blood travels through pulmonary veins to the left atrium.

• it has a smooth posterior wall of left atrium and has openings for four pulmonary veins

separating left atrium from left ventricle = left atrioventricular opening

• covered by left atrioventricular valve also called bicuspid or mitral valve

oxygenated blood flows from left atrium through left atrioventricular opening when valve is open to the left ventricle.

• left atrioventricular valve is forced closed when the left ventricle contracts, preventing backflow of blood into left atrium

Left ventricle:

3x thicker than right ventricular wall; requires thicker wall to make enough pressure to force oxygenated blood that’s returned to the heart from the lungs into the aorta, all through systemic circulation

• aortic semilunar valve marks end of left ventricle and into entrance of aorta

Coronary circulation:

• left and right coronary arteries travel within coronary sulcus of heart to supply blood to cells of heart wall

  → arteries are the only branches of ascending aorta and located in wall of aorta immediately superior to aortic semilunar valve

right coronary artery (right marginal artery)

→ supplies right border of heart

→ inferior interventricular artery supplies posterior surface of both left and right ventricles

left coronary artery (anterior interventricular artery)

→ supplies anterior surface of both ventricles and most of interventricular septum

→ circumflex artery supplies left atrium and ventricle

~both arteries are considered functional end arteries; they both share anastomoses (allows coronary arteries to shunt tiny amount of blood, one artery to another)

Functions:

• acts like end arteries, which don’t have anastomoses and are “end of the line” when coming to arterial blood flow

  EX: if one of the arteries get blocked, anastomoses too tiny to shunt efficient blood to put part of heart supplied by this branch

  • that part dies due to lack of blood flow

venous return occurs through one of several cardiac veins…

• great cardiac vein: alongside anterior interventricular artery

• small cardiac vein: close to right marginal artery

→ all drains into coronary sinus (large vein that lies in posterior aspect of coronary sulcus) when then drains into the right atrium.

most coronary flow happens during ventricular relaxation!

inadequate blood flow causes:

• tachycardia: increased heart rate that shortens diastole

• hypotension: reduces ability of blood to flow through ventricular myocardium (low blood pressure)

Characteristics of cardiac muscle tissue:

• sarcoplasmic reticulum is less extensive and not as organized

• no terminal cisternae

• cardiac muscle t-tubules, overlie the z-discs

• t-tubules are less extensive distribution and reduced association with sarcoplasmic reticulum, allowing delayed onset and prolong contraction of cardiac muscle tissue

myocardium is made of cardiac muscle tissue. the cells are short and branched, only one or two nuclei, and numerous mitochondria for ATP production

Conducting System:

• heart exhibits autorhythmicity (heart itself is responsible for initiating heartbeat)

• special cells specialize in initiating heartbeat

  → sinuatrial (SA) node: located posterior wall of right atrium and act as pacemaker; initiates 70-80 pulses a minute

  • it spreads out via gap junctions to the atrioventricular (AV) node via internodal pathways

    ~also know as Bundle of His and divides into right and left branches, and also purkinje fibers

  • normally slows conduction of impulse as it travels from atria to ventricle

cardiac muscle cells contract as a single unit because muscle impulses are distributed immediately and simultaneously throughout all cells of atria then the ventricles

intercalated discs have desmosomes and gap junctions

• desmosomes prevent cardiac muscle cells from pulling apart

• gap junctions provide low-resistance pathway for ions to move between adjoining cardiac muscle cells; they are required for the synchronous beating of cells

cardiac plexus:

sympathetic innervation

• cardioacceleratory center sends impulses along sympathetic nerves = results in increase in heart rate and force of contraction

parasympathetic innervation

• cardioinhibitory center sends nerve impulses along vagus nerves = results in decrease in heart rate

contraction of heart chamber = systole

relaxation of heart chamber = diastole

Cardiac Cycle:

1. atrial relaxation and ventricular filling

2. atrial contraction and ventricular filling

3. isovolumic contraction

4. ventricular ejection

5. isovolumic relaxation

blood flow through cardiac cycle

1. blood flows from high pressure to low pressure

2. valves keep blood flowing in one direction

3. blood flows veins into atria under low pressure

4. 70% ventricular filling occurs passively when atria and ventricles are relaxed

5. ventricular contraction (systole) increases pressure on blood within ventricles, first closing the AV valves, then forcing semilunar valves open to eject blood into aorta and pulmonary trunk

blood moves through the heart from high pressure to low pressure. contraction of the wall of the heart chamber increases increases the pressure, which causes the blood to move.

development of the heart:

• commences in the third week

  • day 19, two heart (endocardial) tubes form from mesoderm in embryo

  • day 21, paired tubes fuse, forming one tube, developing into… (listed inferior to superior)

    → sinus venosus

    • forms part of left and right atria

    → primitive atrium

    • forms part of left and right atria

    → primitive ventricle

    • forms most of left ventricle

    → bulbus cordis

    • can be further subdivided into a trabeculated part of right ventricle, forming most of right ventricle; conus cordis (forms outflow tracts for the ventricles) and truncus arteriosus (forms ascending aorta and pulmonary trunk)

  • day 22, heart begins to beat; bulbus cord is pulled inferiorly, anteriorly, and to embryo’s right, while ventricle moves left to reposition.

  • day 28, heart tube is S-shaped

• weeks 5-8

  • single heart tube becomes four chambers (two atria two ventricles) and great vessels form

  • common atrium divided into left and right w/ two parts: septum primum and septum secundum

    → they connect to make endocardial cushions

  • opening in septum secundum = foramen ovale