Heart anatomy & embryology

Question 1

R atrium

Answer 1

SVC, IVC, coronary sinus
Crista terminalis - internal divison (smooth muscular ridge) from front of SVC opening to front of IVC
Sinus of venae cavae = smooth muscle wall, posterior to crista terminalis (derived from R. horn of sinus venosus)
Atrium proper = rough musculi pectinati, anterior to crista terminalis (derived from primitive atrium)
Fossa ovalis with prominent limbus fossa ovalis margin

Question 2

RV

Answer 2

Cone-shaped RV outflow tract
inner wall = smooth muscle (derived from bulbus cordis)
Inflow portion = trabeculae carnae
Moderator band = large trabecular carnae crossing ventricular cavity (carries a. bundle branch)
3 papillary muscles attach to tricuspid valve via chordae tendinae

Question 3

LA

Answer 3

4 pulmonary veins into posterior half
inflow = smooth walls (derived from proximal pulmonary veins)
anterior half = musculi pectinati
No crista terminalis
Valve of foramen ovale

Question 4

LV

Answer 4

thickest myocardial layer
LVOT = aortic vestibule - smooth walls (From bulbus cordis)
Trabeculae carnae - more fine and delicate than RV
2 large papillary muscles
chordae tendinae = thicker, less numerous than RV
mitral valve
aortic valve

Question 5

Layers of the heart

Answer 5

from outer:

fibrous layer –> parietal pericardium –> pericardial cavity –> visceral pericardium –> myocardium –> endocardium

Question 6

Orientation of the heart within mediastinum

Answer 6

base of heart (posterior surface): LA, small portion of RA, proximal parts of SVC, IVC, pulmonary veins, esophagus
Apex: inferolateral part of LV
Anterior surface: mostly RV, some RA on right, some LA on left
Diaphragmatic surface (inferior surface): LV, small portion of RV (separated by interventricular groove), separated from the base of the heart by coronary sinus

L. pulmonary surface: LV, portion of LA
R. pulmonary surface: RA

Question 7

Initial development of the heart

Answer 7

Formation of CV system begins in 3rd week of gestation (from mesodermal cells)
- fusion of heart primordia to form a single endocardial tube by day 22
Primitive heart beat by day 22-23, blood circulates by 4th week

Question 8

Endocardial tube

Answer 8

4 distinct regions: sinus venosum, atrium, ventricle, truncus arteriosus
separated by: endocardial cushions, interventricular septum, aorticopulmonary (spiral) septum, interatrial septum

Tube folds up:
atrium and sinus venosum –> superior and posterior
ventricles –> inferior to atria and great vessels
great vessels –> superiorly and somewhat anteriorly
interventricular septum, interatrial septum and spiral septum meet at endocardial cushions

Question 9

Interventricular septum

Answer 9

formed from 2 parts:

1) muscular interventricular septum (most)
2) membranous interventricular septum
- small region at top of IV septum - formed from cells that migrate from inferior edge of spiral septum, endocardial cushions, and muscular IV septum

Question 10

Spiral septum

Answer 10

spirals down truncus arteriosus –> divides it into half - two vessels that are twisted around each other
Lower edge of spiral septum fuses with IV septum and endocardial cushion

Question 11

Interatrial septum

Answer 11

formed from 2 septra –> septum primum and septum secundum
3 openings:
1) foramen (ostium) primum: obliterated when inferior edge of septum primum fuses with endocardial cushions
2) foramen secundum: before primum closes, superior part breaks down leaving foramen (ostium) secundum
3) foramen ovale: septum secundum (stiff) grows from superior part of the atrium on the RHS of the septum primum –> grows towards endocardial cushion but does not reach –> covers foramen secundum
- septum primum is flexible; when RAP > LAP (before birth), blood flows from R –> L
- once lungs are functional, LAP>RAP –> pushes septum primum against septum secundum and seals off foramen ovale –> becomes depression (fossa ovalis) in RA in adults

Question 12

sinus venosus

Answer 12

Right side: incorporated into smooth-walled portion of RA (sinus venarum)
Left side: coronary sinus

Question 13

Pulmonary veins

Answer 13

4 vessels gradually form from one vessel that incorporates into LA (smooth-walled portion of LA)
embryonic atrium forms rough-walled atrial appendage of LA

Question 14

Ductus arteriosus

Answer 14

shunt - passes blood from pulmonary A –> aorta (bypass lungs)
close shortly after birth, leaving ligamentum arteriosum

Question 15

Fetal circulation

Answer 15

1/2 blood returning from placenta via umbilical vein is shunted through ductus venosus directly to the IVC (bypass liver)
Blood in RA –> foramen ovale –> LA
Some blood does not go through foramen ovale; instead, it returns from SVC, goes into pulmonary arteries, then ductus arteriosus (high resistance in pulmonary vessels) –> aorta
(it is important that some blood goes into RV for proper development, prevent hypotrophy)

Question 16

Fetal oxygenation

Answer 16

IVC - returns oxygenated blood from placenta
SVC - returns deoxygenated blood from head & neck
mix in the heart, pumped to supply systemic needs

Question 17

Fetal hemoglobin

Answer 17

HbF - alpha2gamma2
produced after 10-12 weeks of development, witches form embryonic Hb
higher affinity for oxygen than adult hemoglobin
- better access for O from maternal blood at placenta
Predominant form of Hb in utero
Level falls to 1-2% by age 1, but can persist if there is a congenital blood disorder

Question 18

Adult Hemoglobin

Answer 18

HbA - alpha2gamma2 and HbA2 - alpha2delta2
HbA predominant (95%)
switches over from fetal at 6 months, complete by 1 yr

other disease state Hb

Question 19

Transition to adult circulation

Answer 19

First breath, lungs fill with air –> fall in pulmonary resistance
Blood fills the lungs, increase in negative pressure (L pressure becomes greater than R pressure)
Umbilical arteries constrict and umbilical cord is severed
Closure of:
-ductus venosus
-foramen ovale
-ductus arteriosus

Question 20

Regulation of ductus arteriosus

Answer 20

At birth, high concentration of oxygen + reduction in placental vasodilatory prostaglandins results in constriction of the ductus arteriosus