Module 6 (ACTUAL)

Q: Why does the circulatory system begin developing so early? — Slide 1–3

• Begins just after gastrulation (~day 15–16)

• Developing embryo needs connection to maternal blood supply

• Without this connection, embryonic tissues cannot properly develop and grow

Q: How do angioblasts and blood islands form? — Slide 1–3

• Specialized mesoderm cells in yolk sac differentiate → angioblasts

• Angioblasts help build blood-vessel walls

• Angioblasts proliferate → blood islands

• Blood islands = small pockets/groups of angioblasts dispersed through mesoderm

• Yolk sac: days 15–16

• Embryo: ~day 17

• Yolk sac contains endoderm + closely adhering mesoderm; outside embryo this is extraembryonic mesoderm

Q: How do blood islands form the first blood vessels? — Slide 1–3

• Angioblasts organize → form small cavities

• Angioblasts lining cavities → endothelium

• Endothelium = cells lining the inside of blood vessels

• Cavity/space → future lumen

• Lumen = inside of a tube

• Early vessels are called endothelial tubes because they are lined by endothelial cells

Q: How does blood itself develop in the blood islands? — Slide 1–3

• Mesoderm also produces pluripotent blood cells

• Pluripotent = can differentiate into many, but not all, cell types

• These become:

  • RBCs

  • WBCs

  • Platelets

• Blood is physically created inside the angioblast cavities

• It does not come from maternal blood supply

Q: How do the first blood vessels develop into the circulatory network? — Slide 1–3

• Cavities enlarge → fuse together

• Fusing cavities form tubing networks

• Blood cells inside the lumens also continue proliferating

• Small tubes throughout mesoderm fuse → larger tubes

• Eventually all tubes form the circulatory blood-vessel network

• This produces vessels with different sizes and locations, including major vessels such as the aorta and limb arteries

Q: How do the first heart-forming tubes develop? — Slide 4

• Two endothelial tubes lie in the cardiogenic region

• Cardiogenic = cardio (heart) + genic (building/forming)

• Cardiogenic region is initially near the embryo's head

• Endoderm cells in this region send signals

• Signals cause 2 endothelial tubes to specialize → endocardial tubes

• The 2 endocardial tubes fuse → primitive heart tube

Q: What are the 4 dilations of the primitive heart tube? — Slide 4

1. Sinus venosus

2. Atrium

3. Ventricle

4. Bulbus cordis

• These are stretched-out regions of the tube where heart structures will develop

• Sinus venosus + bulbus cordis later contribute to major heart structures/vessels

Q: What happens to the primitive heart tube as it develops? — Slide 4

• Initially, heart = one long tube formed by fusion of 2 tubes

• Tube begins to bend as it grows

• Bending helps form the final heart

• By day 21/end of week 3, heart begins contracting

• Cardiac muscle develops from mesoderm

• Cardiac muscle cells are autorhythmic → contract without nervous-system control

• Early contractions do little because:

  • Heart is still a simple tube

  • Blood has not yet formed

Q: How does the primitive heart develop from days 20–22? — Slide 5

• Day 20: 2 endocardial tubes present

  • Arterial end = future blood exit

  • Venous end = future blood entry

• Day 21: tubes fuse → primitive heart tube

• Day 22: 4 dilations become visible:

  • Bulbus cordis → closest to arterial end

  • Primitive ventricle

  • Primitive atrium

  • Sinus venosus → closest to venous end

Q: What are the developmental fates of the primitive heart regions? — Slide 5

• Sinus venosus → part of right atrium + SA node

• Primitive atria → right + left atria

• Primitive ventricles → right + left ventricles

• Bulbus cordis → primarily right ventricle

• Bulbus cordis also connects to → aorta + pulmonary trunk

• Vessels entering sinus venosus → future veins/vena cavae

• SA node = heart's pacemaker

• SA node sets heart rate through autorhythmic/automatic depolarization

Q: How does the heart achieve its adult orientation? — Slide 5

• Initially, atria are below ventricles

• Adult heart: ventricles are below/anterior, atria above/posterior

• Around day 23, heart begins folding/bending

• Growing tube is constrained by the small pericardial/coelomic space

• This space determines how the heart folds

• Ventricular region moves forward + downward

• Atrial region moves backward + upward

• By day 28, atria + ventricles reach final adult orientation

Q: What do the great vessels develop from? — Slide 5

Vessels connected to sinus venosus → superior + inferior vena cava

• Bulbus cordis → contributes to vessels leaving heart

• Right side → pulmonary trunk

• Left side → aorta

• These become the major great vessels/arteries leaving the heart

Q: What does the fetal heart look like around days 46–50? — Slide 6

• Left ventricle → pumps blood to body through aorta

• Right ventricle → pumps blood toward lungs

• Interventricular septum → muscular wall between ventricles

• Ventricles are highly muscular because they pump blood out

• Left atrium lies above left ventricle

• Right atrium lies above right ventricle

Q: What is the normal blood-flow pathway from newborn to adult? — Slide 6

• Body → superior/inferior vena cava

• → right atrium

• → right ventricle

• → lungs

• → picks up oxygen

• → pulmonary veins

• → left atrium

• → left ventricle

• → aorta

• → body tissues

How is the fetal interatrial septum different? — Slide 6

• Fetal atrial septum is not completely fused

• It has 2 layers:

  • Septum secundum → closer to right atrium; opening more inferior

  • Septum primum → closer to left atrium; opening more superior

• Together → interatrial septum

• Their openings overlap when the septa are pushed together

Q: Why does fetal blood move from the right atrium to the left atrium? — Slide 6

• Fetal lungs are deflated

• Lung vessels are collapsed → high resistance

• Right ventricle cannot easily send blood through lungs

• Blood pressure therefore becomes high on right side

• High right-sided pressure pushes blood through the septal openings

• Openings together form the foramen ovale

• Blood flows right atrium → left atrium

• This bypasses the lungs

• Left ventricle then pumps blood to the body

Q: What is the foramen ovale? — Slide 6

• Opening between the 2 fetal atrial septa

• Created by overlapping openings in septum primum + septum secundum

• “Foramen” = hole/opening

• Allows blood to flow right atrium → left atrium

• High right-sided pressure keeps it open in the fetus

• Allows blood to bypass the lungs

Why does the fetus bypass both the lungs and liver? — Slide 7

• Lungs: fetus does not breathe → lungs are deflated → high resistance

• Therefore blood bypasses lungs

• Liver: placenta performs the liver's filtering role

• Therefore placental blood can bypass the liver

• Blood returning from placenta can enter fetal circulation directly

Q: What triggers the baby's first breath? — Slide 7

• Birth → placenta is detached/cut off

• Maternal oxygen supply stops

• Fetal oxygen levels drop

• Oxygen drop signals the fetal brain → first breath

• Breathing → lungs inflate

• Inflated lungs remove pressure from pulmonary blood vessels

Q: How does the first breath change pressure in the heart? — Slide 7 / Back to Slide 6

• Inflated lungs → pulmonary resistance decreases

• Right heart can pump blood to lungs more easily

• Right-heart pressure decreases

• More blood returns from lungs → left atrium

• Left atrial pressure increases

• Left pressure becomes higher than right pressure

• Septum primum is pushed against septum secundum

• Openings become sealed

• Right atrium can no longer send blood directly to left atrium

Q: How does blood flow through the fetal heart and lungs? — Slide 8

• Body → superior/inferior vena cava → right atrium

• Right atrium → right ventricle

• Right ventricle → pulmonary trunk

• Pulmonary trunk → right + left pulmonary arteries

• Small amount of blood reaches lungs

• Blood returning from lungs → pulmonary veins → left atrium

• Left atrium → left ventricle

• Left ventricle → aorta → body tissues

Q: What are the two ways fetal blood bypasses the lungs? — Slide 8

• Foramen ovale: right atrium → left atrium

• Ductus arteriosus: pulmonary trunk → aorta

• Both bypass the lungs

• Ductus arteriosus also bypasses the left side of the heart

Q: What is the ductus arteriosus? — Slide 8

• Fetal vessel connecting pulmonary trunk → aorta

• Allows blood to bypass lungs

• Blood from right ventricle can enter systemic circulation directly

• Bypasses both lungs + left side of heart

Q: What is the ductus venosus? — Slide 8

• Connects umbilical vein → inferior vena cava

• Allows placental blood to enter fetal circulation

• Bypasses the liver

• Placenta has already filtered the blood

Q: How does oxygenated blood travel from the placenta to the fetus? — Slide 8

• Placenta → umbilical vein

• Umbilical vein carries oxygen-rich blood toward fetus

• Umbilical vein → ductus venosus → inferior vena cava

• It mixes with deoxygenated blood returning from the body

• Blood reaching fetal heart is therefore mixed oxygenated + deoxygenated blood

Q: Why can fetal blood be only partially oxygenated? — Slide 8

• Fetus does not need fully oxygenated blood

• Fetus has relatively low oxygen demand

• Lecture notes humans use ~25% of available oxygen at rest even after birth

• Fetus is not very active → does not require huge oxygen supply

• Mixed blood still provides sufficient oxygen for fetal tissues

Q: What do the umbilical arteries carry? — Slide 8

• Carry oxygen-poor blood away from fetus

• Travel from fetal circulation → placenta

• Pick up oxygen at placenta

• Oxygen-rich blood then returns through umbilical vein

• Arteries = vessels leaving the heart, veins = vessels returning to the heart

Q: What happens to the fetal circulatory shunts after birth? — Slide 9

• Foramen ovale → fossa ovalis

• Ductus arteriosus → ligamentum arteriosum

• Ductus venosus → ligamentum venosum

• Umbilical vein → ligamentum teres

• Umbilical arteries → medial umbilical ligaments

  • Also called chords of the umbilical arteries

• These structures gradually degenerate/close because placental circulation is no longer needed

Q: What happens specifically to the foramen ovale after birth? — Slide 9

• First breath → increased left atrial pressure

• Septa are pushed together → foramen ovale closes

• Anatomically becomes an indentation

• This indentation is called the fossa ovalis

Q: What happens to the ductus arteriosus after birth? — Slide 9

• Pulmonary trunk → aorta connection is no longer needed

• Ductus arteriosus degrades

• Closure is not instantaneous

• Eventually no blood flows through it

• Remnant → ligamentum arteriosum

Q: What happens to the ductus venosus and umbilical vessels after birth? — Slide 9

• Umbilical cord is cut → placental blood flow stops

• Ductus venosus → ligamentum venosum

• Umbilical vein → ligamentum teres

  • “Teres” = round

  • Also called round ligament

• Umbilical arteries → medial umbilical ligaments

  • Also called chords of the umbilical arteries

• Essentially, fetal tubes become ligaments

Q: How does the respiratory system begin developing? — Slide 10

• Begins as a single midline evagination of the foregut

• This is the same type of gut-tube evagination/out-pocket discussed previously

• Initial out-pocket → tracheal bud

• Developing pharynx lies behind it → later becomes throat

• Remaining gut tube → esophagus

• Tracheal bud → trachea

Q: How do the bronchi and lungs develop from the tracheal bud? — Slide 10

• Lung buds branch from developing trachea

• First branching → 2 bronchial buds

• One develops toward each lung

• Bronchi = conducting tubes from trachea → lungs

• Continued branching creates an upside-down tree-like structure

Q: How many generations does the bronchial tree develop? — Slide 11

• Up to 17 generations by birth

• Continues developing after birth

• Up to 24 generations by adulthood

Q: How does bronchial branching progress during weeks 5–7? — Slide 11

• Week 5: trachea → primary bronchi

  • Right primary bronchus

  • Left primary bronchus

• Further branching → secondary bronchi

• Week 6: tertiary bronchi begin

• Week 7: lungs resemble more fully formed fetal lungs

Q: How many lobes do the right and left lungs have? — Slide 11

• Right lung = 3 lobes

  • Superior

  • Middle

  • Inferior

• Left lung = 2 lobes

  • Superior

  • Inferior

• Left has fewer lobes because it must accommodate the heart

• Heart lies slightly left of the midline

• This arrangement remains in the fully developed fetus and adult

Q: What develops around week 8 in the lungs? — Slide 11

• Pleural membrane develops

• It is a serous membrane surrounding/enveloping the lungs

• It surrounds the lung tissue once fully formed

• Important for lung function

Q: What are the key cardiovascular + respiratory developmental dates? — Slides 1–11

• Days 15–16: angioblasts/blood islands begin in yolk sac

• Day 17: blood islands develop in embryo

• Day 20: endocardial tubes

• Day 21: tubes fuse → primitive heart; contractions begin ~day 21

• Day 22: 4 heart dilations visible

• Day 23: heart folding begins

• Day 28: adult heart orientation established

• Days 46–50: fetal heart resembles developed heart

• Week 5: primary bronchi

• Week 6: tertiary bronchi

• Week 7: fetal lung structure resembles mature lungs

• Week 8: pleural membrane develops

• Birth: ~17 bronchial generations

• Adulthood: ~24 bronchial generations