Fetal development and fetal circulation

The fossa ovalis is a shallow oval dip in the interatrial septum, seen from the right atrium. It marks where an opening between the atria was before birth.

• Correct: A shallow dip in the interatrial septum:
• A groove on the outer surface of the heart between atria and ventricles:
• The opening of the coronary sinus:

The right ventricle pumps oxygen-poor blood through the pulmonary valve into the pulmonary trunk, toward the lungs.

• The left ventricle:
• The right atrium:
• Correct: The right ventricle:

SRY, the sex-determining region of the Y chromosome, starts the gonad's development into a testis.

• Correct: SRY:
• The gene for testosterone:
• The gene for FSH:

Normally left atrial pressure is higher, which keeps the septum primum flap pressed shut over the foramen ovale. Straining briefly raises right atrial pressure above left. The flap swings open toward the left, and a clot arriving in the right atrium can cross into the left atrium and travel to the brain.

• Straining raised left atrial pressure, which pushed the clot backward: Higher left atrial pressure presses the flap shut. The flap opens only when right atrial pressure is higher.
• Straining made the ductus arteriosus reopen: The ductus arteriosus closes permanently soon after birth and does not reopen with straining. The opening involved is between the atria.
• Straining sent the clot through the lungs faster than they could trap it: Clots in the lungs lodge in the pulmonary arteries; they do not pass through by speed. This clot bypassed the lungs through the atrial opening.
• Correct: Right atrial pressure briefly rose above left, opening the flap: Correct. The pressure difference reverses and the one-way flap opens right to left.

Oxygen-rich blood leaves the placenta in the umbilical vein, partly bypasses the liver through the ductus venosus, enters the inferior vena cava and right atrium, and is aimed through the foramen ovale into the left atrium. The left ventricle pumps it into the ascending aorta, whose first branches supply the heart and head.

• Correct order: 1. Umbilical vein 2. Ductus venosus 3. Inferior vena cava and right atrium 4. Foramen ovale and left atrium 5. Left ventricle 6. Ascending aorta and the arteries to the head

Prostaglandins hold the ductus arteriosus open. Blocking them late in pregnancy narrows it, so pressure rises in the right ventricle and pulmonary trunk and more blood is forced through the high-resistance lungs. The same drug lowers fetal urine output across another system, shrinking the amniotic fluid. That is why such drugs are avoided late in pregnancy.

• Prostaglandin levels acting on the ductus arteriosus: down. Ibuprofen crosses the placenta and blocks the enzyme that makes prostaglandins in the fetus and placenta.
• Diameter of the ductus arteriosus: down. Prostaglandins keep the ductus's smooth muscle relaxed. Without them it contracts and narrows.
• Pressure in the fetal pulmonary trunk and right ventricle: up. The narrowed ductus resists the right ventricle's outflow, so pressure builds up behind it.
• Blood flow through the fetal lungs: up. With the ductus narrowed, more of the right ventricle's output is forced into the pulmonary arteries.
• Amniotic fluid volume: down. Prostaglandins also help keep fetal kidney blood flow up. Blocking them lowers fetal urine output, which is most of the amniotic fluid late in pregnancy.

Her Sertoli cells work normally and secreted anti-Müllerian hormone before birth, so the paramesonephric ducts, which would have formed the uterus and uterine tubes, broke down. Because no tissue responds to androgens, her mesonephric ducts also broke down and her external genitalia developed as female.

• Correct: Her testes made anti-Müllerian hormone: Correct. Anti-Müllerian hormone acts without androgen receptor proteins, so the uterus never formed.
• Without androgen action, the mesonephric ducts could not form a uterus: The uterus comes from the paramesonephric ducts, not the mesonephric ducts.
• Her ovaries never formed, and ovaries make the uterus: The uterus forms from ducts, not from the ovaries. She has testes, which removed those ducts before birth.
• High testosterone destroyed the uterus after puberty: Her testosterone cannot act on her cells at all, and the uterus was absent from before birth.

The sinus venosus, the inflow chamber the veins drained into, is absorbed into the back wall of the right atrium. It forms the smooth part of that wall, the coronary sinus and the region of the SA node, which is why the node is named for it.

• Bulbus cordis: The bulbus cordis forms most of the right ventricle and the outflow parts of both ventricles.
• Correct: Sinus venosus: Correct. The sinus venosus forms the smooth back wall of the right atrium, where the SA node sits.
• Truncus arteriosus: The truncus arteriosus forms the ascending aorta and pulmonary trunk.
• Primitive ventricle: The primitive ventricle forms most of the left ventricle.

After birth, left ventricular pressure is much higher than right, so oxygen-rich blood is pushed from left to right through the hole. That extra blood recirculates through the lungs and overloads them, causing fast breathing, but it does not send oxygen-poor blood to the body, so the skin stays pink.

• Correct: Left to right, so no oxygen-poor blood reaches the body: Correct. Left-to-right flow overloads the lungs but does not cause cyanosis.
• Right to left, with the lungs still resisting flow as before birth: After birth, the lungs' resistance falls far below its fetal level, so right-sided pressures become lower than left.
• Left to right, driven by a stronger right ventricle: The direction is left to right, but because the left ventricle generates the higher pressure, not the right.
• Right to left, but the lungs then remove the extra carbon dioxide before it reaches the body: Right-to-left flow would send oxygen-poor blood into the aorta and cause cyanosis, which this baby does not have.

At 27 weeks the lungs are between the canalicular and saccular stages. Type II cells are present, but they usually make enough surfactant only by weeks 34 to 36. Without it, surface tension collapses the alveoli, making the lungs stiff. A glucocorticoid given to the mother before the birth would have sped surfactant production.

• Her lungs are still in the pseudoglandular (branching) stage, with no respiratory bronchioles: The pseudoglandular (branching) stage ends around week 17. By 27 weeks respiratory bronchioles and early air sacs have formed.
• Correct: Her type II cells have not yet made enough surfactant: Correct. Too little surfactant makes preterm lungs stiff.
• Her ductus venosus is still open: The ductus venosus carries blood past the liver. It does not affect how stiff the lungs are.
• Her foramen ovale closed too early: Early closure of the foramen ovale would strain the fetal right heart before birth. It would not make the lungs stiff.