Later Development, Develop. Physiol., Dr. D. Penney
Table 3.01 - Changes during Development: The Cardiovascular System is the first to become Functional in Fetus.
Maternal Circulation:
| 3 mos. | Term |
|---|
Uterine blood flow
(ml/min.) | 50 | 500-750 |
| Cardiac output (CO) | 4500 | 6000 |
| Heart Rate = 70 ---> 80 beats per minute; Stroke Volume = 64 ---> 71 ml |
Placenta:
- 60 fetal codyledons
- 20 maternal lobes formed by septa
- At term 50 x 106 villous capillaries
- Blood flow: intervillous space = 65 - 120 ml/100 g/min., fetal = 90 - 190 ml/min.
Fetal Cardiac Output:
- 235 - 525 ml/kg body wt./min. (changes brought about mainly through changes in heart rate)
.... by comparison, the resting cardiac output of an adult human is about 75 ml/kg body wt./min.
| 15 mm embryo | 65 beats/min. |
| 45 days | 125 beats/min. |
| 9 weeks. | 175 beats/min. |
| 15 weeks. | 145 beats/min. |
| 0.5 - 1.0 | 135-155 beats/min. |
Fetal regulation:
- Vagal (vagus nerve) pathway and chemoreceptors on large vessels are present by 6 - 8 weeks
- Bradycardia is a response to maternal hypoxia
- Arterial blood pressure shows a steady rise with fetal growth.
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