Circulation of Amniotic Fluid – Urinary System and Kidney Development

by John McLachlan, PhD

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    00:00 Now, let’s look at the fluid circulation of the amniotic fluid. Amniotic fluid is continuously produced by the amnioblasts. These are the cells that line the amniotic cavity. And there is a steady circulation of this fluid. What happens is that the embryo will swallow this fluid, it will effectively drink the fluid. And it’s then absorbed into its own blood system, and excess fluid is removed by the mother via the placenta. At the same time, the foetus will be passaging fluid through its mesonephros, initially, and then through the metanephros through the true kidney as the true kidney develops. It is important to understand that this fluid is not urine in the normal sense. I have seen it suggested that embryos and foetuses are swimming in their own urine, but that’s not the case.

    00:53 This is not a waste product. The true waste products are being handled by the mother.

    00:59 There’s merely a circulation of fluids swallowed by the embryo, handled by the embryo and removed by the placenta if necessary, and then passed on, excreted through the mesonephros and the metanephros. So there’s a fluid circulation taking place through the embryo mediated by the mother. However, something might go wrong with this process. So for instance, if the baby was unable to swallow properly in the uterus, if there was a blockage of the gut, for instance, and this could be at the level of the oesophagus or further down the gut, then what would happen is that the fluid could not be removed, it could not be shunted out to the maternal circulation and as a result, it would begin to build up. So there’d be an excess of amniotic fluid. Now, the Latin term for this is polyhydramnios, an excess of amniotic fluid. This can be measured by ultrasound, and the measurement would suggest that if in four separate measurements of depth, the total comes to more than 25 centimetres then you would define that as being the condition of polyhydramnios. Conversely however, if there was something wrong with fluid excretion, that could be, for instance, a blockage of urethra or the kidney is failing to develop properly, then the fluid will not pass through the embryo, and it would be as it normally would, and there will be a deficiency of amniotic fluid and the Latin term for this is oligohydramnios. The definition of this is less than five centimetres in total after four successive ultrasound measurements. Together, these measurements represent what’s called the amniotic fluid index or AFI. So, what does this mean in clinical practice? It was actually quite common if we’re going to have oligohydramnios.

    02:49 About 8 percent of pregnancies, the women will show oligohydramnios measurable by the AFI, by the amniotic fluid index. It may of course get better on its own. It may resolve spontaneously.

    03:02 But it might be important to monitor the baby if this was found to be the case. One option might be to consider the possibility of delivering the baby early. The risk of an insufficiency of amniotic fluid is that the baby may not be able to move freely. It’s important to development of the limbs and joints, particularly, that a baby is able to move his arms and legs freely during the course of development. Therefore, if this is inhibited by an absence of amniotic fluid then that might cause problems with the joints or with the limbs later in life.

    03:40 One possibility, of course, is just adding extra fluid to the uterus to increase the amount of fluid that’s present. Polyhydramnios is less common in about 2 percent of women.

    03:52 And of course, the excess amniotic fluid will cause the uterine cavity to swell, and this may affect back pain. It may even begin to impact upon the lungs, so it can cause discomfort and even breathing difficulties as the uterus presses against the diaphragm from the underside.

    04:13 So under these circumstances, an option might be to remove some amniotic fluid. Now, amniocentesis, as this is called, is often carried out as a diagnostic process during the course of development. But in this case, it would be done to remove some amniotic fluid from the total amount present in the amniotic sac.

    About the Lecture

    The lecture Circulation of Amniotic Fluid – Urinary System and Kidney Development by John McLachlan, PhD is from the course System-Specific Embryology with John McLachlan.

    Author of lecture Circulation of Amniotic Fluid – Urinary System and Kidney Development

     John McLachlan, PhD

    John McLachlan, PhD

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