Transfer of Nutrients from Dam to Fetus
Mechanism of placental transfer
- Fetal blood flows through villi in which the capillaries are covered only by fetal tissue which bathes directly in the inter-villus space in a pool of flowing maternal blood.
- Various parts of the placenta are actively involved in the transfer, processing and synthesis of nutrients under the influence of maternal, fetal and placental hormones.
- The blood of fetus and dam never come in to direct contact. The two circulations are close enough at the junction of chorion and endometrium so that oxygen and nutrients can pass from the maternal blood to fetal blood, and waste products in the opposite direction.
- The placental membrane controls the transfer of a wide range of substances by several process.
Simple diffusion:
- The birth weight of a singleton infant correlates significantly with both the wet and dry weight of the placenta.The movement of molecules from an area of high concentration to an area of low concentration.
- Most molecules of physiologic importance are transferred by some active transport, thus they can be “ pumped “ against a concentration gradient allowing the embryo to accumulate higher concentration of nutrients that exist in the maternal blood.
- Water moves very freely between the dam and fetus.
- The total substances absorbed by the fetus 77% is water.
- Several modes of transport across the placenta have been described.
- Relatively lipid soluble molecules such as respiratory gases, anesthetic agents, several drugs and unconjugated bilirubin cross easily by penetrating the cell membrane.
- Small water soluble molecules such as urea and water also cross easily by diffusion or osmosis
- Transfer of glucose is facilitated by a specific carrier molecule but not actively transported.
- Most amino acids, calcium and potassium as well as phosphorous are transported from mother to fetus by specific carrier mediated processes which consume energy, and carry out active transport leading to higher concentration in fetal than in maternal plasma.
- IgG, iron and vitamin B₁₂ are transported by means of a receptor mediated mechanism into the fetal circulation.
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| Maternal fetal exchange |
- Permeability of lipid insoluble substances is hardly affected by changes in the rate of blood flow.
- Their passage is ‘membrane limited’ and control of their transfer is largely by a change in placental tissue carrier mechanisms.
- Large increases in placental blood flow as gestation proceeds are important so that the transport of respiratory gases can be increased to meet the needs of the growing fetus.
- Short term modulation of blood flow does not seem to be an important physiological control mechanism, but reduction may reduce gas transfer.
- The gradient is not directly from maternal to fetal blood, but from maternal blood to the maternal side of the placenta, where proteins, enzymes, nucleic acids, etc. may be synthesized.
- Further conversion and synthesis occurs in the fetal part of the placenta. A multiplicity of factors affect the transport of amino acids across the placenta.
- These are activity and location of amino acid transporter systems, changes in the systems as pregnancy advances, changes in the surface area of the placenta, utero-placental blood flows and concentration of the amino acids in maternal blood.
- Carbohydrate is the principal metabolic fuel of the fetus and is provided in continuous supply by transfer of glucose from the mother through the placenta.
- On the other hand, fat is not a main source of energy in the fetus, so there is only a slow limited transfer of fatty acids across the placenta.
- Sodium is much restricted in the passage through placenta.
- Iron content is much higher in fetus, which is stored in liver, spleen and bone marrow.
- The transfer of trace elements like copper is early effected. This element also accumulates in the liver.
- Calcium and phosphorus enter the fetal blood against the concentrated gradient. Depletion of calcium and phosphorus reserves from the dam occurs in cases of low plane of nutrition.
- Cell growth in the fetus is assumed to result from fetal synthesis of proteins from the amino acids transferred through the placenta.
- It has been consistently observed in several mammalian species that placental size at term always correlates with birth weight.
- This goes to say a great deal about the essential role of the placenta.
- Besides transfer of nutrients it is also a site of major endocrine activity, including synthesis of a broad range of steroid and peptide hormones, growth factors, cytokines and other biologically active compounds.
- The best known are chorionic gonadotrophin and growth hormone (GH) / prolactin like hormones.
- Placental growth hormones are essential for adapting the maternal metabolism in pregnancy, for normal placental development, and thereby for foetal growth.
- Morphologicaly there is a reduced villus surface and thus a diminished area for maternal-fetal exchange.
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Reproduction