T. Estación donde está situado el Puesto Central del T

1.1.3.1. Haemochorial placental blood flow.

There is no intermingling of maternal and foetal blood in the placenta. The placenta is a unique vascular organ. It receives blood supplies from both the maternal and the foetal circulatory systems, which are separated by several tissues called the placental membrane or placental barrier. Some villi anchor the villous tree to the basal plate whereas the bulk of the placenta comprises trees of gas-exchanging terminal villi floating in the maternal blood.

The placental circulation consists of two distinctly different systems (Wang & Zhao 2010). The uteroplacental blood circulation is essential for sufficient exchange of gases as oxygen, water, nutrients and waste products. It’s the terminal villi that are grape-like structures that are important as these are characterized by capillarization and dilated sinusoids. The terminal villi are linked to stem villi that connect to chorionic plate and are characterized by condensed fibrous stroma containing both large vessels with microvessels. As maternal blood flows around terminal villi and into the intervillous space this process occurs via decidual arteries. The in-flowing maternal arterial blood pushes the deoxygenated blood into the endometrial veins. However, it is the uterine veins that drain the deoxygenated blood. The maternal rate of placental blood flow at term is approximately 600–700 ml/minute (80% of the uterine perfusion). The foetoplacental blood circulation involves two umbilical arteries that carry deoxygenated and nutrient-depleted foetal blood into the villous core foetal vessels to the placenta. It is the umbilical vein, which supplies fresh oxygenated and nutrient-rich blood from the placenta to the foetus. Wang & Zhao, (2010), estimated that the surface area of syncytiotrophoblasts is approximately 12m2 and the length of foetal capillaries is approximately 320 kilometres. The terminal villi are essential components of the functional unit of maternal-foetal exchange of oxygen and nutrients (Wang & Zhao, 2010).

1.1.3.2. Transfer between mother and foetus.

The mechanisms by which transfer of substances across the placenta between the mother and foetus occurs include passive diffusion, facilitated diffusion, active transport, endocytosis and exocytosis. The placental transfer or transport involves bidirectional movement of gases, nutrients, waste materials, drugs and other substances across the placenta between maternal and foetal circulations. The process of transfer of substances across the placenta requires blood flow from the foetus via the umbilical vein into the foetal capillary epithelium across the placental membrane, entering the intervillous space and then into the endometrial spiral arteries. Blood flows from the mother via the endometrial spiral arteries, and enters the intervillous space across the placental membrane into the foetal capillary via the umbilical vein. A counter current mechanism exists whereby blood from the maternal circulation as shown in red in Figure 1. (modified from Williams, 2010) is separated from foetal blood containing foetal waste products, as shown in blue, returning back to the maternal circulation via the umbilical vein that carries oxygenated blood to the foetus.

Transport across the placenta increases during the course of gestation due to changes in placental structure (decreasing the distance between maternal and foetal blood), increased foetal and maternal blood flow, and greater foetal demands. Placental transfer increases as the foetal growth rate increases (Wang & Zhao 2010; and Kay et

al, 2011).

Syncytiotrophoblasts are an important site of transport with substances moving from the apical membrane close to the maternal blood in intervillous space, and across the cell wall to the basal membrane of foetal capillary epithelium. Several mechanisms are summarised by which specific substances across are transferred the placenta. Protein carriers and other transporters mediate facilitated diffusion and active transport. For pregnancy the placental syncytiotrophoblast represents the barrier between the maternal and foetal circulations, (Wang & Zhao 2010; Williams, 2010; and Kay et al, 2011).

1.1.3.3a. Simple (passive) diffusion. Diffusion is movement of a substance from higher to lower concentration down electrochemical gradients and is a passive process. Diffusion is the major mechanism of placental transfer. For example water, electrolytes, oxygen, carbon dioxide, urea, simple amines, creatinine, fatty acids,

steroids, fat-soluble vitamins, narcotics, antibodies, barbiturates, and anesthetics. Simple diffusion allows most substances with a molecular mass ≤ 5000 Da to pass through the placental tissue (Johnson, & Everitt, 2007; Wang and Zhao, 2010; and Kay et al, 2011).

1.1.3.3b. Facilitated diffusion. This involves transport via protein carriers and other transporters to move substances like glucose and oxygen across the placental membrane. Transporters are located on both the maternal-facing border sycytiotrophoblast and the foetal-facing basal membrane (Johnson, & Everitt, 2007;

Wang and Zhao, 2010; and Kay et al, 2011).

1.1.3.3c. Active Transport. This utilizes energy-dependent carrier systems and other transporters to move substances against concentration or electrochemical gradients. For example: amino acids, water-soluble vitamins, calcium, iron, and iodine(Johnson & Everitt, 2007; Wang and Zhao, 2010; and Kay et al, 2011).

1.1.3.3d. Pinocytosis. This is nonspecific in the substances that it transports and is used primarily for the absorption of extracellular fluids (ECF) (Johnson, & Everitt, 2007; Wang and Zhao, 2010; and Kay et al, 2011).

1.1.3.3e. Endocytosis and exocytosis. These include the globulins, phospholipids, lipoproteins, antibodies and viruses (Johnson, & Everitt, 2007; Wang and Zhao, 2010; and Kay et al, 2011).

1.1.3.3f. Bulk flow and ‘solvent drag,’ for example, are water and electrolytes. Capillary breaks, for example, are the intact blood cells. However, the maternal leukocytes - organisms such as Treponema pallidum - are known for independent movement (Johnson, & Everitt, 2007; Wang and Zhao, 2010; and Kay et al, 2011). The foetus communicates with the mother via the placenta. Several factors, including maternal health, smoking, hypoxia, and nutritional status, may influence the maternal pregnancy (Johnson, & Everitt, 2007; Wang and Zhao, 2010; and Kay et al, 2011). Hormones produced by the placenta alter maternal metabolism and behavior, nutrient intake, and uterine artery blood flow. These changes are necessary to promote placental development and growth (Johnson, & Everitt, 2007; Wang and Zhao, 2010; and Kay et al, 2011).