The blood brain barrier exists between the circulation and the interstitial fluid in the brain. It consists of the ultrafiltration barrier in the choroid plexuses and the barrier around cerebral capillaries. The latter consists of capillary endothelium, basement membrane and a fenestrated layer of astrocyte endfeet. Tight junctions, impermeable to solutes, join capillary endothelial cells and form a basic component of the blood brain barrier. Water, carbon dioxide and oxygen diffuse freely across the blood brain barrier, but the transport of glucose, the principal brain substrate, and ionized molecules is controlled. Proteins and some drugs cannot cross the endothelium unless it is inflamed. The blood brain barrier has the following functions:
• To provide tight control over ionic (H+, Na+, K+, Ca2+, Mg2+) concentrations in the interstitial fluid because brain cells are extremely sensitive to ion changes
• To protect the brain from transient changes in plasma glucose, the main substrate for the brain
• To protect the brain from endogenous and exogenous toxins in the plasma
• To prevent release of central neurotransmitters into the systemic circulation
The foetus and placenta form a unit in which the placenta enables the foetus to exchange carbon dioxide and metabolic waste products for oxygen and nutrients from the maternal circulation. In the foetus the left and right sides of the heart work in parallel, unlike the adult circulation where the ventricles work in series. The foetal ventricles acting in parallel, pump blood through the systemic vessels and the placenta, which are also arranged in parallel. The output of the foetal heart is split about 60:40, with the majority passing through the placenta, for oxygenation and nutrient—waste exchange (Figure CR.36). The oxygenated blood returns from the placenta, about half of it entering the liver with the portal circulation, while the rest joins the inferior vena cava (IVC) directly. IVC blood is therefore relatively well oxygenated and passes back to the heart where it divides between right and left sides, the majority passing to the left atrium. The right atrium receives superior vena caval blood from the head and upper limbs which mixes with its portion of the oxygenated IVC blood. The right side of the heart provides two-thirds of the foetal cardiac output that goes to supply the lower half of the foetus. The blood filling the left ventricle is nearly all from the IVC, which is therefore better oxygenated than the blood in the right ventricle. The left ventricle output goes to supply the head and upper limbs. The above pattern of foetal circulation is due to the following vascular features in the foetus, which ultimately convert to the adult configuration immediately or soon after birth:
• Umbilical arteries—two arteries that arise from the internal iliac arteries and carry 60% of the cardiac output to the placenta
• Umbilical vein—carries oxygenated blood from the placenta back to the foetal liver and IVC
• Ductus venosus—splits off 50% of the oxygenated blood from the umbilical vein to bypass the liver and enter the IVC directly
• Foramen ovale—communication between right and left atria allows about two-thirds of the oxygenated IVC blood to pass directly to the left atrium
• Pulmonary vasculature—foetal lungs are collapsed and have a high pulmonary vascular resistance only allowing about 10% of the right ventricular output to pass through the lungs
• Ductus arteriosus—connects pulmonary artery to the aorta, allowing the right ventricular output to bypass the lungs and flow down the aorta to the lower foetal body and placenta
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