FETAL CIRCULATION

The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply It has fetal part and maternal part

Fetal part is formed by developing trophoblastic cords from the fetus invaded by the fetal blood vessels Maternal part is formed by decidua basalis

One umblical vein arises from placenta Carries oxygenated blood Bypasses liver by joining ductus venosus Through inferior vena cava enters right atrium Right atrium to left atrium to aorta

From head and neck region blood enters right atrium Through AV valve to right ventricle To pulmonary artery Through ductus arteriosus to aorta Aorta ends into two umblical arteries They end in placenta

Specific Anatomical Structure of the Fetal Circulation Increase in systemic vascular resistance. the pulmonary vascular resistance greatly decreases as a result of expansion of the lungs

Circulatory changes at birth Closure of the Foramen Ovale Closure of the Ductus Arteriosus Closure of the Ductus Venosus

Closure of foramen ovale The low right atrial pressure and high left atrial pressure causes the valve to close In two thirds of all people, the valve becomes adherent over the foramen ovale within a few months to a few years and forms a permanent closure. But even if permanent closure does not occur, the left atrial pressure throughout life normally remains 2 to 4 mm Hg greater than the right atrial pressure, and the backpressure keeps the valve closed.

Closure of the Ductus Arteriosus Blood begins to flow backward from the aorta into the pulmonary artery through the ductus arteriosus, rather than in the other direction as in fetal life. However, after only a few hours, the muscle wall of the ductus arteriosus constricts markedly, and within 1 to 8 days, the constriction is usually sufficient to stop all blood flow. This is called functional closure of the ductus arteriosus

During the next one to four months the ductus arteriosus ordinarily becomes anatomically occluded by growth of fibrous tissue into its lumen. The cause of ductus arteriosus closure relates to the increased oxygenation of the blood flowing through the ductus.

Closure of ductus venosus Ductus venosus closes within few hours after birth.

Cerebral Blood Flow Normal blood flow through the brain of the adult person averages 50 to 65 milliliters per 100 grams of brain tissue per minute. For the entire brain, this amounts to 750 to 900 ml/min, or 15 per cent of the resting cardiac output.

Cerebral Vessels The principal arterial inflow to the brain in humans is via four arteries: two internal carotids and two vertebrals. In humans, the carotid arteries are quantitatively the most significant. The vertebral arteries unite to form the basilar artery, the basilar artery and the carotids form the circle of Willis below the hypothalamus. The circle of Willis is the origin of the six large vessels supplying the cerebral cortex

Cerebral Microcirculation An important structural characteristic of the brain capillaries is that they are much less “leaky” than the blood capillaries in almost any other tissue of the body. One reason for this is that the capillaries are supported on all sides by “glial feet,” which are small projections from the surrounding glial cells

The Blood–Brain Barrier The tight junctions between capillary endothelial cells in the brain effectively prevent proteins from entering the brain in adults and slow the penetration of some smaller molecules as well. This is referred to as the blood–brain barrier.

Areas lacking blood brain barrier Circumventricular organ Neurohypophysis Area postrema Subfornical organ Organ vasculosum of lamina terminalis

Regulation of Cerebral Blood Flow Cerebral blood flow is highly related to metabolism of the tissue. At least three metabolic factors have potent effects in controlling cerebral blood flow: (1) carbon dioxide concentration, (2) hydrogen ion concentration, (3) oxygen concentration.

Autoregulation As seen in other vascular beds, autoregulation is prominent in the brain . This is a process, by which the flow to many tissues is maintained at relatively constant levels despite variations in perfusion pressure, In the brain, autoregulation maintains a normal cerebral blood flow at mean arterial pressures of 65 to 140 mm Hg.

Role of the Sympathetic Nervous System in Controlling Cerebral Blood Flow. The cerebral circulatory system has strong sympathetic innervation that passes upward from the superior cervical sympathetic ganglia in the neck and then into the brain along with the cerebral arteries. This innervation supplies both the large brain arteries and the arteries that penetrate into the substance of the brain. However, transection of the sympathetic nerves or mild to moderate stimulation of them usually causes very little change in cerebral blood flow because the blood flow autoregulation mechanism can override the nervous effects.

Cerebral “Stroke” Occurs When Cerebral Blood Flow Is Obstructed Cerebral infarction/Cerebral Hemorrhage Causes Hypertension Atherosclerosis Diabetes mellitus

Cerebral Ischemia Unconsciousness occurs in 5 to 10 seconds if the blood flow to the brain is completely cut off. It has been estimated that neuronal function ceases after about 1 minute and that irreversible changes start to occur after about 4 minutes.

Splanchnic Circulation Splanchnic circulation constitutes Mesenteric circulation Splenic circulation Hepatic circulation (1500 ml/min, 30% of CO)

Regulation of Splanchnic circulation Auto regulation GIT activity Hormones like gastrin and cholecystokinin Excess CO2, H ions and O2 lack Nervous factors