Circulation Through Special Regions - Pulmonary, Placental and Fetal Circulation PDF

Circulation Through Special Regions Pulmonary, Placental and Fetal circulation Learning Outcomes • Describe the blood flow through lung’s functional zones. • Describe the maternal cardiovascular changes. • Explain the role of placenta on fetal blood flow • Explain the difference between fetal hemo...

Circulation Through Special Regions Pulmonary, Placental and Fetal circulation Learning Outcomes • Describe the blood flow through lung’s functional zones. • Describe the maternal cardiovascular changes. • Explain the role of placenta on fetal blood flow • Explain the difference between fetal hemoglobin and adult Hb in terms of oxygen affinity. Pulmonary Circulation • The pulmonary trunk arises from the right ventricle and divides into right and left pulmonary arteries, which convey the deoxygenated blood to the right and left lung, respectively. • The blood circulates through a capillary plexus intimately related to the walls of the alveoli and receives oxygen from the alveolar air. • This blood, which is now oxygenated is returned to the heart (left atrium) through pulmonary veins. Bronchial Circulation • Blood also flows to the lungs through small bronchial arteries that originate from the systemic circulation, amounting to 1% to 2% of the total cardiac output. • This bronchial arterial blood is oxygenated, in contrast to the partially deoxygenated blood in the pulmonary arteries. • It supplies the supporting tissues of the lungs, including the connective tissue, septa, and large and small bronchi. • After this bronchial and arterial blood passes through the supporting tissues, it empties into the pulmonary veins and enters the left atrium, rather than passing back to the right atrium. • Therefore, the flow into the left atrium and left ventricular output are about 1% to 2% greater than that of the right ventricular output. Relationship between the bronchial and pulmonary circulations. The pulmonary artery supplies pulmonary capillary network A. The bronchial artery supplies capillary networks B, C, and D. Blue-colored areas represent blood of low O2 content. Lymphatic Circulation • Lymph vessels are present in all the supportive tissues of the lung, beginning in the connective tissue spaces that surround the terminal bronchioles, coursing to the hilum of the lung, and then mainly into the right thoracic lymph duct. • Particulate matter entering the alveoli is partly removed by these lymph vessels, and plasma protein leaking from the lung capillaries is also removed from the lung tissues, thereby helping to prevent pulmonary edema. Characteristics of Pulmonary Circulation • The lung has two circulations, a high-pressure, low-flow circulation and a low-pressure, high-flow circulation. • The high-pressure, low-flow circulation supplies systemic arterial blood to the trachea, bronchial tree (including the terminal bronchioles), supporting tissues of the lung, and outer coats (adventitia) of the pulmonary arteries and veins. • The bronchial arteries, which are branches of the thoracic aorta, supply most of this systemic arterial blood at a pressure that is only slightly lower than the aortic pressure. • The low-pressure, high-flow circulation supplies venous blood from all parts of the body to the alveolar capillaries where oxygen (O2) is added and carbon dioxide (CO2) is removed. • The pulmonary artery, which receives blood from the right ventricle, and its arterial branches carry blood to the alveolar capillaries for gas exchange, and the pulmonary veins then return the blood to the left atrium to be pumped by the left ventricle through the systemic circulation. Pulmonary Blood Volume • Pulmonary vessels contain about 600 mL of blood at rest. • Since the pulmonary vessels act as capacitance vessels their blood content can vary from 200 to 900 mL. • In pathological conditions like haemorrhage transfer of blood from pulmonary vessels to systemic circulation can partly compensate for the blood loss. • Thus, the pulmonary vessels act as a reservoir of blood. Regulation Of Pulmonary Blood Flow Neural Control • Efferent sympathetic vasoconstrictor nerves richly innervate the pulmonary blood vessels. But these nerves have no resting discharge and tone, which means they can only show an increase in activity when stimulated. These participate in the vasomotor reflexes, e.g.: • Baroreceptor stimulation produces reflex dilatation of pulmonary vessels, while • Chemoreceptor stimulation causes pulmonary vasoconstriction. Afferent control through vagus is mediated through following receptors: • Pulmonary baroreceptors. • Pulmonary volume receptors are the vagal mechanoreceptors present at the junction of pulmonary vein with left atrium. • Stimulation of these receptors produces tachycardia and diuresis, which help in regulating blood volume. • J receptors (juxtapulmonary receptors) are present adjacent to pulmonary capillaries in the alveolar interstitial space. Stimulation of J receptors leads to reflex tachypnoea (increases in rate of breathing) and reduction of skeletal muscle tone. Chemical Control • Local hypoxia is responsible for most significant alterations in pulmonary blood flow by producing vasoconstriction • Hypercapnia and acidosis also produce vasoconstriction. • The effects of pCO2 and acidosis on pulmonary vessels are just opposite to those in the systemic vessels, where these stimuli produce vasodilation. The functional significance of this response is the same as in the case of local hypoxia. • Chronic hypoxia, as occurs in high-altitude dwellers, is associated with a marked increase in pulmonary arterial pressure (pulmonary hypertension). Placental and Fetal Circulation Placental and Fetal Circulation Uterine Circulation • The blood flow of the uterus parallels the metabolic activity of the myometrium and endometrium and undergoes cyclic fluctuations that correlate with the menstrual cycle in nonpregnant women. Placental and Fetal Circulation Uterine Circulation • During pregnancy, blood flow increases rapidly as the uterus increases in size. • Vasodilator metabolites are undoubtedly produced in the uterus, as they are in other active tissues. • In early pregnancy, the arteriovenous O2 difference across the uterus is small, and it has been suggested that estrogens act on the blood vessels to increase uterine blood flow in excess of tissue O2 needs. Changes in uterine blood flow and the amount of O2 in uterine venous blood during pregnancy. Placental and Fetal Circulation However, even though uterine blood flow increases 20-fold during pregnancy, the size of the conceptus increases much more, changing from a single cell to a fetus plus a placenta that weighs 4 to 5 kg at term in humans. Consequently, more O2 is extracted from the uterine blood during the latter part of pregnancy, and the O2 saturation of uterine blood falls. Corticotrophin-releasing hormone appears to play an important role in up-regulating uterine blood flow, as well as in the eventual timing of birth. Changes in uterine blood flow and the amount of O2 in uterine venous blood during pregnancy. Placenta • The placenta is a unique vascular organ that receives blood supplies from both the maternal and the fetal systems and thus has two separate circulatory systems for blood: • (1) the maternal-placental (uteroplacental) blood circulation, and • (2) the fetal-placental (fetoplacental) blood circulation. Placenta • The uteroplacental circulation starts with the maternal blood flow into the intervillous space through decidual spiral arteries. • The exchange of oxygen and nutrients takes place as the maternal blood flows around the terminal villi in the intervillous space. • The in-flowing maternal arterial blood pushes deoxygenated blood into the endometrial and then uterine veins back to the maternal circulation. • The fetal-placental circulation allows the umbilical arteries to carry deoxygenated and nutrient-depleted fetal blood from the fetus to the villous core fetal vessels. • After the exchange of oxygen and nutrients, the umbilical vein carries fresh oxygenated and nutrient-rich blood circulating back to the fetal systemic circulation. Placenta • O2 is taken up by the fetal blood and CO2 is discharged into the maternal circulation across the walls of the villi in a fashion analogous to O2 and CO2 exchange in the lungs. • However, the cellular layers covering the villi are thicker and less permeable than the alveolar membranes in the lungs, and exchange is much less efficient. • The placenta is also the route by which all nutritive materials enter the fetus and by which fetal wastes are discharged into the maternal blood. Fetal Circulation • Fifty-five percent of the fetal cardiac output goes through the placenta. • The blood in the umbilical vein in humans is believed to be about 80% saturated with O2, compared with 98% saturation in the arterial circulation of the adult. • The umbilical vein transports blood rich in oxygen and nutrients from the placenta to the fetus. • This vein enters the body and travels along the anterior abdominal wall to the liver. About half the blood it carries passes into the liver, and the rest enters a vessel called the ductus venosus, which bypasses the liver. • The ductus venosus extends a short distance and joins the inferior vena cava. • There, oxygenated blood from the placenta mixes with deoxygenated blood from the lower parts of the fetal body. This mixture continues through the inferior vena cava to the right atrium. • In an adult heart, blood from the right atrium enters the right ventricle and is pumped through the pulmonary trunk and arteries to the lungs. • The fetal lungs, however, are nonfunctional, and blood largely bypasses them. • Much of the blood from the inferior vena cava that enters the fetal right atrium is shunted directly into the left atrium through an opening in the atrial septum called the foramen ovale. • Blood passes through the foramen ovale because blood pressure is somewhat greater in the right atrium than in the left atrium. • Furthermore, a small valve on the left side of the atrial septum overlies the foramen ovale and helps prevent blood from moving in the reverse direction. *Patent foramen ovale (PFO) is a hole between the left and right atria (upper chambers) of theheart. This hole exists in everyone before birth, but most often closes shortly after being born. • The rest of the fetal blood entering the right atrium, including a large proportion of the deoxygenated blood entering from the superior vena cava, passes into the right ventricle and out through the pulmonary trunk. • Only a small volume of blood enters the pulmonary circuit because the lungs are collapsed and their blood vessels have a high resistance to blood flow. • However, enough blood does reach lung tissues to sustain them. • Most of the blood in the pulmonary trunk bypasses the lungs by entering a fetal vessel called the ductus arteriosus, which connects the pulmonary trunk to the descending portion of the aortic arch. • As a result of this connection, blood with a relatively low oxygen concentration, returning to the heart through the superior vena cava, bypasses the lungs. At the same time, it is prevented from entering the portion of the aorta that branches to the heart and brain. • The more highly oxygenated blood that enters the left atrium through the foramen ovale mixes with a small amount of deoxygenated blood returning from the pulmonary veins. • This mixture moves into the left ventricle and is pumped into the aorta. • Some of it reaches the myocardium through the coronary arteries, and some reaches the brain tissues through the carotid arteries. Blood carried by the descending aorta includes the less oxygenated blood from the ductus arteriosus. • Some of the blood is carried into the branches of the aorta that lead to the lower regions of the body. The rest passes into the umbilical arteries, which branch from the internal iliac arteries and lead to the placenta. There the blood is reoxygenated. • At birth, the fetal cardiovascular system must adjust when the placenta ceases to function and the newborn begins to breathe. Diagram of the circulation in the fetus, the newborn infant, and the adult. DA, ductus arteriosus; FO, foramen ovale. Fetal Respiration • The tissues of fetal and newborn mammals have a remarkable but poorly understood resistance to hypoxia. • However, the O2 saturation of the maternal blood in the placenta is so low that the fetus might suffer hypoxic damage if fetal red cells do not have a greater O2 affinity than adult red cells. • The concentration of oxygen-carrying hemoglobin in fetal blood is about 50% greater than in maternal blood, and fetal hemoglobin has a greater attraction for oxygen than does adult hemoglobin • The fetal red cells contain fetal hemoglobin (hemoglobin F), whereas the adult cells contain adult hemoglobin (hemoglobin A). • Some hemoglobin A is present in blood during fetal life. After birth, production of hemoglobin F normally ceases, and by the age of 4 mo, 90% of the circulating hemoglobin is hemoglobin A. Dissociation curves of hemoglobin in human maternal and fetal blood. Changes In Fetal Circulation & Respiration At Birth • Because of the patent ductus arteriosus and foramen ovale, the left heart and right heart pump in parallel in the fetus rather than in series as they do in the adult. • At birth, the placental circulation is cut off and the peripheral resistance suddenly rises. • The pressure in the aorta rises until it exceeds that in the pulmonary artery. • Meanwhile, because the placental circulation has been cut off, the infant becomes increasingly asphyxial. • Finally, the infant gasps several times, and the lungs expand. • The markedly negative intrapleural pressure (–30 to –50 mm Hg) during the gasps contributes to the expansion of the lungs, but other factors are likely also involved. • The sucking action of the first breath plus constriction of the umbilical veins squeezes as much as 100 mL of blood from the placenta (the “placental transfusion”). • Once the lungs are expanded, the pulmonary vascular resistance falls to less than 20% of the value in utero, and pulmonary blood flow increases markedly. Blood returning from the lungs raises the pressure in the left atrium, closing the foramen ovale by pushing the valve that guards it against the interatrial septum. • The ductus arteriosus constricts within a few hours after birth, producing functional closure, and permanent anatomic closure follows in the next 24–48 h due to extensive intimal thickening. • https://www.youtube.com/watch?v=zTXmaVgobNw