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The Circulation of blood
Chapter 15
Types of Blood vessels o Arteries —carry blood away from the heart and toward capillaries o Veins —carry blood toward the heart and away from capillaries o Capillaries — The smallest blood vessels. Carry blood from the arterioles to the venules Oxygenated blood is pumped from the left ventricle of heart through a series of large distribution vessels— the arteries. The largest artery in the body is the aorta. Arteries subdivide into vessels that become progressively smaller called arterioles that control the flow into microscopic exchange vessels called capillaries (the smallest blood vessels). In the so-called capillary beds , the exchange of nutrients and respiratory gases occurs between the blood and cell. Blood is drained, from the capillary beds and then enters the small venules , which join with other venules and increase in size, becoming veins. The largest veins are the superior vena cava and the inferior vena cava which finally return deoxygenated blood into right atrium. Sequence of blood flow : arteries —>arterioles —>capillaries —>venules —>veins
Structure of blood vessels : Both arteries and veins have three layers or coats. Arteries a. Tunica intima—inner smooth layer of endothelial (epithelial) cells of squamous type. No valve present. b. Tunica media—smooth muscle layer, thicker in arteries than vein; some elastic tissue; important in blood pressure regulation c. Tunica externa—outer layer of fibrous connective elastic tissue. Capillaries —microscopic vessels and has only one thick layer of endothelium—the tunica intima, receive blood from arterioles and are the sites for exchange of substances between cell and blood. Veins a. Tunica intima—inner layer of endothelial cells squamous type; valves present in veins prevent retrograde movement of blood. This single layer of cells provides a very smooth lining that prevents the accidental formation of blood clots. b. Tunica media—smooth muscle layer; thin in veins than artery. The thicker muscle layer in the artery wall is able to resist great pressures generated by ventricular systole. In arteries, the tunica media plays a critical role in maintaining blood pressure and controlling blood distribution. c. Tunica externa—heavy layer of fibrous connective tissue in many veins
Ischemia : a condition developed due to reduced blood supply to a tissue because of the narrowing of blood vessels, involves the gradual death of cells and may lead to complete tissue death—a condition called necrosis. Infarction is tissue death (necrosis) due to inadequate blood supply to the affected area. It may be caused by artery blockages, rupture, mechanical compression, or vasoconstriction.. Ischemia of heart muscle can lead to myocardial infarction. Gangrene : If a large section of tissue becomes necrotic, it may begin to decay leading to gangrene. Gangrene can be caused by a serious bacterial infection also. In general, arteriosclerosis develops with advanced age, diabetes, high-fat and high-cholesterol diets, hypertension (high blood pressure), and smoking. Arteriosclerosis can be treated by drugs called vasodilators that trigger the smooth muscles of the arterial walls to relax, thus causing the arteries to dilate. May be corrected by vasodilators or angioplasty (mechanical widening of vessels by stenting or balloon angioplasty), or surgical replacement (bypass surgery) o Aneurysm —abnormal widening of arterial wall due to damage of the arterial wall by atherosclerosis or other factors. It promotes formation of thrombi that may obstruct blood flow to vital tissues. Also, the arterial walls may burst, resulting in life-threatening hemorrhaging o Cerebrovascular accident (CVA), or stroke —A stroke results from ischemia of brain tissue caused by an embolism or ruptured aneurysm or sudden interruption of blood flow to brain. B. Disorders of veins —low-pressure vessels o Varicose veins (varices) — Varicose veins are veins in which blood tends to pool rather than continue on toward the heart. Also called varices most commonly occur in superficial veins near the surface of the body. The great saphenous vein, the largest superficial vein of the leg, often becomes varicose in people who stand for long periods. The force of gravity slows the return of venous blood to the heart in such cases, causing blood-engorged veins to dilate enlarged veins in which blood pools. Varicose veins can be treated by supporting the dilated veins from the outside. For instance, support stockings can reduce blood pooling in the great saphenous vein. Surgical removal of varicose veins can be performed in severe cases o Hemorrhoids — Hemorrhoids or piles, are varicose veins in the rectum. Excessive straining during
defecation can create pressures that cause hemorrhoids. Treatments of the varicose vein include supporting affected veins or surgical removal of veins. o Phlebitis and Thrombophlebitis : P hlebitis or vein inflammation is commonly caused by the irritation by an intravenous catheter. Thrombophlebitis is acute phlebitis caused by clot (thrombus) formation. Veins are more likely sites of thrombus formation than arteries because venous blood moves more slowly and is under less pressure. If a piece of a clot breaks free, it may cause an embolism when it blocks a blood vessel. Pulmonary embolism, results when an embolus lodges in the circulation of the lung which can lead to death quickly if too much blood flow is blocked. Why blood do not clot inside the body? o Due to presence of heparin (anti-coagulant) in the blood. o Due to the smooth endothelial inner wall of the blood vessels. Circulation of Blood Blood circulation —refers to the flow of blood through all the vessels, which are arranged in a complete circuit or circular pattern. Different types of blood circulation are: Systemic circulation o Carries blood throughout the body. o Blood flow from the left ventricle of the heart through blood vessels to all parts of the body and back to the right atrium of the heart is described as the systemic circulation o Path goes from left ventricle through aorta, smaller arteries, arterioles, capillaries, venules, venae cavae, to right atrium Pulmonary circulation o Carries blood to lung from heart and from the lungs to heart. o Arteries deliver deoxygenated blood to the lungs for gas exchange o Path goes from right ventricle through pulmonary arteries, lungs, pulmonary veins, to left atrium.
- Fetal circulation (Figure 15-12) o Circulation in the body before birth differs from circulation after birth because the fetus secures oxygen and food from maternal blood instead of from its own lungs and digestive organs. Specialized blood vessels must carry the fetal blood to the placenta , where the exchange occurs, and then return it to the fetal body. Three vessels as part of the umbilical cord accomplish this purpose. They are the two small umbilical arteries and a single, much larger umbilical vein. o The movement of blood in the umbilical vessels may seem unusual at first in that the umbilical vein carries oxygenated blood from mother to the baby, and the umbilical artery carries oxygen-poor blood from baby to mother. o Ductus venosus -Another structure unique to fetal circulation is called the ductus venosus. It is actually a continuation of the umbilical vein. It serves as a shunt, allowing most of the blood returning from the placenta to bypass the immature liver of the developing baby and empty directly into the inferior vena cava. o Foramen Ovale and Ductus arteriosus -Two other structures in the developing fetus allow most of the blood to bypass the developing lungs, which remain collapsed until birth. The foramen ovale shunts blood from the right atrium directly into the left atrium, and the ductus arteriosus connects the aorta and the pulmonary artery. o At birth, the baby's specialized fetal blood vessels and shunts must be rendered nonfunctional. When the newborn infant takes its first deep breaths, the circulatory system is subjected to increased pressure. The result is closure of the foramen ovale and rapid collapse of the umbilical blood vessels, the ductus venosus, and ductus arteriosus. o Several congenital disorders result from the failure of the circulatory system to shift from the fetal route
of blood flow at the time of birth. The ductus arteriosus may fail to close, for example, and allow deoxygenated blood to bypass the lungs. Similarly, the foramen ovale may fail to close and remain as a so-called hole in the heart that allows blood to bypass the pulmonary circulation. In such cases, a light-skinned baby may appear bluish because of the lack of oxygen in the systemic arterial blood. This condition of bluish tissue coloration is called cyanosis.
for example, about the rate strength, and rhythmicity of the heartbeat. Major pulse points named after arteries over which they are felt (Figure 15-15) Three pulse points are located on each side of the head and neck :
- Superficial temporal artery in front of the ear
- Common carotid artery in the neck along the front edge of the sternocleidomastoid muscle. Carotid pulse is used during emergency situations and when performing cardiopulmonary resuscitation (CPR).
- Facial artery at the lower margin of the mandible at a point below the corner of the mouth A pulse is also detected at three points in the upper limb :
- Axillary artery in the armpit
- Brachial artery at the bend of the elbow along the inner or medial margin of the biceps brachii muscle. This pulse site is the most commonly used site to obtain blood pressure measurements
- Radial artery at the wrist. The radial pulse is the most frequently monitored and easily accessible in the body. The pulse also can be felt at four locations in the lower extremity :
- Femoral artery in the groin
- Popliteal artery behind and just proximal to the knee
- Posterior tibial artery just behind the medial malleolus (inner bump of the ankle)
- Dorsalis pedis artery on the front surface of the foot, just below the bend of the ankle joint Apical pulse - Found at the apex of the heart, located at the fifth intercostal space left side, mid clavicular line, that is, between the fifth and sixth ribs perpendicular to the middle of the clavicle, left of the sternum. A stethoscope is required to obtain an apical pulse.
Hypertension (HTN) o Occurs when blood pressure exceeds 140/90 mm Hg (Figure 15-14) o Types: Primary-essential hypertension: 90% of HTN cases are primary-essential (idiopathic) with no known cause. Secondary Hypertension: HTN can be caused by kidney disease, hormonal problems, pregnancy or other causes o Many risk factors for HTN, including genetics, age, stress, obesity, lack of exercise, smoking, alcohol and more o Untreated HTN may contribute to heart disease, kidney failure, and stroke. Hypertension is known as ‘silent killer’ Circulatory Shock o Circulatory shock—failure of the circulatory system to deliver oxygen to the tissues adequately, resulting in cell impairment o When the cause is known, shock can be classified as follows: Cardiogenic shock—caused by heart failure Hypovolemic shock—caused by a drop in blood volume due to blood loss that causes blood pressure
