Copstead-Kirkhorn: Pathophysiology Chapter 15 Alterations in Blood Flow PDF

**Copstead-Kirkhorn: Pathophysiology, 4^th^ Edition** **Chapter 15: Alterations in Blood Flow** **KEY POINTS** **ORGANIZATION OF THE CIRCULATORY AND LYMPHATIC SYSTEMS** - Arteries and veins have three distinct layers. The intima, the innermost layer, is composed of a single layer of endo...

**Copstead-Kirkhorn: Pathophysiology, 4^th^ Edition** **Chapter 15: Alterations in Blood Flow** **KEY POINTS** **ORGANIZATION OF THE CIRCULATORY AND LYMPHATIC SYSTEMS** - Arteries and veins have three distinct layers. The intima, the innermost layer, is composed of a single layer of endothelial cells. The media, or middle layer, is composed of smooth muscle and elastin. Media is thicker in arteries than in veins. The adventitia, the outermost layer, is composed of supporting connective tissue. - Capillaries have only a single layer of endothelial cells attached to a basement membrane. The permeability of capillaries is determined by how tightly the endothelial cells join together. - Lymphatic vessels resemble veins, having thin walls and valves. **PRINCIPLES OF FLOW** - Physical laws govern the flow of blood through the circulatory system. Predictions regarding blood flow, blood pressure, and resistance to flow can be made using these laws. The important relationships may be summarized as follows: 1. 1\. Flow = pressure/resistance 2. 2\. Blood pressure = flow (cardiac output) × resistance 3. 3\. Resistance = pressure/flow - The main factors affecting resistance to flow are the radius and length of the vessels, and blood viscosity and turbulence. Usually, the radius of the vessel is the most important determinant of resistance. It affects resistance inversely and to the fourth power. A small decrease in radius results in a large increase in resistance. - The velocity of blood flow varies inversely with the total cross-sectional area of the vascular bed. The capillaries have the greatest total cross-sectional area and, therefore, the slowest flow. - Laplace's law describes the relationships among wall tension, distending pressure, and vessel radius *(P = T/r; T = Pr)*. An increase in radius or distending pressure results in increased wall tension. At critical closing pressure, wall tension overwhelms distending pressure and blood flow ceases. - The transcapillary exchange of fluid and nutrients is accomplished by the processes of diffusion and filtration. Diffusion refers to movement of solute and is determined by capillary permeability and the size of the concentration gradient. Filtration refers to movement of fluid and is affected in the following way: 1. 1\. Increased capillary fluid pressure and interstitial fluid colloid osmotic pressure enhance filtration. 2. 2\. Increased interstitial fluid pressure and plasma colloid osmotic pressure oppose filtration. 3. 3\. Increased permeability (K) enhances filtration. **CONTROL OF FLOW** - The blood flow through a particular vascular bed is regulated centrally by the autonomic nervous system and locally by the organ or tissue. - In most vascular beds, the SNS causes constriction, which increases resistance and reduces flow. Smooth muscle cells in these vascular beds have α-1 receptors that bind the SNS neurotransmitter norepinephrine, causing contraction. There is no significant parasympathetic innervation of systemic vessels. - Autoregulation refers to a tissue's ability to regulate its own flow. Autoregulation allows a tissue to maintain optimal flow despite changes in blood pressure or metabolic demands. In instances of high blood pressure or decreased metabolic demand, the arterioles and precapillary sphincters that control flow to the tissue constrict, reducing flow. In instances of low blood pressure or high demand, vessels dilate, increasing flow. - Lymphatic vessels maintain flow by contracting when stretched with lymph. Intraluminal valves prevent backflow. External compression by contracting muscles enhances lymph flow. **GENERAL MECHANISMS THAT CAUSE ALTERED FLOW** - Altered blood flow results from obstructive processes. Obstruction results in reduced flow beyond the obstruction (downstream) and increased pressure before the obstruction (upstream). - In the arterial system, obstruction manifests primarily as distal ischemia. In the venous system, obstruction manifests as edema. - The causes of vessel obstruction include thrombi, emboli, vasospasm, external compression (e.g., compartment syndrome), and structural alterations (e.g., atherosclerotic plaques, aneurysms). - Alterations in pressures within the circulatory system or interstitium produce edema, whereas an impairment of the lymphatic system results in lymphedema. **ALTERATIONS IN ARTERIAL FLOW** - Common causes of arterial obstruction are atherosclerosis, inflammation, vasospasm, and aneurysms. Emboli are the usual cause of acute arterial occlusion. - Atherosclerosis is the most common cause of chronic progressive arterial obstruction. Several risk factors for the development of atherosclerosis have been proposed, among them smoking, hyperlipidemia, male gender, advancing age, sedentary lifestyle, obesity, glucose intolerance, and a family history of cardiovascular disease. - Acute arterial obstruction is accompanied by the classic manifestations known as the six Ps: pallor, paresthesia, paralysis, pain, pulselessness, and polar (cold to touch). **ALTERATIONS IN VENOUS FLOW** - Common causes of venous obstruction are incompetent valves (as may occur with obesity, pregnancy, right heart failure, or prolonged standing), producing varicose veins and chronic venous insufficiency, and obstruction by deep vein thrombosis. - Edema, venous stasis ulcers, and pain usually accompany chronic venous obstruction. - Deep vein thrombosis is potentially life threatening because of the likelihood of embolization to the pulmonary circulation. It is treated aggressively with the administration of anticoagulants. **ALTERATIONS IN LYMPHATIC FLOW** - Obstruction of lymph flow is most commonly the result of surgical removal of, or radiation damage to, lymphatic vessels during treatment of cancer. - Manifestations of lymphatic obstruction include regional edema and thickened subcutaneous tissue.