4. Regulation of fluid balance in the microcirculation JC - Tagged.pdf

Life Sciences & Medicine Regulation of fluid balance in the microcirculation Prof James Clark School of Cardiovascular and Metabolic Medicine and Sciences...

Life Sciences & Medicine Regulation of fluid balance in the microcirculation Prof James Clark School of Cardiovascular and Metabolic Medicine and Sciences PHYSIOLOGY AND ANATOMY OF SYSTEMS Learning Outcomes  Describe the pathways by which water and other substances move across the capillary wall  Describe the Starling principle of fluid balance in the microcirculation  Describe the role of the lymphatic system in maintaining interstitial fluid balance  List four important factors responsible for oedema formation The microcirculation The part of the vascular system comprising the terminal arterioles, capillaries, and post- capillary venules The part of the circulation where gases, water, nutrients, waste materials, and other substances are exchanged between the blood and body tissues, via the interstitial compartment Made up of a 3-D meshwork of blood vessels 3 (absorption + lymphatic flow) This can occur due to four main factors: 1. Increased capillary hydrostatic pressure  In heart failure, there is an increase in venous pressure which backs up into the capillaries  Venous obstruction due to thromboembolism increases capillary pressure 2. Increased capillary or venular permeability  Inflammation increases venular permeability, causing proteins to leak out into the interstitium. 3. Decreased plasma oncotic pressure  Malnutrition, burns and liver dysfunction can reduce the plasma protein concentration 4. Lymphatic obstruction reduces lymphatic flow  Can be caused by lymph node removal, lymphatic inflammation or lymphatic invasion by parasites Capillary Pressures REMEMBER: There is a pressure gradient through the capillaries: Pressure at the arterial end is ~30 mmHg. At the venous end it is ~10 mmHg. Thus NFP is positive at the arterial end and negative at the venous end. This means that some fluid leaves the capillaries at the arterial end and returns into the venous capillaries. About 90% of the lost fluid returns into the venous capillaries. The rest is removed by the lymphatics. he Lymphatic System The lymphatic system Preserves fluid balance, transfers fat absorbed in the small intestine to the circulatory system, transports foreign materials to the lymph nodes for immunosurveillance. Lymphatic capillaries contain intercellular clefts which allow the one-way entry of fluid (containing water, salts, proteins, bacteria) driven by tissue compression. one-way valve lymphangion Afferent lymphatic trunks run next to major blood vessels. Contain smooth muscle which contracts rhythmically. lymphatic Each lymphangion has pacemaker cells and acts like a miniature blood capillary heart, pumping the fluid forward. Need to compress to 40-50 mmHg capillary/ to prevent pumping (important for stopping envenomation) venule Lymphatic drainage afferent lymphatic efferent lymphatic Left or right Subclavian vein lymphocyte thoracic or right subclavian duct lymph node: reabsorption of some lymph fluid occurs here. Lymphocytes cycle from blood to lymph glands to blood, can become activated in the lymph glands by antigens entering via the afferent lymphatics Learning Outcomes  Describe the pathways by which water and other substances move across the capillary wall  Describe the Starling principle of fluid balance in the microcirculation  Describe the role of the lymphatic system in maintaining interstitial fluid balance  List four important factors responsible for oedema formation Recommended reading: Silverthorne’s Human Physiology: An Integrated Approach, Global Edition Pages 160-162, 531-536