MICROCIRCULATION

1. MICROCIRCULATION CAPILLARY CIRCULATION Dr. AmelEassawi

2. OBJECTIVES The student should be able to: • Understand the essential role of microcirculation as the site of exchange between blood and tissue. • Know the basic structure of the capillary membrane. • Know the basic composition of the interstitium. • Understand the formula and factors of fluid filtration through the capillary membrane. • Understand the movements of fluid at the arterial and venous ends of the capillaries. • Understand the role of the lymphatic system in the interstitial fluid balance. • Identify the causes and factors affecting abnormal interstitial fluid accumulation (Edema).

3. Microcirculation The term Microcirculation refers to the functions of the capillaries and the neighboring lymphatic vessels. 5 % of circulating blood volume is present in the capillaries at any given time. This takes part into the exchange of nutrients, gases and waste products between the blood & tissues. • Important in the transport of nutrients to tissues • Site of waste product removal • Over 10 billion capillaries with surface area of 500-700 square meters • Solute and fluid exchange

4. Capillary Network

5. Microcirculation Exchange of substance across the capillary wall occurs by: • Simple diffusion • Bulk Flow Particular Capillary Pores: • Brain: tight junction allow only very small molecules to pass through, blood-brain barrier • Liver: sinusoids (wide-open pore) • Gastrointestinal: intermediate size • Renal glomeruli: fenestration within cells

7. Microcirculation Diffusion: Motion of water and dissolved particles across capillary membrane, driven by concentration differences. Lipid-soluble particles diffuse through the cell membrane: oxygen, carbon Dioxide. Water soluble particles go only through the pores. Permeability: Water soluble particles go only through the pores, and the permeability of a particle depends on particle diameter versus diameter of the pore. Capillary permeability to albumin is almost zero, except for the liver sinusoids. Permeability varies between capillary beds. Interstitium and interstitial fluid: Interstitium is extracellular and extravascular space. Comprises collagen fibers, proteoglycans and free fluid.

9. Microcirculation • Arteriole  Meta arteriole  Capillaries Venules. • Pre capillary sphincter is present at the junction where the capillary arises from the Meta arteriole. This opens and closes the entrance of capillary and hence regulates the blood flow through the capillary. • The capillary wall is thin & consists of a single layer of endothelial cells on basement membrane. Pores are present between the endothelial cells that allow transport of substances including water.

10. STARLINGS FORCES The rate of filtration at any point along the capillary depends on a balance of forces – STARLINGS FORCES. 1. Capillary hydrostatic pressure Arterial end = 37 mmHg Venous end = 17 mmHg 2. Plasma colloid osmotic pressure = 25 mmHg 3. Interstitial hydrostatic pressure = 0 -1 mmHg 4. Interstitial colloid osmotic pressure = 0 mmHg

11. Fluid movement = Kf [ (Pc – Pi) – ( Πc - Πi ) Kf = filtration coefficient

12. STARLINGS FORCES Arteriolar end fluid moves out into tissue spaces Venous end fluid enters into capillaries. Any decrease in plasma proteins (hypoproteinemia) or increase in capillary hydrostatic pressure (cardiac failure) causes edema. (abnormal increase in interstitial fluid volume ) Histamines, bradykinin increases capillary permeability  edema.

14. Lymphatic Circulation

15. Lymphatic circulation • Lymphatic system is responsible for bringing the interstitial fluid to vascular compartment. • Normal 24 hrs lymph flow is 2-4 L • Lymphatic capillaries lie in interstitial fluid close to vascular capillaries ,these capillaries merge into large lymphatic vessels & eventually into largest vessel, thoracic duct which empties into large veins . • The interstitial fluid enter lymphatic capillaries through loose junctions between endothelial cells . • Lymph flow back to the thoracic duct is promoted by contraction of smooth muscle in wall of lymphatic vessels & contraction of surrounding skeletal muscle . • Failure of lymphatic drainage can lead to Edema

17. Edema • Accumulation of fluid beneath the skin or in a body cavity • Palpable swelling produced by expansion of the interstitial fluid volume

18. Causes of Edema 1. Increase capillary pressure • Increase vascular volume • Heart failure • Kidney disease • Pregnancy • Environmental heat stress • Venous obstruction • Thrombosis • Liver disease 2. Increase capillary permeability • Inflammation • Allergic reaction • Tissue injury • Malignancy

19. Causes of Edema 3. Decrease colloidal osmotic pressure • Increase loss of proteins • Nephrotic syndrome • Burns • Decrease production • Starvation, Malnutrition • Liver disease 4. Obstruction of lymphatic flow • Surgical removal of lymph nodes • Malignant obstruction • Infection ( filariasis)

20. References • Human physiology, Lauralee Sherwood, seventh edition. • Text book physiology by Guyton &Hall,11th edition. • Physiology by Berne and Levy, sixth edition.