Microcirculation, Lymphatic system PDF

Dr. Noor-un-Nisa Memon Assistant Professor Physiology DEPT JSMU  Microcirculation The most important function of circulation occurs in the microcirculation that is transport of nutrients to the tissues and removal of cell excreta Walls of capillaries are t...

Dr. Noor-un-Nisa Memon Assistant Professor Physiology DEPT JSMU  Microcirculation The most important function of circulation occurs in the microcirculation that is transport of nutrients to the tissues and removal of cell excreta Walls of capillaries are thin made up of single layer of endothelial cells, so substance exchange quickly b/w tissue and blood Whole body has 10 billion capillaries with 500 to 700 sq meter surface area  Microcirculation The microcirculation is the blood flow through blood vessels smaller than 100 µm Components of microcirculation Meta arterioles Arterioles Capillaries Postcapillary venules  CAPILLARIES Smallest of a body's blood vessels, measuring 5-10 μm Connect arteries and veins Closely interact with tissues "CAPILLARY BED" the network of capillaries supplying an organ. Usually carries no more than 25% of the amount of blood  STRUCTURE OF CAPILLARIES
Walls composed of a unicellular layer of endothelium Surrounded by a basement membrane 0.5 micrometer thick Lipids soluble substances and gases can pass easily through diffusion Pores or intercellular cleft 6-7 nano m represents 1/1000th of total surface area Allows for the passage of water soluble molecules except for proteins Plasmalemmal vesicles– play a role in transporting material from one surface of capillaries to other  TYPES OF CAPILLARIES Continuous Have a sealed epithelium only allow small molecules, water and ions to diffuse Example blood brain barrier Fenestrated Have openings that allow larger molecules to diffuse Examples-- Renal glomeruli,GIT Sinusoidal Special forms of fenestrated capillaries Have larger openings in the epithelium allowing RBCs and serum proteins to enter Example liver, lymphoid tissue, endocrine organs, and hematopoietic organs (bone marrow, spleen)  VASOMOTION
Intermittent flow of blood in the capillaries due to intermittent constriction and dilation of regional arterioles Oxygen concentration in tissue is the most important factor regulating circulation  CAPILLARY FUNCTIONS Exchange of water and nutrients b/w blood and interstitium Mechanism of exchange Diffusion Filtration (bulk flow)  DIFFUSION Principal mechanism of micro vascular exchange Rate of diffusion is directly proportional to -Change in concentration -Surface area -Solubility Inversely proportional to -Thickness of membrane -Molecular weight -Lipid soluble – through endothelial cell membrane. -Water soluble – through intercellular pores Size Dependent  INTERSTITIUM Spaces between the cells Fluid in this space– interstitial fluid 2/3rd of ECF, 1/6th of TBW Composed of -Collagen fibers -Proteoglycan filaments  INTERSTITIAL FLUID
Interstitial fluid---same composition as of plasma except for lower concentration of proteins Composed of Interstitial gel Free fluid Gel--- fluid that is entrapped in the small spaces among proteoglycan filaments Difficult to flow easily Free fluid----fluid that is free from proteoglycan Can flow easily In the form of free fluid vesicle and rivulets 
FILTRATION AND REABSORPTION
FILTRATION Movement of fluid from plasma into interstitium REAPSORPTION Movement of fluid from interstitium to plasma STARLING FORCES Hydrostatic and osmotic forces that determines direction of flow  STARLING FORCES NFP= Pc – Pif –Πp+ Πif  CAPILLARY HYDROSTATIC PRESSURE Pressure in capillaries that favors the outward fluid movement At arterial end : 30 mm Hg At venous end : 10 mm Hg The gradient favors filtration at arterial end and reabsorption at venous ends Mean capillary pressure is sum of two 17 mmHg  INTERSTITIAL FLUID HYDROSTATIC PRESSUE Slightly negative Promotes filtration Intrapleural space -8mmHg Epidural space -4 to -6mmHg Average pressure in subcutaneous interstitial fluid is -3mmHg Due to pumping of fluid by lymphatic system Rises if the interstitial fluid volume increases e,g edema Positive pressure prevents filtration from capillaries  PLASMA OSMOTIC COLLOID PRESSURE
Osmotic pressure within the capillary is principally determined by plasma proteins that are relatively impermeable. Referred to as the "oncotic“ Tends to favor reabsorption Average value 28mmhg 19mm hg comes from protein 9mm hg by DONNAN effect i.e. Osmotic pressure exerted by sodium potassium and other ions held in plasma by proteins Albumin generates about 70% of the oncotic pressure. Increases along the length of the capillary,. INTERSTITIAL FLUID COLLOID OSMOTIC PRESSSURE
Proteins cannot pass through endothelium into interstitium Only a very small amount leaks through pores 3gm/dl of protein exerts a pressure of 8 mmHg Favors filtration of fluid MOVEMENT OF FLUID AT ARTERIAL END OF CAPILLARY
Pressures moving fluid out of the capillary: -Capillary pressure 30 -Pressure of interstitial fluid 3 -Oncotic pressure of ISF 8 41 Pressures moving fluid into the capillary: -Oncotic pressure of plasma 28 Together 41-28=13 mmHg in direction out of the capillary MOVEMENT OF FLUID AT VENOUS END OF CAPILLARY Pressures going out of the capillary: -Capillary pressure 10 -Pressure of interstitial fluid 3 -Oncotic pressure of ISF 8 21 Pressures going into the capillary: -Oncotic pressure of plasma 28 Together 28-21=7 mmHg in direction into the capillary MOVEMENT OF FLUID THROUGHOUT CAPILLARY
Pressures going out of the capillary: -Mean Capillary pressure 17.3 -Pressure of interstitial fluid 3 -Oncotic pressure of ISF 8 28.3 Pressures going into the capillary: -Oncotic pressure of plasma 28 Together 28-28.3= 0.3 mmHg in direction out of the the capillary  Net Filtration
Reabsorption pressure is considerably less than filtration pressure Venous capillaries are more numerous and more permeable than arterial Capillaries at their venous ends absorb 9/10th of fluid filtered at arterial end Remaining is absorbed by lymphatic Normal rate of net filtration ( 0.3 mm Hg ) in entire body is only 2 ml / minute  LYMPHATIC SYSTEM Provide accessory route through which fluid flow from interstitial space to blood Returning albumin and other interstitial macromolecules to the circulatory system Recovers 200 gm proteins daily Without lymphatic drain we would die in 24 hrs Normal lymph flow 2L/day for entire body  MECHANISM OF LYMPH FLOW Lymphatic fluid is pumped out of the tissues by contraction of large lymph vessels and skeletal muscle Flow towards the heart because of presence of valves in vessels This fluid is then transported to progressively larger lymphatic vessels culminating in the right lymphatic duct (for lymph from the right upper body) and the thoracic duct (for the rest of the body); these ducts drain into the circulatory system at the right and left subclavian veins. Lymphatic System An acces s or y route by which fluid and protein can flow from interstitial spaces to the blood Important in preventing edema Lymph is derived from interstitial fluid that flows into the lymphatics  FUNCTIONS OF THE LYMPHATIC SYSTEM: 1. It carries excess of interstitial fluid from interstitium into the blood. Rate of lymph flow is more than 3 liters/day. So this amount is drained by lym phatic system. 2. It drains proteins and electrolytes from Interstitial space into lymphatic system. Lymphatic system drain 195 grams of blood proteins from interstitium back into the blood. 3. It provides lymphocytes and antibodies into the circulation 4. Removes bacteria and other microorganisims from the tissues. 5. Lacteals are involved in the absorption and transport of lipids. 6. Many large enzymes which are produced in the tissues get entry into the circulation through lymphatic system like histaminases and lipase. 7. It maintains the negative interstitial fluid hydrostatic pressure. Determinants of Lymph Flow: Figure 16-11; Guyton and Hall The degree of activity of the lymphatic pump - smooth muscle filaments in lymph vessel cause them to contract - external compression also contributes to lymphatic pumping  Determinants of Lymph Flow Pressure on Lymphatics from outside: 1.Skeletal Muscle Contraction 2.Movements of different parts of body 3.Presure by objects which come in contact on the outer surface of the body 4.Pulsations of near by arteries  FACTORS CONTROLLING THE FORMATION OF TISSUE FLUID 1. Starlings Forces 2. Capillary permeability 3. Lymphatic obstruction  Causes of Extracellular Edema I. Increased capillary pressure A. Excessive kidney retention of salt and water 1. Acute or chronic kidney failure 2. Mineralocorticoid excess B. High venous pressure and venous constriction 1. Heart failure 2. Venous obstruction 3. Failure of venous pumps (a)Paralysis of muscles (b)Immobilization of parts of the body (c) Failure of venous valves C. Decreased arteriolar resistance 1. Excessive body heat 2. Insufficiency of sympathetic nervous system 3. Vasodilator drugs  Causes of Extracellular Edema II. Decreased plasma proteins A.Loss of proteins in urine (nephrotic syndrome) B.Loss of protein from denuded skin areas 1. Burns 2. Wounds C.Failure to produce proteins 1. Liver disease (e.g., cirrhosis) 2. Serious protein or caloric malnutrition  Causes of Extracellular Edema III. Increased capillary permeability A. Immune reactions that cause release of histamine and other immune products B. Toxins C. Bacterial infections D. Vitamin deficiency, especially vitamin C E. Prolonged ischemia F. Burns IV. Blockage of lymph return A. Cancer B. Infections (e.g., filaria nematodes) C. Surgery D. Congenital absence or abnormality of lymphatic vessels