Capillary Fluid Exchange-Edema PDF

Capillary Fluid Exchange-Edema Faize Elif Bahadır (PhD) Bahcesehir University School of Medicine Department of Physiology Upon completion of this lecture, the students should be able to Describe how the capillaries are the sites for the exchange of water and solutes between...

Capillary Fluid Exchange-Edema Faize Elif Bahadır (PhD) Bahcesehir University School of Medicine Department of Physiology Upon completion of this lecture, the students should be able to Describe how the capillaries are the sites for the exchange of water and solutes between the bloodstream and the interstitial fluid Describe how is pulsatile blood flow in large arteries converted into steady flow in the microcirculation Explain mechanisms of capillary exchange, the hydrostatic and osmotic factors that underlie Starling’s hypothesis for capillary function Explain the contribution of each physiological changes alter the normal balance of Starling forces and increase the filtration of water out of capillaries Describe the mechanism of edema formation in different clinic situations Classify the general causes of oedema The microcirculation is defined as the circulation of blood through the smallest vessels of the body: arterioles, capillaries, and venules Arterioles give rise directly to capillaries or, in some tissues, to metarterioles, which then give rise to capillaries Metarterioles can bypass the capillary bed and connect to venules, or they can connect directly to the capillary bed The capillaries form an interconnecting network of tubes with an average length of 0.5 to 1 mm. microcirculation In metabolically active organs, such as the heart, skeletal muscle, and glands, capillary density is high Blood flow in capillaries depends chiefly on the contractile state of arterioles The average velocity of blood flow in capillaries is ~ 1 mm/s it can vary from 0 to several mm/s. in the same vessel within a brief period These changes in capillary blood flow may be rhythmic►is caused by contraction and relaxation (vasomotion) of the precapillary vessels Capillaries fall into three groups, based on their degree of leakiness Continuous capillary► is the most common form with interendothelial junctions 10 to 15 nm wide (skeletal muscle)  Fenestrated capillary►In these capillaries, the endothelial cells are thin and perforated with fenestrations These capillaries most often surround epithelia (small intestine, exocrine glands) Discontinuous capillary► In addition to fenestrae, these capillaries have large gaps. Discontinuous capillaries are found in sinusoids (liver)  Small molecules like gases, lipids, and lipid-soluble substances directly diffuse through capillary wall endothelial cell membranes Glucose, amino acids, and ions, including sodium, potassium, calcium, and chloride, use transporters for facilitated diffusion via membrane-specific channels Glucose, ions, and bigger molecules may also pass through intercellular clefts Larger molecules can transit through fenestrated capillary pores, and sizeable plasma proteins can go through the large gaps in the sinusoids Some large proteins in the blood plasma can enter and exit the endothelial cells in vesicles via endocytosis and exocytosis Water moves by the process of osmosis  Bulk flow, more efficient than mere diffusion, drives fluids into and out of capillary beds This movement involves two pressure-induced mechanisms: Filtration► where fluid moves from a higher-pressure area in a capillary bed to a lower-pressure area in the tissues Reabsorption► where fluid moves from a high-pressure area in the tissues into a low-pressure area in the capillaries Both these mechanisms involve the interaction of hydrostatic and osmotic pressures Hydrostatic pressure, defined as the pressure of any fluid in an enclosed space, is the primary force driving fluid transport between capillaries and tissues When fluid exits a capillary and enters tissues, the hydrostatic pressure in the interstitial fluid increases Osmotic pressure drives reabsorption—the movement of fluid from the interstitial fluid back into the capillaries While hydrostatic pressure forces fluid out of the capillary, osmotic pressure draws it back in  The movement of water into a capillary is called reabsorption The movement of water in the opposite direction, from the capillary plasma into the interstitial fluid, is called filtration  The flux ought to be proportional to the magnitude of the The amount of solute concentration difference across the capillary wall and that it that crosses a ought to be bigger in leakier capillaries particular surface area The cerebral vessels have unique characteristics that of a capillary per unit constitute the basis of the blood-brain barrier►don’t time is called a flux allow to pass easily These ideas are embodied in a form of Fick's law The passive movement of a small solute (X) across any surface can be described by Fick's law: Flux of solute (moles · cm −2 · s −1 )► D is the diffusion coefficient in cm 2 · s −1 , and ∂[X]/∂ z is the concentration gradient of X ( units: moles · cm −3 · cm −1 ) along the z axis.  The plasma proteins consist of albumin, globulin, and fibrinogen fractions and most capillary walls are relatively impermeable to these, so they exert an osmotic force across the capillary wall that pulls water into the blood  The capillary wall constitutes a semipermeable membrane Water can readily pass through capillary pores, but the pores in continuous capillaries are too small to permit the passage of plasma proteins The concentration of plasma proteins is typically 7 (g/dL) within the capillary plasma but only 0.2-2 g/dL in the interstitial fluid These dissimilar protein concentrations create an osmotic imbalance As a consequence, water molecules tend to move by osmosis from the interstitial fluid into the capillary blood plasma  Most of the other plasma proteins are synthesized in the liver Synthesis plays an important role in the maintenance of albumin levels Plasma protein levels are maintained during starvation until body protein stores are markedly depleted However, in prolonged starvation and in malabsorption syndromes, plasma protein levels are low (hypoproteinemia) They are also low in liver disease, and in nephrosis, because large amounts of albumin are lost in the urine Starling forces► Hydrostatic and Colloid The capillary hydrostatic pressure (Pc)► Osmotic Forces which tends to force fluid outward through the Determine Fluid capillary membrane Movement Through the The interstitial fluid hydrostatic pressure (Pif) Capillary Membrane ► which tends to force fluid inward through the capillary membrane when Pif is positive but outward when Pif is negative The capillary plasma colloid osmotic pressure (Πp) ► which tends to cause osmosis of fluid inward through the capillary membrane The interstitial fluid colloid osmotic pressure (Πif)►which tends to cause osmosis of fluid outward through the capillary membrane  The normal osmotic pressure created by the proteins in the plasma is 25 mm Hg ►it is also called plasma oncotic pressure or colloid osmotic pressure However, because changes in hydrostatic pressure versus plasma oncotic pressure, there is a small, net filtration of water out of the capillaries This water would simply accumulate in the interstitial spaces and cause swelling there (edema formation) if not for the lymph vessels, which collect excess interstitial fluid Interstitial Fluid About 1/6 of the total volume of the body consists of spaces between cells►interstitial fluid It contains almost the same constituents as plasma except for much lower concentrations of proteins The interstitium contains two major types of solid structures: collagen fiber bundles and proteoglycan filaments which composed of about 98% hyaluronic acid and 2% protein  Analysis of the Forces Causing Filtration at the Arterial End of the Capillary The approximate average forces operative at the arterial end of the capillary that cause movement through the capillary membrane are shown as follows► 28 Guyton  Analysis of Reabsorption at the Venous End of the Capillary The low blood pressure at the venous end of the capillary changes the balance of forces in favor of absorption as follows: Guyton Chapter 16, 193-204  Intracellular Edema► fluid accumulation inside the cell Extracellular Edema ►outside the cell Edema formation►can be due to different factors including ✓ increased vascular hydrostatic pressure ✓ decreased plasma osmotic pressure ✓ lymphatic obstruction ✓ inflammation Intracellular Edema Hyponatremia Depression of the metabolic systems of the tissues If the blood flow becomes too low to maintain normal tissue metabolism, the cell membrane ionic pumps become depressed → sodium ions that normally leak into the interior of the cell can no longer be pumped out of the cells and the excess intracellular sodium ions cause osmosis of water into the cells Lack of adequate nutrition to the cells Inflammation usually increases cell membrane permeability → allows sodium and other ions to diffuse into the interior of the cell, with subsequent osmosis of water into the cells Extracellular edema Abnormal leakage of fluid from the plasma to the interstitial spaces across the capillaries Failure of the lymphatics to return fluid from the interstitium back into the blood, often called lymphedema Decreased plasma osmotic pressure Decreased production of albumin by the liver (in cirrhosis or other forms of generalized liver damage) A decreased level of albumin results in edema through decreased plasma osmotic pressure Also, the decreased intravascular volume that accompanies edema stimulates an elevated level of aldosterone which can promotes sodium and water retention However, because the patient with cirrhosis is hypoalbuminemic, the retained water enters the interstitial space, further contributing to the formation of edema Increased vascular hydrostatic pressure Heart failure: blood backs up in the veins due to reduced cardiac output, increasing venous pressure and causing systemic or pulmonary edema Cirrhosis: Fibrous scarring of the liver that impairs return of blood through the portal vein, thereby increasing venous pressure in portal vein tributaries and causing fluid to leak into the peritoneal cavity Venous obstruction: For example, a tumor pushing on a vein will cause back up of blood, eventually with leakage of fluid into the interstitium Prolonged Standing or Sitting: Gravity increases venous pressure in the lower limbs when standing or sitting for extended periods, leading to localized edema  In heart failure, the arterial pressure tends to fall, causing decreased excretion of salt and water by the kidneys, which causes still more edema Also, blood flow to the kidneys is reduced in persons with heart failure, and this reduced blood flow stimulates secretion of renin, causing increased formation of angiotensin II and aldosterone, which both cause additional salt and water retention by the kidneys  In advanced heart failure, increased secretion of ADH stimulates water reabsorption by the renal tubules, leading to hyponatremia as well as intracellular and extracellular edema Thus, in persons with untreated heart failure, all these factors acting together can cause serious generalized edema /doi.org/10.3389/fcvm.2022.933215 During kidney diseases, urinary excretion of salt and water is disturbed and large amounts of sodium chloride and water are added to the extracellular fluid Most of this salt and water leaks from the blood into the interstitial spaces, but some remains in the blood  The pulmonary vascular pressures, including pulmonary capillary pressure, rise far above normal, causing serious and life-threatening pulmonary edema When untreated, fluid accumulation in the lungs can rapidly progress, causing death within a few hours Thank you…