Cardiovascular Physiology (Microcirculation) - Summer 2024 PDF

CARDIOVASCULAR PHYSIOLOGY 7. Microcirculation Andre Azevedo, DVM, MSc Assistant Professor of Veterinary Physiology [email protected]  I mostoxygena...

CARDIOVASCULAR PHYSIOLOGY 7. Microcirculation Andre Azevedo, DVM, MSc Assistant Professor of Veterinary Physiology [email protected]  I mostoxygenatedtoless Structure of the microcirculation ifÉÉÉ ines akaterminalarterio IIIaries The microcirculation of each organ is organized to serve the organ’s specific needs Each artery entering an organ branch 6-8 times before becoming small enough to be called arterioles Arterioles branch 2-5 times until they become the TERMINAL ARTERIOLES that supply blood to capillaries Terminal arterioles are called METARTERIOLES  2 Structure of the microcirculation TIF meta coat havecontinuousm uscular arterioles METARTERIOLES do not have a continuous muscular coat Smooth muscle fibers encircle the vessel at intermittent points At the point where each true capillary originates from a metarteriole, a smooth muscle fiber usually encircles the capillary PRECAPILLARY SPHINCTER Open or close the entrance of the capillary of perfusedpassage blood Not all capillaries are always perfused with blood It depends on metabolic needs why  3 Structure of the microcirculation weaker stronger bloodvessel what largersmaller VENULES are larger than arterioles and have a much weaker muscular coat The pressure is lower and allows constriction despite the weak muscle  4 Structure of the microcirculation The CAPILLARY WALL is composed of a single layer of endothelial cells and a thin basement membrane  5 Structure of the microcirculation Special types of PORES occur in the capillaries of certain organs BRAIN – space between endothelial cells has tight junctions that allow only extremely small molecules to pass (BBB) LIVER – In sinusoidal capillaries, the clefts between cells are wide open to allow almost all dissolved substances of the plasma to pass from the blood into the liver KIDNEY – numerous small oval fenestrations penetrate all the way through the middle of the endothelial cells. Large amounts of small molecules and ionic substances can filter through the glomeruli without having to pass through the clefts between endothelial cells  6 Capillary blood flow Blood does not flow continuously through the capillaries bic on depends demand It is an intermittent flow that turns on and off every few seconds or minutes VASOMOTION = intermittent contraction of metarterioles and precapillary sphincters The degree of constriction or relaxation markedly affects blood flow Also determines the TOTAL PERIPHERAL RESISTANCE Metarterioles serve as a vascular shunt when precapillary sphincters are closed (thoroughfare channel)  7 Capillary blood flow O2 is the most important factor affecting the degree of opening and closing of the metarterioles/precapillary sphincters When the rate of oxygen usage by the tissue is great (O2 decreases) the blood perfusion of capillaries increases d epend local mostly on factors oxygenis impo most Factor  8 Capillary diffusion DIFFUSION is the most important way by which substances are transferred between the plasma and interstitial fluid Recall: lipid-soluble substances cross the capillary wall by dissolving in and diffusing across the endothelial membrane Recall: water-soluble substances (ex: ions) cross capillary wall through water-filled clefts or large pores (fenestrated capillaries)  9 with.fi ueiiesietrateofdiffusionthroughmembrane Capillary diffusion Recall that the concentration difference influences the net rate of diffusion through the membrane The net rate of diffusion is proportional to the concentration difference of the substance between the 2 sides of the membrane Large quantities of O2 usually move from blood toward the tissues CO2 moves into the blood and is carried away from the tissues  whattwo things theinterstitialFluid form Capillary fluid exchange The filtration and diffusion from the capillaries form the INTERSTITIAL FLUID INTERSTITIAL FLUID = Fluid present in the INTERSTITIUM (spaces between cells) About 1/6 of the total volume of the body consists of the interstitium Interstitium is made up of collagen fiber bundles and proteoglycan filaments Provide most of the tensional strength of the tissues Interstitial fluid is usually entrapped within proteoglycans filaments forming a “tissue gel” Small amounts of fluid encountered as a “free fluid” EDEMA = large amounts of free interstitial fluid  is of dueto rememberResorption process losingasubstance it else by absorbed something thatsubstancebeing Capillary fluid exchange hinttorememberresorptionvsreabsorption if reabsorption losesletters aand b resorptiondescribesloss The fluid filtration across capillaries is determined by hydrostatic and oncotic pressures (Starling forces) FILTRATION = movement of water from capillary plasma into the interstitial fluid REABSORPTION = movement of water into a capillary of absorbed being process something repeatedly FILTRATION REABSORPTION  12 Capillary fluid exchange CAPILLARY HYDROSTATIC PRESSURE (Pc) Pressure that drives fluid out of the capillaries into the interstitial space – FAVORS FILTRATION It is highest at the arteriolar end of the capillary and low at the venular end Equal to blood pressure! Capillary arterial pressure = 35 mmHg Hydrostatic pressure is the pressure that any fluid exerts Capillary venous pressure = 17 mmHg in a confined space INTERSTITIAL FLUID HYDROSTATIC PRESSURE (Pif) Pressure that drives fluid out of the interstitial space into the capillaries Opposes filtration It is typically near zero or slightly negative FILTRATION REABSORPTION  13 mostproteinscarry chargesomore generally p roteinsm more wouldattract ions Nat inthis case soitattractinnat.itzofollowsbioit.iohas Natattra strong Capillary fluid exchange Each gram of Albumin molecules This creates a strong CAPILLARY ONCOTIC PRESSURE (πp) albumin can carry many charged attraction of Na ions. Water hold 18 ml of amino acid residues is pulled into the vasculature water within the (net charge of -17 at a because of the sodium Also called COLLOID OSMOTIC PRESSURE normal pH) attraction intravascular space Pressure exerted by plasma proteins (80% by albumin) NORMALLY FAVORS REABSORPTION Arterial osmotic pressure = 26 mmHg Venous osmotic pressure = 26 mmHg INTERSTITIAL FLUID ONCOTIC PRESSURE (πif) FILTRATION REABSORPTION Force favoring filtration DETERMINED BY THE INTERSTITIAL FLUID PROTEIN CONCENTRATION Normally quite low because there is little loss of protein in the interstitial fluid  14 Capillary fluid exchange FLUIDS: HYDROSTATIC PRESSURE – FLUID ONCOTIC PRESSURE – PROTEIN BLOOD PUSHES FLUID INTERSTITIAL FLUID PULLS FLUID OUT OF THE COMPARTMENT INTO THE COMPARTMENT COMPARTMENTS: CAPILLARY LUMEN INTERSTITIUM Arterial side Venous side remember Pislower Capillary OXIGENATED BLOOD DEOXIGENATED BLOOD Capillary hydrostatic Capillary Oncotic Capillary hydrostatic Capillary Oncotic pressure Pressure pressure Pressure NET FILTRATION PRESSURE 0 1 0 1 17 NET FILTRATION PRESSURE ARTERIAL SIDE 35 Interstitial VENOUS SIDE 26 Interstitial Oncotic Interstitial Interstitial 26 Hydorstatic Pressure Hydorstatic Oncotic (35 + 1) – (26 + 0) = 10 Pressure Pressure Pressure (17 + 1) – (26 + 0) = -8 FAVORS FILTRATION Interstitium FAVORS REABSORPTION  15 The hydrostatic pressure (blood pressure) pushes fluid out of the capillaries The osmotic pressure (proteins) pulls fluid inside of the capillaries If the result of all these forces (net filtration pressure - NFP) is positive, there will be a net fluid filtration across the capillaries If the result of the starling forces is negative, there will be a net fluid The difference between pressures at The difference between pressures at the arterial end (high BP) of the the venous end (low BP) of the reabsorption from the interstitial capillary moves water out capillary moves water in fluid into the capillaries Vídeo: https://www.youtube.com/watch?v=VKkrFvZqpag  Capillary fluid exchange 16 PERMEABILITY OF THE CAPILLARY WALL also influences the fluid movement in the capillaries HYDRAULIC CONDUCTANCE (Kf) = the water permeability of the capillary wall Varies among different types of tissues Size of the cleft between cells Whether the capillaries are fenestrated Permeability is increased in capillary injury Ex: toxins, burns, inflammation Increase permeability to water  17 Capillary fluid exchange The amount of fluid filtering outward from the arterial ends of capillaries equals almost exactly the fluid returned to the circulation by absorption The slight desequilibrium that does occur (favoring filtration) accounts for the fluid that is eventually returned to the circulation by way of the lymphatics 17 LITERS PER DAY REABSORBED The remaining fluid returns to 20 LITERS PER the circulation through DAY FILTRATED lymphatic capillaries  18 Lymphatic system The lymphatic system is an accessory route through which fluid can flow from the interstitial spaces into the blood Lymphatics carry proteins and large particle matter away from the tissue spaces Neither of each can be removed by absorption directly into the blood capillaries Essential for life! Lymphatic capillaries lie in the interstitial fluid close to the vascular capillaries The lymphatic system plays a key role in controlling interstitial fluid protein concentration, volume and pressure  19 Lymphatic system The lymphatic capillaries possess one-way flap valves which permit interstitial fluid and protein to enter but not to leave Almost all tissues of the body have special lymph channels that drain excess fluid directly from the interstitial spaces Lymphatic capillaries merge into larger lymphatic vessels/thoracic duct Lymph flow depends on contraction of the smooth muscle in the lymph vessels and compression of the lymph vessels by activity of the surrounding skeletal muscle  20 Lymphatic system The lymphatic system has another functions: Fluid balance Fat transport Absorbs fats from the intestines and transports to the blood stream Immune system Transports foreing material to lymph nodes Involved in development of immune cells  H the Clinical application: edema and effusion Changes in Starling forces can influence the direction and magnitude of fluid movement 1. EDEMA It is an increase in interstitial fluid volume (swelling) 2. EFFUSION It is an abnormal collection of fluid in a hollow space or between tissues (Ex: pleural effusion)  22 Clinical application: edema and effusion THE BLOOD INCREASE IN CONGESTIVE CANNOT BE “ACCUMULATES” CAPILLARY HEART FAILURE PROPERLY BACKWARDS HYDROSTATIC PUMPED PRESSURE FLUID MOVES OUT OF THE VESSEL CAUSING EDEMA OR EFFUSION ABDOMINAL https://heartsmart.vet.tufts.edu/treatment PULMONARY PERIPHERAL EFFUSION -of-congestive-heart-failure/ EDEMA EDEMA (ASCITES) Constanzo. Physiology 4th 2009  23 Clinical application: edema and effusion PROTEIN DECREASE IN LOSING DECREASE IN PLASMA CAPILLARY ENTEROPATHY PROTEINS CONCENTRATION ONCOTIC PRESSURE FLUID MOVES OUT OF THE VESSEL CAUSING EDEMA OR EFFUSION Dr. Vieira Constanzo. Physiology 4th 2009  24 Clinical application: edema and effusion INCREASE IN LOCAL RELEASE OF VASOACTIVE PERMEABILITY OF INFLAMMATION MEDIATORS THE CAPILLARY (Ex: histamine and cytokines) WALL FLUID MOVES OUT OF THE VESSEL CAUSING EDEMA OR EFFUSION https://davidbessler.wordpress.com/2006/05/19/so-put- Credit: Cassidy Mahoney, Red Semester on-a-happy-face/ Class 2026 Snake bite Ari ate a Bee Constanzo. Physiology 4th 2009  25 Clinical application: edema and effusion MASS EFFECT IMPARING FLUID FROM CAUSING THE NORMAL LYMPHATIC FILTRATION EDEMA OR LYMPHOMA DRAINAGE OF FLUID ACCUMULATES EFFUSION FROM THE TISSUE https://www.vet.purdue.edu/pcop/canine-lymphoma-research.php Constanzo. Physiology 4th 2009  FYI Clinical correlation – Head trauma Mean Intracranial Cerebral arterial perfusion pressure pressure pressure CPP = MAP - ICP Hypoxia ICP CPP Caused by Ischemia brain edema