Lecture 18 Capillary bulk flow and lymphatic system PDF

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Taught by Dr. Erikson, almost note for note from his presentation, part of exam 4...

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Capillaries ● Capillaries are microscopic blood vessles that connevt arterioles to venules. Capillaries form extensive branching networks called capillary beds or capillary networks. Each organ is served by its own capillary network. ○ At least 1 capillary is found within 0.1 mm of almost every cell of the animal’s body ○ Cardiovascular system is a closed system, but some fluid leaks out of the capillaries ● Function of capillaries is to allow the exchange of nutrients, gases, and waste between the blood and tissue cells ○ Capillary vessel walls are extremely thin, consisting of single layer of squamous endothelial cells attached to a basement membrane. Thus, chemical substances (O2, CO2, nutrients, urea) moving from the blood into tissues (and vis versa) have to pass through or between just 1 layer of endothelial cells to enter or leave tissues

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Transcapillary exchange: Solutes (and water) in the plasma move into the interstitial fluid by crossing the capillary walls using a process called transcappilary exchange, which is driven by the induvidual concentration gradients for the various solutes. ○ The 2 ECF comparments (plasma and IF) are separated by the endothelium that makes of up the capillary wall, which is an exchange epithelium. The loose tight junction between these cells make the endothelium of most capillaries highly permable to water and small solutes like ions, glucose, amino acids, and gases. However, larger molecules like proteins cannot easily cross the capillary wall. ○ Because of the permibility of the capillary wall, the compostions of the 2 ECF compartments (plasma and IF) are almost identical. The major difference between these 2 ECF comparments is that plasma has a higher concentration of proteins than IF. ■ Plasma proteins (ex. Albumin, some globulins, fibrogen) are synthesized by liver cells and then move into the plasma by crossing the wall of the discontinous capillaries (sinusolds) in the liver. In other tissues, the plasma proteins stay in the plasma because they cannot cross thw all of the continuous capillaries found in most other tissues.

Cappilary filtration (Bulk Flow): Is the mass movement of fluids and solutes between the blood and IF as a result of the hydrostatic and osmotic pressure gradients that exist across the walls of cappilaries. ● This process is referred to as “Bulk Flow” because the various constituents of the ECF (water and most solutes) are moving in bulk in contrast to the discrete diffusion of induvidual ions and molecules (ex. O2, CO2, glucose, various ions) down their concentration gradients.

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Starling Forces ○ Bulk Flow or capillary filtration across the cpillary walls is driven by 4 forces collectively called “Starling Forces” 1. Cappilary hydrostatic pressure (PC) 2. Interstitial Fluid Hydrostatic pressure (PIF) 3. Plasma Colliod Osmotic Pressure (𝛑P) 4. Interstitial Fluid Colliod Osmotic Pressure (𝛑IF)

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Net Exchange Pressure: Across capillary walls at any point in a capillary at any point in a capillary can be determined using the following equation referred to as the Starling Forces 𝛑P) ○ Net Exchange Pressure= (PC+𝛑 𝛑IF)- (PIF+𝛑 ■ (PC+𝛑IF) is the net outward driving pressures ■ (PIF+𝛑P) is the net inward driving pressure Ultrafiltration: Is bulk flow from blood plasma into interstitial fluid and occurs when net exchange pressure is positive Reabsorption: Is bulk flow from interstitial fluid into blood plasma and occurs when net exchange pressure if negative

Hydrostatic Pressure ● Pressure exerted on vessles walls by fluid (blood or interstitial fluid) ○ Net filtration pressure (NFP)= PC-PIF ● Capillary Hydrostatic Pressure (PC) (going out of capillary): is the blood pressure in the capillary bed. It varies depending on arterial pressure venous pressure, precapillary and post capillary resistance ○ In general, PC is highest on the arteriolar end of the capillary bed (37 mm HG) and lowest on the venular end (17 mm Hg). ● Interstitial Fluid Hydrostatic Pressure (PIF) (going in: Is extremely low (about 1 mm Hg) throughout the capillary bed under normal conditions. Colloid Osmotic (Oncotic) Pressure ● Osmotic pressure created by the presence of non-filterable proteins in the plasma ○ Plasma collid osmotic pressure (𝛑P)= 25 mm Hg (going into the blood capillary) ○ Interstitial Collid Pressure (𝛑IF)= o mm Hg (going out of capillary) ■ Other than proteins, most solutes (like ions and small molecules) diffuse easily beween plasma and IF, and therefore do not contribute to colliod osmotic pressure 𝛑P) Net Exchange Pressure= (PC+𝛑 𝛑IF)- (PIF+𝛑 ● Under normal conditions ○ On the ateriolar end of capillary bed ■ Net Exchange Pressure (37+0)- (1+25)= 11 mm Hg ● The positive exchange pressure means that ultrafiltration is occuring on the arteriolar end of the capillary bed. ○ On the venular end of capillary bed ■ Net Exchange Pressure= (17+0)- (1+25)= -9 mm Hg ● The negative net exchange pressure means that reabsorption is occuring on the venular end of the capillary bed.

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In most capillary beds, net ultrfiltration is greater than net reaborption resulting in accumulation of fluid in the interstitial space. On average, ultrafiltration from all capillary beds in humans exceeds reaborption by about 3 liters/day

Summary of Bulk Flow ● On average, about 28% (stays filtered) of the plasma moving through a capillary bed will filter out the arteriolar end of a capillary bed into the interstitial fluid, and 85% (reabsorbed) of  that will be reabsorbed on venular end of the capillary bed. The rest (aproximately 3 liters/day) has to be picked up by the lymphatic system and returned to the circulation. If fluid accumulates in the interstitial space of a tissue, edema results and the tissue swells

Edema:Excessive accumulation of interstitial fluid in a tissue ● Reasons, some common causes ○ High arterial blood pressure ○ Venous obstruction ○ Leakage of plasma proteins into interstitial fluid ○ Loss of plasma proteins in the urine due to leakage of plasma ○ Decreased plasma protein concentration due to severe starvation or liver failure ■ Kwashiorkor: A form of severe protein energy malnutrition observed in malnurished children. One sign of this disease is a severly swollen belly, which is primarily due to abdominal edema (ascites). The edema results from lower concentration ofplasma proteins and the corresponding lower plasma colliod osmotic pressure (𝛑P) ○ Obstruction o lyphatic drainage= lymphedema

Lymphatic system ● 3 general functions 1. Transports excess interstitial (tissue) fluid back into the bloodstream 2. Transports absorbed fats from small instestine to the bloodstream 3. Help provide immunological defenses against pathogens in lymphatic tissues like lymph nodes, tonsils, and spleen ●

Anatomy ○ Found in all tissues except cartilage, bone, epithelium, and CNS tissues

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The smallest vessles of the lymphatic system are the lymphatic capillaries, which are closed ended vessles found widely dispersed in the capillary beds of most tissue ■ Lymphatic capillaries are highly permeable to almost all dissolved chemicals (along with particulates, microbes, and cells) because that are no tights between endothelial cells. ■ Lymphatic capillaries are attaches by fine filaments to surounding connective tissue. When muscles contract, the fine filaments pull on the lymphatic capillaries to open the space between the endothelial cells and permit entry of of fluid, small solutes, large proteins, particles microbes, and cells into the lymph system ■ Once interstitial fluid containing its solutes, microbes, cells and particulates enters the lymphatic capillaries, it is referred to a lypm Lymphatic capillaries merge to form successively larger lyph messles called lymphatics. As, the lymph moes these vessles, it circulates through lymph nodes, which are oval or kindey shaped organs that act as lymph filter to remove foreign patricles like bacteria and viruses and to help orchestrate immune responses to pathogens The lymphatic eventrually merge into the right and left thoracic ducts that drain into the subclavain veins near where these veins connect with the internal jugular vein Lymphatics contain a system of one way valves and movement of lymph is drive by skeletal muscle pumping (milking), lymphatic vessle smooth muscle contrations, and tissue pressure ■ There is no dedicated pump by lymp circulation in mammals. Throughout the course of a day, flow of lymph varies considerably. It is highest during exercise and lowest during periods of inactivity

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○ ○ Lymphatic circulation ● Average lymph flow per day is about equal to the animal’s volume plasma (3 liter/day in humans) ● Lymphedema results from blockage of the lyphatics leading to edema in the most complient tissues (usually subcutaneous) in the area of the body where the blockage occurs

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Elephantiasis is what happens during severe lymphatic blockage. It is cases by the larae of the filariasis worm, which invade the block the lymphatic system causing massive lympedema in extremities and other soft tissues....