CAPILLARY FUNCTIONCAPILLARY FUNCTION

KHAING ZAY AUNGKHAING ZAY AUNG

CAPILLARIES CAPILLARIES

•Capillus – hairCapillus – hair

•Smallest ones of the body’s blood vessels that Smallest ones of the body’s blood vessels that make up the microcirculation make up the microcirculation

•Connects the arterioles and venulesConnects the arterioles and venules

• the endothelial linings are only one cell layer the endothelial linings are only one cell layer thickthick

•Diameter – 5-10 µm in diameterDiameter – 5-10 µm in diameter

•Main function of capillaries is ‘exchange’Main function of capillaries is ‘exchange’•Water, electrolytes, gases, nutrients & waste Water, electrolytes, gases, nutrients & waste productsproducts

DEVELOPMENTDEVELOPMENT

•Development through the process known as Development through the process known as vasculogenesisvasculogenesis

• Formation of de novo production of Formation of de novo production of endothelial cellsendothelial cells

FUNCTIONFUNCTION

• Capillaries do not function as their ownCapillaries do not function as their own

• Instead in the Instead in the capillary bedcapillary bed

• An interweaving network of capillaries supplying An interweaving network of capillaries supplying the organs or tissuesthe organs or tissues

• Two types of vessels – true capillaries & Two types of vessels – true capillaries & metarteriolesmetarterioles

• Total area of capillary walls in the body exceeds Total area of capillary walls in the body exceeds 6300 m26300 m2

• In many beds the endothelial gaps can permit In many beds the endothelial gaps can permit molecules up to 10 nm in diametermolecules up to 10 nm in diameter

• Capillaries & post-capillary venules have pericytes Capillaries & post-capillary venules have pericytes around their endothelial cellsaround their endothelial cells

• Have long processes that wrap around vesselsHave long processes that wrap around vessels

• Contractile & release vasoactive substancesContractile & release vasoactive substances

• Regulation of flow through the junction between Regulation of flow through the junction between endothelial cellsendothelial cells

3 TYPES OF CAPILLARIES3 TYPES OF CAPILLARIES

• Continuous capillaries Continuous capillaries • Skeletal muscle, gonads, skin, and CNS Skeletal muscle, gonads, skin, and CNS

• Fenestrated capillariesFenestrated capillaries• Renal glomerulus, pancreas , intestines & endocrine glands Renal glomerulus, pancreas , intestines & endocrine glands

• Sinusoidal capillaries Sinusoidal capillaries • Bone marrow, lymph nodes, liver & spleen Bone marrow, lymph nodes, liver & spleen

FUNCTIONFUNCTION

• Exchange of water, electrolytes, gases, nutrients Exchange of water, electrolytes, gases, nutrients and waste products across the vesselsand waste products across the vessels

• At one time, only about 5% of the circulating blood At one time, only about 5% of the circulating blood is in the capillariesis in the capillaries

CAPILLARY PRESSURE & FLOWCAPILLARY PRESSURE & FLOW

• Vary considerablyVary considerably

• Typical value in nail bed capillaries (32 & 15 mmHg)Typical value in nail bed capillaries (32 & 15 mmHg)

• Capillaries are short but blood moves slowly Capillaries are short but blood moves slowly (0.07cm/s)(0.07cm/s)

• Average transit time 1-2 sAverage transit time 1-2 s

EQUILIBRATION WITH INTERSTITIAL FLUIDEQUILIBRATION WITH INTERSTITIAL FLUID

• Paracellular transport - transfer of substances Paracellular transport - transfer of substances across the epithelium by passing through the across the epithelium by passing through the intercellular spaces between the cellsintercellular spaces between the cells

• Diffusion Diffusion

• Filtration Filtration

• Transcellular transport – travel through the cell by Transcellular transport – travel through the cell by passing through the apical membrane or basolateral passing through the apical membrane or basolateral membrane membrane

• PinocytosisPinocytosis

• Exocytosis Exocytosis

DIFFUSIONDIFFUSION

• Diffusion is the net movement of substances from a Diffusion is the net movement of substances from a region of high concentration gradient to a region of low region of high concentration gradient to a region of low concentration gradient concentration gradient

• Substances diffuses via the capillaries are mostly gases Substances diffuses via the capillaries are mostly gases and water and water

• For oxygen – from plasma to tissues For oxygen – from plasma to tissues

• For carbondioxide – from tissues to plasmaFor carbondioxide – from tissues to plasma

FILTRATION FILTRATION

• Filtration is the process by which fluid is forced Filtration is the process by which fluid is forced through a membrane or other barrier because of through a membrane or other barrier because of difference in pressure of two sidesdifference in pressure of two sides

• Mainly in glomerula capillaries Mainly in glomerula capillaries

• Sodium and potassium ions pass readily however Sodium and potassium ions pass readily however plasma proteins and glucose do not.plasma proteins and glucose do not.

SCHEME OF FILTRATION BARRIER (BLOOD-URINE) IN THE KIDNEY.SCHEME OF FILTRATION BARRIER (BLOOD-URINE) IN THE KIDNEY. A. THE ENDOTHELIAL CELLS OF THE GLOMERULUS; 1. PORE (FENESTRA). A. THE ENDOTHELIAL CELLS OF THE GLOMERULUS; 1. PORE (FENESTRA).B. GLOMERULAR BASEMENT MEMBRANE: 1. LAMINA RARA INTERNA 2. LAMINA B. GLOMERULAR BASEMENT MEMBRANE: 1. LAMINA RARA INTERNA 2. LAMINA DENSA 3. LAMINA RARA EXTERNADENSA 3. LAMINA RARA EXTERNAC. PODOCYTES: 1. ENZYMATIC AND STRUCTURAL PROTEIN 2. FILTRATION C. PODOCYTES: 1. ENZYMATIC AND STRUCTURAL PROTEIN 2. FILTRATION SLIT 3. DIAPHRAGMASLIT 3. DIAPHRAGMA

• Exocytosis Exocytosis

• Endocytosis Endocytosis

• The rate of filtration at any point along a capillary The rate of filtration at any point along a capillary depends on a balance of forces k/a depends on a balance of forces k/a

““Starling’s Forces”Starling’s Forces”

• Comprises the hydrostatic pressure gradient and Comprises the hydrostatic pressure gradient and oncotic pressure gradientoncotic pressure gradient

Fluid movement = k [(Pc − Pi) − (Fluid movement = k [(Pc − Pi) − (ππc − c − ππi)]i)]

wherewhere

•k = capillary filtration coefficientk = capillary filtration coefficient

•P c = capillary hydrostatic pressureP c = capillary hydrostatic pressure

•P i = interstitial hydrostatic pressureP i = interstitial hydrostatic pressure

•π c = capillary colloid osmotic pressureπ c = capillary colloid osmotic pressure

•π i = interstitial colloid osmotic pressureπ i = interstitial colloid osmotic pressure

• 24 litres of fluid is filtered thr’ capillaries each day24 litres of fluid is filtered thr’ capillaries each day

• About 0.3% of cardiac outputAbout 0.3% of cardiac output

• 85% has reabsorbed85% has reabsorbed

• The remainder are reabsorbed by lymphaticsThe remainder are reabsorbed by lymphatics

Location Pc (mmHg)[3] Pi (mmHg)[3] σπc (mmHg)[3] σπi (mmHg)[3]

arteriolar end of capillary

+35 −2 +28 +0.1

venular end of capillary

+15 −2 +28 +3

• Flow-limited exchangeFlow-limited exchange• Total diffusion can be increased by increasing the flow rateTotal diffusion can be increased by increasing the flow rate

• Diffusion-limited exchange Diffusion-limited exchange • Transfer of substances that do not reach the equilibrium with the Transfer of substances that do not reach the equilibrium with the

tissues during their passage through the capillaries tissues during their passage through the capillaries

• Capillaries that form part of the blood–brain barrier Capillaries that form part of the blood–brain barrier however only allow for transcellular transport as however only allow for transcellular transport as tight junctions between endothelial cells seal the tight junctions between endothelial cells seal the paracellular space.paracellular space.

• tight junctions have a net negative charge, and are tight junctions have a net negative charge, and are believed to preferentially transport positively-believed to preferentially transport positively-charged moleculescharged molecules

• also known to be size-selectivealso known to be size-selective

TIGHT JUNCTIONSTIGHT JUNCTIONS

• Also k/a zonula occludens Also k/a zonula occludens

• Surrounds the apical margins of the cells in epithelia Surrounds the apical margins of the cells in epithelia s/a s/a intestinal mucosa, walls of renal tubules and intestinal mucosa, walls of renal tubules and choroid plexus choroid plexus

• Important to endothelial barrier functionImportant to endothelial barrier function

GAP JUNCTIONS GAP JUNCTIONS

• Forms a cytoplasmic tunnel for diffusion of small Forms a cytoplasmic tunnel for diffusion of small molecules (<1000 Da)molecules (<1000 Da)

• Made up of protein units called connexons Made up of protein units called connexons

• 25 – 3 nm25 – 3 nm