CVS: Capillary Structure and Function, and Solute and Fluid Movement Flashcards
What does metabolism create a need for?
Transport of solutes + fluids
To do this, solutes + fluids must move across cell membranes which often acts as barriers
What controls the rate of solute transport?
- Properties of passive and active transport across membranes
- Fick’s law
- Properties of capillaries
Together, these form the concept of permeability - Allowing solutes/fluids to cross capillary membranes
Describe passive transport
- Movement molecules down conc/pressure/osmotic gradient
- DOESN’t need energy
- Simple (O2/CO2) or facilitated (ions,glucose)
Describe active transport
- Movement molecules against conc gradient
- NEEDS energy (ATP)
- e.g. ATP-dependent pumps, endocytosis, exocytosis
Describe the 4 passive transport processes
- Diffusion
- Concentration gradient -e.g. O2 uptake from lungs into blood
- Regulated by distance, time taken ∝ distance squared
- Fast over micro m and slow >1 mm
- Convection
- Pressure gradient -e.g. Circulation
- Requires pressure gradient, functioning heart - appropriate CO etc.
- Osmosis
- Osmotic pressure (water) gradient - e.g. water uptake by cells
- Requires balance of filtration, reabsorption and functioning lymphatics
- Electrochemical flux
- Electrical and concentration gradient - e.g. Ion flow during AP
- Requires - Active/other transport mechanisms to create electrochemical gradients and ion channels to provide ion movement across membranes
What does Fick’s law describe?
Properties of solutes and membranes affecting transport
Solute movement - Mass per unit time, m/t (Js)
Determined by 4 factors:
Js = - D A (deltaC/x)
D = Diffusion coefficient of solute – how easy it moves through solvent
A = Area
**DeltaC / x = Concentration gradient (C1-C2) across distance
x, negative value : flowing ‘down’ a concentration gradient**
What controls diffusion rate?
Linked to Fick’s law:
- Increased blood flow
- Increases concentration solutes transported to capillaries
- Fall in intracellular concentration (more solute used, metabolism)
- Increase concentration difference (greater concentration gradient)
- Recruitment of capillaries
- Dilation of arterioles - ⬆️number capillaries perfused
- Increases total SA for diffusion (Fick’s law)
- Shortens diffusion distance (faster diffusion)
At what vessel does most solute and fluid movement occur? Describe it
Capilaries:
- Smallest diameter BV
- Extension of inner lining of arterioles
- Endothelium only 1 cell thick
- Semi-permeable
- Vessels that connect arterioles to venules
- Found near every cell in body but higher density in highly active tissue
- Solute movement (due to passive/active transport and filtration), e.g. O2, glucose, amino acids, hormones, drugs
- Fluid movement (due to pressure gradients, osmotic pressure), e.g. regulation of plasma, interstitial, intracellular fluid
Describe the properties and function of continuous capillaries
- Moderate permeability
- Tight gaps b/w neighbouring cells
- Constant basement membrane e.g. blood-brain barrier
Describe the properties and function of fenestrated capillaries
- High water permeability
- Fenestration structures
- Modest disruption of basement membrane
- E.g. ‘high water turnover’ tissues such as salivary glands, kidney, synovial joints, choroid plexus (cerebrospinal fluid)
Describe the properties and function of discontinuous capillaries
- Very large fenestration structures
- Disrupted basement membrane
- E.g. when movement of cells is requires such as RBCs in liver, spleen, bone marrow
How does permeability change as you go from continuous → fenestrated → discontinuous capillaries?
Increasing permeability to solutes + fluids
What are the general properties of all capillaries that can influence solute transfer?
- Intracellular cleft - 10-20nm wide
- Glycocalyx - Covers endothelium charged material, acts as sieve for solute permeation
- Caveolae + vesicles - Movement of large molecules, e.g. plasma proteins, lipoproteins
What are the different routes of solute transport?
- Big gaps in inflammation
- Trans-cellular channels
- Vesicles
- Trans-cellular
- Inter-cellular
- Fenestral route
- Water channels
What is the dominant route of solute transport?
- Diffusion, for example, filtration only accounts for 2% glucose transport
How does fluid move at the capillary wall?
- Capillary wall semi-permeable - allows H2O pass through
- Fluid moves across membrane into interstitial space due to capillary BP
- Large molecules (e.g. plasma proteins) can’t pass, instead they exert osmotic pressure termed oncotic pressure
- Oncotic pressure creates suction force to move fluid from interstitial space into capillary
- Fluid movement depends on balance b/w capillary BP and oncotic pressure across capillary wall - revised Starling’s principle of fluid exchange
Describe the revised Starling’s principle of fluid exchange
The Starling Principle states that fluid movements between blood and tissues are determined by differences in hydrostatic and colloid osmotic (oncotic) pressures between plasma inside microvessels and fluid outside them
Jv = Lp A { ( Pc - Pi ) - sigma (pp - pg) }
Jv (net filtration) µ Blood/fluid pressures difference (Pc - Pi )
- Osmotic pressure difference (pp - pg)
LP - Conductance of endothelium
A - Endothelium plasma membrane area
Sigma - Reflection coefficient - related to intracellular gaps
Fraction (sigma) of osmotic pressure is exerted by gaps
Effective osmotic pressure = Sigma x potential osmotic pressure
Sigma for plasma protein = 1 so no conduction across
Sigma for plasma protein = 0 so free conduction across
Plasma proteins move from lumen into interstitial space via vesicle system, NOT via intercellular spaces as glycocalyx acts as barrier
Stream of fluid filtration into interstitial space carries plasma proteins away from endothelium into pii, creating low pg (subglycocalyx region)
Pp = Pi
Hence, osmotic gradient is Pp - Pg
What does Revised Starling’s principle mean?
Revised Starling principle - Balance of pressures cannot halt fluid exchange because microvessels are permeable to macromolecules
- Less Pc than at arterial end
- Means plasma proteins diffuse into subglycocalyx region
- Pii - PiG which are also same as PiP
- Filtration continues at venous end as even though Pc is low it is still greater than osmotic gradient to absorb
- Filtration occurs across length of capillaries
Describe the role of lymphatic circulation in constant filtration
- Lymphatic circulation returns excess tissue fluid/solutes back to cardio-vascular system
- Lymph vessels have valves and smooth muscle
- Spontaneous contractions of smooth muscle contributes to lymph flow
- Surrounding skeletal muscle contractions/relaxation also contributes to lymph flow
What happens if Pc becomes very low?
Hypovolemia:
- Drop in blood volume leads to drop in CO and BP, therefore Pc is reduces
- Pip becomes dominant
What does ECF balance depend on?
- Capillary filtration
- Capillary reabsorption
- Lymphatic system
What occurs when the balance between filtration, reabsorption and lymphatic function is not maintained?
Oedema - Excessive fluid in interstitial space
Why does increased capillary pressure lead to oedema?
Increased Pc across capillaries (gravitational from standing too long, cardiac failure, DVT) causes increased flitration
Why does decrease osmotic pressure lead to oedema?
Decreased osmotic pressure can be caused by low protein oedema or malnutrition/malabsorption, hepatic failure, nephrotic syndrome (this is protein loss in urine that is much greater than the amount of protein being produced by the liver)
Liver disease - not enough albumin being made
- Reduced plasma protein concn
- Reduce plasma oncotic pressure, greater influence of Pc
- Excessive fluid filtration from capillaries into interstitial fluid
- Oedema
Describe inflammatory-mediated oedema
- Swelling triggered by local chemical mediators of inflammation
- Increase capillary permeability - become ‘leaky’
What are problems caused by lymphatic dysfunction?
Lymphatic obstruction:
- Filariasis/elephantitis - nematode infestation, larvae migrate to lymphatic system, grow/mate/form nests - block lymphatic drainage
Lymphatic removal:
- Lymphoedema - caused by surgery, removal of lymphatics
Continued filtration leads to build up of fluid in interstitial space
