29. microcirculation, exchange of substances, venus circulation Flashcards Preview

2nd Midterm Essay 17-32 > 29. microcirculation, exchange of substances, venus circulation > Flashcards

Flashcards in 29. microcirculation, exchange of substances, venus circulation Deck (7)
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1
Q

Microcirculation

A
  • The very high number of smaller vessels results in a 600-1000 times higher total cross section area for the total capillary bed than for the aorta.
  • Exchange of materials between blood and ECF is made possible by permeable capillaries:
    • The continuous type of capillaries is most common.
    • Tissues taking part in secretion and resorption display fenestrated capillaries. o Kidneys: porous capillary
    • Liver: capillaries form sinusoids
  • Arteries –> Arterioles –> Metarterioles –> Capillaries
    • At the point of branching of metarterioles into capillaries, precapillary sphincters are found. The majority of the sphincters are closed during rest (5-10% is open).
    • Between arterioles and venules a shunt may be present (arteriovenous anastomosi).
2
Q

Diffusion

A
  • The rate of diffusion depends on the concentration gradient, permeability, and the surface area.
  • Water exchange via filtration/resorption: 0,06ml/min/100g tissue.
  • Water exchange with diffusuion: 300 ml/min/100g tissue
  • Gases and small molecular substances are mostly exchanged by diffusion.
  • Two types of transport by diffusion:
    • Flow limited: For small molecules ot is only the rate of blood flow that limits the transport.
    • Diffusion limited: For large molecules (e.g. polypeptides) it is the rate of diffusion that limits the transport.
  • Diffusion of gases:
    • Partial pressure of the gas drops in both directions: towards the end of the capillary, and towards distant cells. The higher the actual oxygen consumption of the tissue, the higher will be the gradient for oxygen.
    • Cells getting less oxygen release more regulatory signals, and these open more capillaries in the vicinity of these cells. This local autoregulation is a very important way of insuring an even distribution of gases.
3
Q

Exchange of substances by filtration/resorption

A
  • Hydrostatic pressure and oncotic pressure of the blood and of the tissue determine the pressure gradient for the fluids.

Phcap = hydrostatic pressure in the capillary

Point = oncotic pressure in the interstitium

Pocap = oncotic pressure in the capillary

Phint= hydrostatic pressure in interstitium

  • Effective pressures are the differences of the blood and tissue pressures

Pheffective = Phcap - Phint

Poeffective = Pocap - Point

  • The final effective filtration is the difference of he effective hydrostativ and effective oncotic pressures

Peffective = Pheffective - Poeffective

  • The effective filtration pressure shows towards the tissue at the arterial side of the capillary: filtration may occur.
  • On the venous end of the capillary the effective filtration pressure is negative, i.e. it shows towards the lumen of the capillary: resorption may occur.
  • Volume flow, Q:
    • The flow of fluid depends on the effective pressure and on the permeability
    • In rest Qfiltrated is greater than Qresorbed
4
Q

Transport mechanisms

A
  • Formation of ISF:
    • Water, electrolytes and anelectrolites with small molecular weight can permeate the capillary wall without restriction.
    • Only for colloids is the capillary wall a considerable barrier. There is only little transport occurring in the continuous capillaries with the help of specific carrier systems (however, sinusoids in the liver are permeable for proteins).
  • Pathes of transport:
    • Through fenestration
    • Through interendothelial ways
    • In a transcellular way
    • By cytosis (endo-, exo-, pyno)
  • Forces determining transport:
    • Diffusion: It is the most important factor to insure transport of substances. The greatest part of substances crossing the capillary membrane is transported in this way.
    • Osmotic forces: The osmotic effect of the proteins in the solution makes water to move. The water content inside will increase. This so-called oncotic pressure continuously gets the water to move into the intravasal compartments from the ISF.
    • Electric forces: If the movement of some components is restricted between two compartments, the concentration of the diffusible ions will be different in the two compartments after the balance develops:
      • Electroneutrality: The total amount of anions and cations have to be equal in certain compartments
      • Thermodynamic rule: The product of the concentrations of diffusible ions must be equal on both sides of the membrane.
    • Hydrostatic pressure: The hydrostatic pressure coming from blood is not the same at the arterial and the venous “end” of the capillary, that is why the net fluid flow is different. (Itsbiggeronarterialside)
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6
Q

The venus circulation

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  • Function is determined by: structure of the wall and venous valves.
  • The pressure in veins drop from the venules (10 – 15 mmHg) to the Right Atrium (0 -3) continuously.
  • Both, pressure and flow rate change with certain rhytmicity in the veins, due to the valves and the change of tissue pressure (i.e. muscle pump), and gravitation
  • Characteristics of Venous System: capacitance-system (reservoir), 55 – 75 % of circulating blood reside in the veins.
  • In case it is needed, the vasomotor mechanism „redistribute” the blood (toward the resistance segment).
  • Distensibility is large (but the collagen network sets the limit; there are only few elastic elements in the veins).
7
Q

Factors Maintaining Venous Circulation

A
  • The work of the heart
  • Gravitation
  • Venous valves
  • Skeletal muscle pump (rhytmic changes of muscle tension „pumps” the blood, direction of flow is always centripetal due to the valves; in case of weak muscle tension, blood accumulates in the vein, creating retrograd capillary flow – increased pressure – leading to edema!
  • Changing pressure in chest and in abdomen („chest pump”; during inspiration intrathoracal pressure decreases and facilitates the filling of right atrium (RA), while abdominal pressure changes the transmural pressure of veins running through the abdomen)
  • Pressure in the Right Atrium and in the hollow veins is normally about 0 mmHg. Due to the cardiac cycle, however, positive pressure changes can be detected in the right atrium (it is called central venous pressure or central venous pulse, CVP, having 3 waves called: acv waves.