29. microcirculation, exchange of substances, venus circulation

Question 1

Microcirculation

Answer 1

• 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).

Question 2

Diffusion

Answer 2

• 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.

Question 3

Exchange of substances by filtration/resorption

Answer 3

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

Ph_cap = hydrostatic pressure in the capillary

Po_int = oncotic pressure in the interstitium

Po_cap = oncotic pressure in the capillary

Ph_^int= hydrostatic pressure in interstitium

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

Ph_effective = Ph_cap - Ph_int

Po_effective = Po_cap - Po_int

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

P_effective = Ph_effective - Po_effective

• 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 Q_filtrated is greater than Q_resorbed

Question 4

Transport mechanisms

Answer 4

• 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)

Question 6

The venus circulation

Answer 6

• 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).

Question 7

Factors Maintaining Venous Circulation

Answer 7

• 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.