Topic 29 - Microcirculation, exchange of substances, venous circulation

Question 1

Words to inculde

Answer 1

• Microcirculation
• Capillaries
  
  • Continous type of capillaries
  • Fenestrated capillaries
    
    • Tissues
      
      • Secretion
      • Resorption
  • Porous capillaries
    
    • Kidney
  • Capillaries form sinusoids
    
    • Liver
• Arteries arterioles metarterioles capillarie
  
  • Precapillary sphicters
• Shunt
  
  • Arterioles
  • Venules
  • Arteriovenous anastamosis
• Substance exchange
• Diffusion
  
  • Gas exchange
  • Ion exchange
  • Small substances
  • Flow limited
  • Diffusion limited
  • Concentration gradient
  • Permability
  • Surface of the capillaries
  • Gases
    
    • Partial pressure
    • Regulatory signals
    • Local autoregulation
  • Oxygen → tissues
• Filtration / resorption
  
  • Starling forces
  • Hydrostatic pressure difference (P_h)
  • Permability
  • Oncotic pressure (P_onc)
  • Pressure of the surrounding tissue (P_interst)
  • Effective filtration pressure
    
    • Plasma flow
      
      • P_eff = (P_hydrostatic - P_oncotic -P_tissue)
  • Arterioles
  • Venules
  • Capillary pressure
  • Net filtration
  • Net reabsorption
• Transport mechanisms
  
  • Water
  • Electrolytes
  • Anelectrolytes
  • Paths:
    
    • Fenestration
    • Interendothelial ways
    • Transcellular way
    • Cytosis
      
      • Endocytosis
      • Exocytosis
      • Pynocytosis
  • Diffusion
  • Osmotic forces
  • Electric forces
  • Electroneutrality
    
    • Anions
    • Cations
  • Thermodynamic rule
  • Hydrostatic pressure

Venous circulation

• Reservior
  
  • Capitance-system
• Postcapillary venule
• Collecting venule
• Muscular venule
• Redistribution
  
  • Vasomotor mechanism
• Distensibility
  
  • Collagen network
• Venous valves
• Distensible walls
• Right atrium
• Venous circulation
  
  • “Vis a tergo”
    
    • Work of the heart
  • Gravitation
  • Venous valves
    
    • Centripetal flow direction
    • Retrograde capillary flow
    • Edema
  • Canging pressure in chest & abdomen
    
    • Chest pump
  • Central Venous Pressure (CVP)
    
    • 0 mmHg

Question 2

Microcirculation

Capillaries, types

Answer 2

• Exchange of materials between blood and ECF is made possible by permeable capillaries
• Type of capillaries:
  
  • Continous capillary
    
    • Skin
    • Lung
    • Muscle
    • CNS
  • Fenestrated capillary
    
    • Interstitial mucosa
    • Endocrine glands
  • Porous capillary
    
    • Renal glomerulus
  • Sinusoid
    
    • Liver
    • Blood-forming organs

Question 3

Microcirculation

Morphology

Answer 3

• 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
  • Between arterioles and venules a shunt may be present (arteriovenous anastomosis)

Question 4

Exchange of substances

Types of substance exchange

Answer 4

1. Diffusion
2. Filtration /resorption

Question 5

Exchange of substances

Diffusion

Answer 5

• Most of the exchange
• Materials:
  
  • Gas exchange
  • Ion exchange
  • Small substances
• Types:
  
  • Flow limited (small substances)
  • Diffusion limited (bigger molecules)
• Factors influencing diffusion:
  
  • Concentration gradient
  • Permeability
  • Surface of the capillary
• Diffusion of gases:
  
  • Partial pressure of the gas drops in both directions: towards the end of the capillary, and towards distant cells
  • 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
• Diffusion of oxygen to tissues:
  
  • That drops towards the end of the capillary
  • Need for more oxygen: more capillaries will open

Question 6

Exchange of substances

Filtration /resorption

Answer 6

• Starling forces
• Major forces:
  
  • Hydrostatic pressure difference (P_h)
  • Permeability
  • Oncotic pressure (P_onc)
    
    • Constant in the capillary
  • Pressure of the surrounding tissue (P_interst)
• Direction of the substances is determined by the effective filtration pressure
• Direction of plasma flow:
  
  • P_eff = (P_hydrostatic - P_oncotic - P_tissue)
• Hydrostatic pressure constantly drops to the venous end of capillary
• Hydrostatic pressure higher than oncotic = net filtration
• Hydrostatic pressure lower than oncotic = net reabsorption

Question 7

Transport mechanisms

Formation of ISF

Answer 7

• 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

Question 8

Transport mechanisms

Paths of transport

Answer 8

• Through fenestration
• Through interendothelial ways
• In a transcellular way
• Cytosis
  
  • Endocytosis
  • Exocytosis
  • Pynocytosis

Question 9

Transport mechanisms

Forces determining transport

Answer 9

1. Diffusion:
   
   • The greatest part of substances crossing the capillary membrane is transported in this way
2. Osmotic forces:
   
   • The osmotic effect of the proteins in the solution makes water to move
   • Water inside ↑
   • Oncotic pressure continuously gets the water to move into the intravasal compartments from the ISF
3. Electric forces:
4. Electroneutrality:
   
   • The total amount of anions and cations have to be equal in certain compartments
5. Thermodynamic rule:
   
   • The product of the concentrations of diffusible ions must be equal on both sides of the membrane
6. 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. (Its bigger on arterial side)

Question 10

Venous circulation

Types of venola

Answer 10

1. Post capillary venule
2. Collecting venule
3. Muscular venule

Question 11

Venous circulation

Characteristics of venous system

Answer 11

1. Reservior - capitance system
   
   • The majority of circulating blood reside in the veins
2. Resistance: 0
3. Redistribution
   
   • Vasomotor mechanism
4. Distensibility is large
   
   • Collagen network sets the limit
5. Function is determined by structure of the wall and venous valves
   
   • ​Posses venous valves and thin distensible walls
6. The pressure in veisn drops from the venules to the right atrium

Question 12

Venous circulation

Factors maintianing venous circulation

Answer 12

1. “Vis a tergo”
   
   • Work of the heart (force from behind)
2. Gravitation
3. Venous valves
4. Skeletal muscle pump
   
   • In case of weak muscle tension, blood accumulates in the vein, creating retrograde capillary flow – increased pressure – leading to edema
5. Changing pressure in chest and in abdomen
   
   • Chest pump
6. Central Venous Pressure (CVP)
   
   • 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