Microcirculation and Lymphatics

Question 1

what are metarterioles?

Answer 1

Arteries -> Arterioles -> METARTERIOLES-> capillaries-> venules ->vein

Question 2

what is the structure of a capillary wall?
What special features do capillaries contain in specific tissue?

Answer 2

A single layer of endothelial cells, with intercellular clefts, which form pores between the endothelial cells.
Brain capillary endothelial cells have tight junctions, while the liver endothelial has very large pores that allow filtration of most constituents.

Question 3

what are the haemodynamics of blood flow through the capilaries ?
How does blood flow vary

Answer 3

Blood flow is never continuous in the capillaries; rather, it is intermittent. Vasomotion occurs, which is the intermittent contraction of the metarterioles and pre-capillary sphincter that can start and stop blood flow through the capillaries.

When O2 usage is high, the opening and closing of the metarterioles and precapillary sphincter is geared towards prolonging blood flow in an effort to deliver more oxygen.

Question 4

How do nutrients and solutes leave the capillary blood and enter the interstitial space?

Answer 4

Diffusion is the main preocess. Lipid soluble products will diffuse directly through the capillary walls. Lipid insoluble substances pass through the clefts between the endothelial cells.

Question 5

What determines net diffusion of solutes from the capillaries into the extracellular space?

Answer 5

net diffusion is directly proportional to the concentration gradient

Question 6

what are the primary factors that affect the diffusion rate?

Answer 6

1) pore size
2) Molecular size of diffusing substance
3) concentration difference of the substrate between he two sides of the membrane

Question 7

What are the major components of the interstitial space?

Answer 7

collagen fiber bundles, water and proteoglycans for a tissue gel. There is only a very small amount of free water which is not associated with proteoglycans in the interstitium

Question 8

What changes occur in the interstitium with oedema?

Answer 8

There is an accumulation of water which is not bound to free water.

Question 9

what determines the plasma and interstitial fluid volumes?

Answer 9

The protein concentration within this

Question 10

What is the role of the lymphatics in maintaining interstitial and intravascular volume?

Answer 10

As protein concentration regulates interstitial fluid and intravascular volume primarily, it is is the job of the lymphatics to absorb proteins out of the interstitium and transport these into the general circulation

Question 11

What determines fluid movement in the capillaries?

Answer 11

• Capillary pressure - determines how much is pushed out
• Interstitial fluid pressure - determines how much is pushed into the capillaries
• plasma colloid osmotic pressure - determines how much is pushed into the capillaries
• interstitial colloid osmotic pressure determines how much is drawn out of the capillaries

Question 12

What is normal capillary pressure?

Answer 12

17mmHg

Question 13

What is the interstitial fluid pressure and what determines this?

Answer 13

They are usually negative (bellow atmospheric pressure) and the lymphatic system is responsible for the negative pressures .

Question 14

What are the roles of the lymphatics?

Answer 14

• determine protein concentration gradient between the interstitial fluid and the intravascular space
• transport lipids, proteins, and immune cells
• maintain a negative interstitial pressure

Question 15

What is the Plasma colloid osmotic pressure (COP) ?
How much of this is due to the Donnan effect and what is that?

Answer 15

28mmHg
9mmHg is due to the Donnan effect. The Donnan effect is cause by the retention of Cations (Esp Na+) with the large negatively charged proteins

Question 16

What are the primary proteins that determine the COP ?

Answer 16

Albumin, globulin and fibrinogen

Question 17

Which molecules have the greatest effect on COP?

Answer 17

The ones with low molecular weights (i.e. albumin instead of fibrinogen) as osmolality is determined by the number of osmotically active particle rather than their sixe.e

Question 18

What is the starling equilibrium for capillary exchange?

Answer 18

The driving force for fluid movement is out of the capillaries into the tissues and from the tissues into the venules . The capillary pressure is 13mmHg while it is only 7mmHg in the vein.
The starling equilibrium is the average of all the forces regarding fluid exchange at the capillaries (but it does not take into account arteriole and venuole forces mentioned above). This results ina. slight net diffusion (0.3mmHg) into the interstitium from the capillary. This is balanced out due to the presence of lymphatics.

Question 19

How is lymph flow affected by interstitial fluid pressure?

what is the rate of lymph flow?

Answer 19

Any factor that increases interstitialpressure flow will also increase lymphatic flow

product of interstitial fluid pressure and the activity of the lymphatic pump

Question 20

what are lymphatic valves?

Answer 20

Otherwise known as a lymphatic pump, when the pressure in the lymphatic vessels elevates due to the presence of fluid, the smooth muscle contracts and moves the lymph forward.

Question 21

What happens to lymphatic flow when protein concentrations build up in the interstitial fluid?

Answer 21

When proteins leak from the capillaries into the interstitium, the interstitial osmotic pressure rises . this will result in the opening of the lymphatic channels and the removal of the proteins and accumulated fluid.