U3 L7 Microcirculation and Capillary Exchange Notes

Capillaries

• Smallest blood vessels.
• Walls consist of a thin tunica interna, one cell thick.
• Lumen allows only a single RBC to pass at a time.

Types of Capillaries

Continuous Capillaries

• Abundant in the skin and muscles.
• Endothelial cells provide an uninterrupted lining.
• Adjacent cells are held together with tight junctions.
• Intercellular clefts of unjointed membranes allow the passage of fluids.

Fenestrated Capillaries

• Found wherever active capillary absorption or filtrate formation occurs (e.g., small intestines, endocrine glands, and kidneys).
• Characterized by fenestrations (pores).
• More permeable to solutes and fluids than continuous capillaries.

Sinusoids

• Highly modified, leaky, fenestrated capillaries with large lumens.
• Found in the liver, bone marrow, lymphoid tissue, and in some endocrine organs.
• Allow large molecules (proteins) and blood cells to pass between the blood and surrounding tissues.

Microcirculation

• Interwoven networks of capillaries.
• Consists of:
  • Vascular shunts (metarterioles, or a thoroughfare channel) connecting an arteriole directly with a postcapillary venule.
  • True capillaries that branch off the metarteriole and return to the thoroughfare channel at the distal end of the bed.
• Precapillary sphincters contain smooth muscle cells that surround each true capillary and regulate blood flow into it.
• Blood flow through the capillary bed is regulated by vasomotor nerves and local chemical conditions, so it can either bypass or flood the capillary bed.
• Venules are formed when capillary beds unite.

Capillary Exchange

• $O<em>2$, $CO</em>2$, nutrients, and metabolic wastes diffuse between the blood and interstitial fluid along their concentration gradients.
• $O<em>2$ and nutrients pass from the blood to tissues; $CO</em>2$ and metabolic wastes pass from tissues to the blood.
• Water-soluble solutes pass through clefts and fenestrations, whereas lipid-soluble molecules diffuse directly through endothelial membranes.

Capillary Filtration

• Direction and amount of fluid flow depend upon the difference between hydrostatic pressure (HP) and colloid osmotic pressure (OP).

Capillary Hydrostatic Pressure ($HP_c$)

• Pressure of blood on the capillary walls.
• Forces fluids through the capillary walls.
• Greater at the arterial end of a bed than at the venular end because of resistance.

Interstitial Hydrostatic Pressure ($HP_i$)

• Pressure applied on the capillary wall by the interstitial fluid.
• Very small due to very little fluid in the interstitial space.

Capillary Osmotic Pressure ($OP_c$)

• Created by non-diffusible plasma proteins.
• Draw water toward themselves.

Interstitial Osmotic Pressure ($OP_i$)

• Created by the few proteins in the interstitial space.
• It is small.

Net Filtration Pressure (NFP)

• Considers all the forces acting upon a capillary bed.
• $NFP = P<em>{out} – P</em>{in} = (HP<em>c + OP</em>i) – (HP<em>i +OP</em>c)$

At the arterial end of a capillary bed:

• $OUT: HP<em>c + OP</em>i = 35 mmHg + 3 mmHg = 38 mmHg$
• $IN: HP<em>i + OP</em>c = 1 mmHg + 25 mmHg = 26 mmHg$
• $NFP = OUT – IN = 38 mmHg – 26 mmHg = 12 mmHg$
• Filtration takes place (out of the capillary).

At the venular end of a capillary bed:

• $OUT: HP<em>c + OP</em>i = 15 mmHg + 3 mmHg = 18 mmHg$
• $IN: HP<em>i + OP</em>c = 1 mmHg + 25 mmHg = 26 mmHg$
• $NFP = OUT – IN = 18 mmHg – 26 mmHg = -8 mmHg$
• Reabsorption takes place (into the capillary).