Unit Four: The Circulation

Chapter 16: The Microcirculation and Lymphatic System: Capillary Fluid

Exchange, Interstitial Fluid, and Lymph Flow

Guyton and Hall, Textbook of Medical Physiology, 12 edition

Structure of the Microcirculation/Capillary Systems

• Microcirculation- transport of nutrients to the tissuesand the removal of cell wastes

a. The microcirculation of each organ is organized specifically to meet that organ’s needs.

Fig. 16.1

Structure of the Microcirculation/Capillary Systems

Fig. 16.2 Structure of the capillary wall

Structure of the Microcirculation/Capillary Systems

• Special Types of Pores in Capillaries

a. Brain-tight junctions between the capillary endothelialcells so only small molecules pass in and out

b. Liver-clefts are wide open so that almost all dissolved substances can pass

c. GI tract-midway between those of the muscle and liver

d. Kidney-fenestrated; small openings to increase filtration through the glomerulus

Flow of Blood in the Capillaries

• Regulation of Vasomotion

• Average Function of the Capillary System

Exchange of Water, Nutrients, etc. Between the Bloodand Interstitial Fluid

• Diffusion Through the Capillary Membrane

Fig. 16.3

Exchange (cont)

a. Lipid soluble substances diffuse directly throughthe cell membranes of the capillary endothelium

b. Water soluble, non-lipid soluble, substancesdiffuse through intercellular “pores” in the membrane

c. Effect of molecular size on passage through thepores

Exchange (cont)

Substance Molecular Weight

Permeability

Water 18 1.00

NaCl 58.5 0.96

Urea 60 0.80

Glucose 180 0.60

Sucrose 342 0.40

Inulin 5000 0.20

Myoglobin 17600 0.03

Hemoglobin 68000 0.01

Albumin 69000 0.001

Table 16.1 Relative permeability of skeletal muscle capillary pores to different size molecules

Exchange (cont)

d. Effect of concentration difference on net rate ofdiffusion through the capillary membrane

1. Net rate is proportional to the concentrationdifference between the two sides of the membrane

Interstitium and Interstitial Fluid

Fig. 16.4 Structure of the interstitium; containing collagen bundle fibers and proteoglycan filaments

Interstitium (cont.)

• “Gel” in the Interstitium - because of the proteoglycan filaments and the fluidsentrapped within them, the consistencyis that of a “gel.” The fluid diffuses but not flow.

• Free Fluid in the Interstitium – usually <1% unless there is edema and then it expandsrapidly

Filtration Across Capillaries: Determined by Hydrostatic and Colloid Osmotic Pressures, as well as the Capillary

Filtration Coefficient

a. Hydrostatic pressure in the capillaries tends to force

fluid and its dissolved substances through the poresinto the interstitial spaces.

b. Colloid osmotic pressure caused by the plasmaproteins tends to cause fluid to move into the blood.

Fluid Filtration (cont.)

• Hydrostatic and Colloid Osmotic Forces Determine Fluid Movement Through the Capillary Membrane

Fig. 16.5 Fluid pressure and colloid osmotic pressure forces operate at the capillary membrane, moving fluid either outward or inward.

Fluid Filtration (cont.)

a. Capillary pressure tends to force fluid outward

b. Interstitial fluid pressure tends to force fluid inward

c. Capillary plasma colloid osmotic pressure tends tocause osmosis of fluid inward

d. Interstitial fluid colloid osmotic pressure tends to causeosmosis of fluid outward

The sum of these forces—the net filtration pressure+ net fluid filtration- fluid absorption

Fluid Filtration (cont.)

• Interstitial Fluid Hydrostatic Pressure- pumping by the lymphatic system is the basic cause of negative interstitial fluid pressure

• Plasma Colloid Osmotic Pressure - caused by plasmaproteins

Fluid Filtration (cont.)

Proteins g/dl Plasma colloidOsmotic pressur

Albumin 4.5 21.8 mm Hg

Globulins 2.5 6.0 mm Hg

Fibrinogen 0.3 0.2 mm Hg

Total 7.3 28.0 mm Hg

Effect of different plasma proteins on colloid osmotic pressure

Lymphatic System

Fig. 16.7 Lymphatic System

Lymphatic System (cont.)

• Lymph channels of the body

• Terminal lymphatic capillaries and their permeability

• Formation of lymph

Fig. 16.8 Special structure of the lymphatic capillaries that permits passage of substances of high molecular weight into the lymph.

Lymphatic System (cont.)

• Rate of Lymph Flow- effect of interstitial fluid pressure on lymph flow

Fig. 16.9

Lymphatic System (cont.)

• Factors That Increase Interstitial Fluid Flow

a. Elevated capillary hydrostatic pressureb. Decreased plasma colloid osmotic pressurec. Increased interstitial fluid colloid osmotic

pressured. Increased permeability of the capillaries

Lymphatic System (cont.)

• Lymphatic Pump Increases Lymph Flow

Fig. 16.10 Structure of lymphatic capillaries and a collecting lymphatic, showing also the lymphatic valves.