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 tissues and the removal of cell wastes 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 Brain-tight junctions between the capillary endothelial cells so only small molecules pass in and out Liver-clefts are wide open so that almost all dissolved substances can pass GI tract-midway between those of the muscle and liver 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

Diffusion Through the Capillary Membrane Exchange of Water, Nutrients, etc. Between the Blood and Interstitial Fluid Diffusion Through the Capillary Membrane Fig. 16.3

Exchange (cont) Lipid soluble substances diffuse directly through the cell membranes of the capillary endothelium Water soluble, non-lipid soluble, substances diffuse through intercellular “pores” in the membrane Effect of molecular size on passage through the pores

Exchange (cont) Substance Molecular Weight Permeability Water 18 1.00 Table 16.1 Relative permeability of skeletal muscle capillary pores to different size molecules 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

Exchange (cont) Effect of concentration difference on net rate of diffusion through the capillary membrane Net rate is proportional to the concentration difference 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 fluids entrapped within them, the consistency is that of a “gel.” The fluid diffuses but not flow. Free Fluid in the Interstitium – usually <1% unless there is edema and then it expands rapidly

Filtration Across Capillaries: Determined by Hydrostatic and Colloid Osmotic Pressures, as well as the Capillary Filtration Coefficient Hydrostatic pressure in the capillaries tends to force fluid and its dissolved substances through the pores into the interstitial spaces. Colloid osmotic pressure caused by the plasma proteins 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.) Capillary pressure tends to force fluid outward Interstitial fluid pressure tends to force fluid inward Capillary plasma colloid osmotic pressure tends to cause osmosis of fluid inward d. Interstitial fluid colloid osmotic pressure tends to cause osmosis 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 plasma proteins

Fluid Filtration (cont.) Effect of different plasma proteins on colloid osmotic pressure Proteins g/dl Plasma colloid Osmotic 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

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 Elevated capillary hydrostatic pressure Decreased plasma colloid osmotic pressure Increased interstitial fluid colloid osmotic pressure d. 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.