Role of intestinal lymphatics in interstitial volume regulation and transmucosal water transport

Abstract

Two of the principal functions of intestinal lymphatics are to assist in the maintenance of interstitial volume within relatively normal limits during alterations in capillary filtration (e.g., acute portal hypertension) and the removal of absorbed water and chylomicrons. The contribution of lymphatics to the prevention of interstitial overhydration or dehydration during alterations in transcapillary filtration is similar in the small intestine and colon. While the lymphatics of the small intestine contribute substantially to the removal of absorbed water (particularly at low and moderate absorption rates), the contribution of colonic lymphatics to the removal of the fluid absorbate is negligible. This difference is attributed to the relative caliber and location of lymphatics in the mucosal layer of the small and large intestines. In the small intestine, large lacteals lie in close proximity to transporting epithelium, while colonic lymph vessels are rather sparse and confined to the basal portion of the mucosa. In the small intestine, the lymphatics assume a more important role in removing absorbed water during lipid absorption than during glucose absorption.

Keywords: colon; lymphatics; secretion; small intestine; transcapillary fluid exchange; water absorption.

Figure 1

A. The capillary and blood circulations of the intestinal villus. (Adapted from Guyton, A.C. & J.E. Hall. 2006. Textbook of Medical Physiology, Chapter 65. pg 813. Elsevier. Philadelphia). B. The mucosal-submucosal lymphatic organization of the small and large intestine. Adapted from Kvietys et al. C. Anchoring filaments of the initial lymphatics. Adapted from Leak and Burke.

Figure 2

Starling forces and capillary membrane parameters in the small intestine under control (non-transporting) conditions. J_v,c,, rate of trancapillary fluid movement; K_f,c, capillary filtration coefficient; P_c, capillary hydrostatic pressure; P_t, interstitial hydrostatic pressure; σ_d, osmotic reflection coefficient; π_c, plasma oncotic pressure; π_t, interstitial oncotic pressure; NFP, net capillary filtration pressure. Adapted from Granger et al.

Figure 3

The safety factors against interstitial edema in the cat intestine and dog colon for an increment in capillary pressure of 12.0–13.2 mmHg as well as the safety factors against interstitial dehydration in the small intestine during a decrement in capillary pressure of 6.5 mmHg. Adapted from Granger and Barrowman.

Figure 4

Interstitial fluid pressure, hydraulic conductance, and macromolecular exclusion as a function of interstitial volume. Adapted from Granger et al.

Figure 5

Interstitial hydrostatic (P_t) and oncotic (π_t) pressures as a function of net fluid absorption rate. Adapted from Granger et al.

Figure 6

Percent of interstitial volume absorbed by the blood and lymph capillaries as a function of net fluid absorption. Adapted from Granger and Taylor.

Figure 7

Effects of net fluid absorption on Starling forces and membrane parameters in the small intestine. J_v,c,, rate of trancapillary fluid movement; K_f,c, capillary filtration coefficient; P_c, capillary hydrostatic pressure; P_t, interstitial hydrostatic pressure; σ_d, osmotic reflection coefficient; π_c, plasma oncotic pressure; π_t, interstitial oncotic pressure; NAP, net capillary absorptive pressure. Adapted from Granger et al.

Figure 8

Relationship between intestinal lymph flow and villus contraction frequency.

Figure 9

Effects of active (solute-coupled) fluid secretion on Starling forces and capillary membrane parameters in the small intestine. J_v,c,, rate of trancapillary fluid movement; K_f,c, capillary filtration coefficient; P_c, capillary hydrostatic pressure; P_t, interstitial hydrostatic pressure; σ_d, osmotic reflection coefficient; π_c, plasma oncotic pressure; π_t, interstitial oncotic pressure; NFP, net capillary filtration pressure. Adapted from Granger et al.