Digestive System

Abstract

To understand digestion we need first a gross, qualitative description of the fate of ingesta; what is secreted in response to the meal, how ingesta are transformed by reaction with secretions and by the propulsive mechanism, what is absorbed and what rejected. This should be followed by an exact quantitative and temporal description in the style of chemical kinetics in which net changes are reported in terms of two-way fluxes. Coupled with this should be a detailed account of the physiological and morphological basis of secretion, transformation, and absorption reaching down to the deepest levels at which anatomy, biochemistry, and biophysics become indistinguishable one from another. Finally we require a description complete at all levels of controlling processes. Then we can put the fragments of our knowledge to sether to characterize what happens in the gut.2Descriptive, analytical, and synthetic aspects of our knowledge of diges tion are incomplete, for gastroenterological physiology suffers from difficul ties inherent in all biological science. Even summary descriptions must fre quently be revised as we gain new tools and insights, and scientific attack does not proceed in an orderly manner. Before we had learned to describe di gestion in simple terms there were triumphs of analysis, as for example the discovery of the secretin mechanism, but we cannot say how individual parts cooperate to perform the total function. In addition, gastroenterological physiology has its peculiar troubles. What is ingested may range from a ten course dinner served at the Duchess of Guermantes' to repulsive mixtures of mud and mush appropriately called "test meals." There are wide species dif ferences, and even within one species the digestive apparatus shows great functional variability. Consequently, when we come to describe what actu ally occurs during digestion we must specify what meal was eaten by what animal under what circumstances. SECRETION FROM THE SALIVARY GLANDSNot only do salivary glands differ from species to species, but several glands in one animal secrete juices of different compositions. The parotid 1 This review considers selected aspects of the recent literature available in the University of Michigan library at the end of May, 1958. 2 Papers on the following phases of digestion are cited for completeness but will not be discussed: on secretion (161 to 172); on the pancreas (173 to 177); on bile ( 178 to 182); on absorption (183 to 199); on cholesterol absorption (200 to 210); on neuro physiology (211 to 217); on smooth muscle (218 to 220); on the esophagus (221 to 228); on regulation (218 to 229); on the effects of drugs and 5-hydroxytryptamine (229 to 242); on growth, injury, and repair (24.3 to 249); on blood flow (250 to 255); and on methods (257 to 263). 183Annu. Rev. Physiol. 1959.21:183-210. Downloaded from www.annualreviews.org Access provided by Western Michigan University on 02/03/15. For personal use only.Quick links to online content Further ANNUAL REVIEWS 184 DAVENPORT gland of dog and man s...

The digestion and metabolism of sucrose in young milk-fed calves

“…Since the tissues do not contain an enzyme for utilizing either lactose or sucrose, both of these disaccharides if absorbed as such are excreted in the urine (Hawk et al, 195^-; Davenport, 1959)o Wallace et al (1951) reported an apparent digestion coefficient of 93^ for sucrose in a ration which contained 32^ sucrose and 50^ dried skimailk. However, Ruber et al» (1958) found that only 57^ of the sucrose was digested in a diet containing 0.7 g sucrose per pound of body weight per feeding.…”