Lesson 21

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Outline: Basic Problems / Basic Concepts

Need for feeding and its link to digestion, metabolism, gas exchange and excretion Nutrient exchange with the environment Delivery of nutrients between environment and tissues Co-evolution of respiratory, circulatory, excretory and digestive systems Need for control

• Overview of the Digestive System • Ontogeny (Embryonic Origin) of the Digestive System • Basic Form (Components) of the Digestive System • Basic Function of the Digestive System

Objectives: At the end of this lesson you should be able to:

Describe the link between feeding, digestion, metabolism, gas exchange and excretion Describe the problems that arise as animals increase in size for exchange of nutrients and any other substance with the environment Discuss the co-evolution of respiratory, circulatory, excretory and digestive systems

• Describe the role of the digestive system and the basic processes involved in digestion

• Describe the embryonic origins of the alimentary canal • Describe the basic components of the digestive system • Describe the basic functions of the digestive system

References: Chapter 12: 265-291

Reading for Next Lesson: Chapter 12: 265-291

Basic Problems / Basic Concepts

1) Needs for feeding and its link to digestion, metabolism, gas exchange and excretion

Most nutrients procured by animals are used to obtain energy. When these compounds are broken down, over 90% of the available energy contained in their chemical bonds can only be obtained through cellular respiration. This process, performed by the mitochondria, requires the presence of oxygen. This enables the complete oxidation of the compounds to carbon dioxide and water. A few organisms can exist indefinitely in the total absence of oxygen while many others can survive for limited periods under anaerobic conditions. In this case, the catabolism of nutrients stops far short of completion and the end products, usually lactic acid or ethanol, still contain much chemical energy. While these organisms are capable of colonizing otherwise uninhabitable environments, or surviving short periods of oxygen deprivation, they are usually sessile or relatively inactive. Most organisms are totally dependent on the presence of oxygen, many to the point that oxygen deprivation for more than a few minutes leads to death. Associated with the fundamental problem or procuring oxygen, for these organisms, is the problem of eliminating the waste product of cellular respiration, carbon dioxide. Changes in the level of this compound lead to changes in acid-base balance. The performance of all functional proteins, from enzymes to receptors and transport proteins, depends on the pH of their environment and carbon dioxide is one of the three variables that regulate pH. Thus the needs of animals to exchange gases are tightly linked to their need to obtain and digest the food that they ultimately metabolize for energy stores, as well as to their need to excrete waste products from that process. Animals must obtain sufficient O2 to meet their metabolic needs and must excrete sufficient CO2 to maintain acid-base balance.

2) Nutrient exchange with the environment Nutrient exchange between any living cell and its environment is by diffusion across a moist membrane. There are two potential problems here.

O2

CO2 Nutrients

Chemical Wastes

O2

CO2 Nutrients

Chemical Wastes

First, nutrients must go into aqueous solution before they can move across a living membrane. Second, diffusion in water is slow.

3) Delivery between environment and tissues As animals grow in size and complexity, maintenance of an exchange surface of adequate dimensions to serve the entire volume of the animal becomes a problem. With increasing size in three dimensions, the surface area (a square function) increases much more slowly than volume (a cubic function). The problem is more acute for more active animals. As a consequence, animals must develop specialized exchange surfaces where folding and other morphological changes can be employed to increase surface area in excess of that dictated by increased size alone. Another complexity associated with increasing three-dimensional size is that most cells will be deep within the body of the organism and far removed from the environment. Diffusion alone is incapable of moving gases in adequate concentration deep into tissues. It is estimated that simple diffusion through extracellular fluid could only support tissues up to a depth of 1 millimeter from the surface. Some form of a delivery system is essential for moving nutrients from the exchange surface to the individual cells of the animal, as well as for moving gases, wastes, etc. between tissues and their exchange sites.

Most animals use an intermediate system, the circulatory system, to transport oxygen and carbon dioxide between tissues and the external medium. Circulatory systems invariably require a muscular, internal pump to function. These solutions, a highly developed exchange surface linked to all the other tissues of the body by a circulatory system with a pump, and containing transport proteins for carrying oxygen, raise a further problem. To enhance the ability of the circulatory system to exchange gases with the environment, it is important to keep the distance between the blood or circulating fluids and the external medium to a minimum. As a result, exchange surfaces are generally very thin and delicate. The external surfaces of animals, on the other hand, have usually evolved to be relatively impermeable and protective. As a result, exchange surfaces are usually limited to a restricted region of the body. Often this restricted area is protected by an external covering or develops as an inward-oriented extension of the body surface. This is particularly true for terrestrial animals where there is also the need to keep this membrane moist and prevent desiccation and excess water loss. Finally, this latter solution to the problem of protecting thin, moist delicate exchange surface creates a problem for ensuring good communication between the environment and the exchange surface. It creates the need for mechanisms to deliver the nutrients to the exchange surfaces – another pump. 4. Co-evolution of respiratory, circulatory, excretory and digestive systems Because of the tight link between the need for digestion, diffusive exchange in the gut, circulation and diffusive exchange at the tissues, it is not surprising that the individual components of the systems involved in the transport of nutrients between tissues and the environment have co-evolved so that their capacities roughly match. From the perspective of nutrient exchange, there are two levels to this process. In the first instance, the capacity of the digestive system to provide fuel, the capacity of the respiratory system to provide the oxygen to metabolize this fuel, the capacity of the excretory system to eliminate the waste products of this process and the capacity of the circulatory system to move all substances between exchange sites, must match. In the second instance, the movement of each compound (O2, CO2, lipids, carbohydrates, nitrogenous wastes, etc.) between exchange surfaces is a multi-step process and the capacity for flux at each step must also match.

The systems must have evolved together (can’t select for a phenotype that is not expressed). Let's review this by looking at some of the trends we see.

5. Need for control With the development of specialized surfaces in specific locations, and the need for separate pumps for internal and external transport, there is a need to control these processes to ensure efficient function. There are costs associated with digestion and transport by the circulatory system and these costs can be high. Each process in the pathway for transport between tissues and the atmosphere must be regulated to ensure efficiency. Energy saved in the process that can be used to improve reproductive fitness is truly adaptive. These are the basic problems faced by all animals. Given this, let’s now examine how different animals meet these basic problems and some the unique new problems that arise as a result of the solutions they use.

Digestive System Overview of the Digestive System The role of the digestive system is to provide the bodies of animals with substances that can be used either as building blocks for manufacturing needed molecules or as sources of chemical bonds that can be broken to obtain energy.

The necessary sequence for obtaining nutrients and energy is feeding, digestion and metabolism.

The alimentary system plays the essential role of digesting and absorbing food and voiding indigestible material.

Digestion consists of rendering food, whatever the type, into molecules that can enter the bodies of animals by passing across the epithelial membrane of the alimentary system. Remember that to do so, they must either be water soluble, dissolved in water and small enough to pass through pores, lipid soluble and small enough to pass through the membrane, or have a transporter present in the membrane to help them across (active or passive). In all but the simplest organisms, digestion is extracellular.

Remember the distinction between things that are within the body and things that are "in" the body. The lumen of the alimentary canal is continuous with the outside environment at both ends and things that are within the canal have not yet crossed a membrane and have not truly "entered" the body.

Different demands will be placed on the design of the digestive systems of animals that feed on blood, plant fluids, meat (carnivores), plants (herbivores) or a combination of everything (omnivores).

The design of the alimentary canal will also reflect the quality of the food in the diet. Quality here refers to several factors:

The cost of obtaining the food. The rate at which the food can be digested. The amount of energy released by digestion and metabolism of the food.

Higher quality foods release more energy at a faster rate and require less time for digestion. In general, digestive systems must be designed to:

- receive food - store food - mechanically digest food - chemically digest food - bacterially digest food - absorb nutrients and water - process and expel indigestible material.

Think about the order these must be done in and why!

Think about the advantages of having material pass in one direction along a continuous digestive canal.

Think about what is required to move substances in one direction along such a canal.

Think about how much of the digestive system must be devoted to each component of this process for animals with different diets. Embryonic Origins In the embryo, the gut is a simple tube of endoderm from which the pharynx and alimentary canal arise, along with their associated digestive glands. During embryological development, invaginations from the surface ectoderm come into contact with the endodermal gut at opposite ends of the body. The anterior invagination is the stomodeum, which meets the foregut. A temporary buccopharyngeal membrane forms between them initially, but eventually ruptures so that a lumen forms between them. The stomodeum forms the buccal cavity.

The pharynx develops from the foregut and gives rise to pharyngeal pouches that become the pharyngeal or gill slits. This region becomes the source of many organs that will be discussed in later lectures. The posterior invagination of ectoderm is the proctodeum, which meets the hindgut. A cloacal membrane forms between them which eventually ruptures creating the cloaca. (Note: the blastopore does not technically become the anus!). During embryonic development, the endoderm gives rise to the lining of the gut, and the surrounding mesoderm forms smooth muscles, connective tissue, and blood vessels. Distinctive regions then become delineated. Basic Form (Components) of the Digestive System - General Features The adult digestive tract is a tubular passage that extends through the body from the opening of the mouth to the anus, or cloacal opening. Primary Regions - on the basis of embryological derivation and histological differences in intrinsic luminal glands, the digestive tract can be divided into:

- buccal cavity - pharynx - alimentary canal. Primary Organs - on the basis of histological differences the alimentary canal can be divided into:

- esophagus - stomach - small intestine - large intestine.

Accessory Digestive Glands - are extrinsic (outside the walls of the digestive tract), but secrete enzymes for digestion, via ducts, into the lumen of the digestive tract:

- salivary glands / oral glands - liver / gall bladder - pancreas. Basic Function of the Digestive System - General Features Processes: Ingestion Storage

Mechanical Digestion churning

Propulsion swallowing peristalsis Chemical Digestion Absorption Defecation Functional Concepts Digestion actually occurs outside the body. Substances are mechanically and chemically broken down. Substances then enter the body by transport across mucosal cells of the alimentary canal. Digestion consists of the breakdown of substances in a controlled area outside (but within) the body so that the products of the breakdown can be efficiently absorbed.