The Digestive and Excretory Systems

All animals require some sort of food source to survive. Whatever the food source, it is broken down in the body by acid and enzymes within the digestive system, and the resulting nutrients are absorbed into the body. The digestive system consists of many different organs that aid in the metabolic processing of food for nutrients and energy. Animals are classified into different types according to the food source they require for survival. Herbivores consume only plant-based food sources, carnivores eat other animals, and omnivores consume animals as well as plant-based foods.

Simple organisms like hydra have a basic digestive system. It consists of a single compartment, called a gastrovascular cavity, with a single opening. This cavity aids in digestion, as well as the distribution of nutrients through the body, by way of the vascular system. More complex animals have digestive systems that resemble the human digestive system: a tube (gastrointestinal tract) with two openings, the mouth and the anus. The gastrointestinal (GI) tract includes the mouth, esophagus, stomach, small intestine, and large intestine. In addition, there are several accessory organs, such as the liver, pancreas, and gallbladder, that provide secretions to help break down food.

Digestion is the process of breaking food down into nutrients the body can use. Many molecules are obtained from food, including proteins, vitamins, carbohydrates, and fats. In order for cells of the body to receive these molecules, specific processes must take place within the digestive tract to break down food.

Digestion begins in the mouth with both mechanical and chemical digestive processes. Chewing mechanically breaks food down into smaller pieces and mixes the digestive enzyme amylase, found in saliva, with the food. The enzymes in saliva begin the breakdown of starches into sugars. The chewed food is then swallowed and travels down the esophagus to the stomach.

The stomach contains acid that converts an inactive precursor enzyme, pepsinogen, into the active enzyme pepsin, used to break down proteins. The churning of the stomach pushes the now-liquid food into the small intestine in small doses. Within the small intestine, various enzymes from accessory organs act to further digest the food into nutrient components.

The pancreas releases enzymes that degrade proteins, fats, nucleic acids, and carbohydrates within the small intestine. In addition, the liver produces bile that non-enzymatically aids in the breakdown of fats in the small intestine. The bile is concentrated and stored in the gall bladder until needed. The nutrients released through these digestive processes are absorbed through the epithelial cells of the villi that line the intestine. Villi are fingerlike projections of epithelial cells on the surface and a network of capillaries behind them. Nutrients enter the bloodstream through this capillary network. Once they are in the bloodstream, the nutrients can be delivered to the rest of the body. Undigested waste is passed through the large intestine, where water is removed and the remainder is eliminated from the body as solid matter called feces.

The function of the excretory system is to dispose of metabolic wastes and control body fluid composition in response to external or internal stimuli. These functions are central to homeostasis, the body's processes to physiologically regulate its internal environment with response to fluctuations that occur in the internal or external environment. The excretory system consists of structures of the urogenital system, such as the kidneys, urinary bladder, and urethra.

Through homeostasis, the body regulates its internal temperature and blood electrolytes by movement of internal fluids. For example, when mammals sit in the hot sun, their body must maintain a stable internal temperature despite a rise in the external temperature. One mechanism is the release of water as sweat from glands in the skin to cool the body down.

Mammals produce fluid waste called urine, which is removed from the body by the urinary system. Production and excretion of urine occurs through a series of steps involving the excretory system and urinary system working in tandem with the blood capillaries. In mammals, urine is produced in the kidneys, where filtrate that comes from the blood is collected. Because the capillaries are immediately adjacent to structures within the kidney, water and solutes travel between the capillaries and a tubule called the nephron. Substances that are needed by the body are reabsorbed from the filtrate. They are sent from the nephron back to the bloodstream for use. Substances such as toxins or excess salts are removed from the bloodstream by the nephron. This collection of water, solutes, toxins, excess ions, and other substances are concentrated as urine.
In order to excrete urine from the body, several organs must work together. Each kidney is connected to a duct called the ureter, where urine that is produced from each kidney travels to the bladder. The urinary bladder stores the urine until it is expelled through the urethra, a tube that empties urine outside the body.