Anatomy and Function of Digestive & Respiratory Systems
The Human and Avian Digestive System Anatomy
Pharynx: Swallowing and Lymphocyte Maturation
The Pharynx is a muscular tube that leads into the next portions of the digestive tract: the esophagus and trachea. The tonsils are used for the maturation of lymphocytes. Below the cartilaginous fold belonging to the larynx is the epiglottis, which covers the glottis during swallowing to prevent food from passing into the trachea. The medulla controls swallowing, including closing the nostrils by raising the soft palate, preventing the food bolus from moving upward.
Esophagus and Peristaltic Movement
The Esophagus is a straight duct that moves the food bolus through contact with its walls. Peristaltic movements, caused by muscle contractions in the esophageal walls, propel the food. In birds, the crop stores food temporarily.
Stomach Structure and Gastric Digestion
The stomach is the most extensive portion of the digestive tract. Its entry is called the cardia (due to its proximity to the heart) and its exit is the pylorus. The stomach begins true digestion. The wall contains glandular cells that secrete HCl, which activates gastric enzymes (like pepsin) that break down peptides. These cells also secrete the intrinsic factor necessary for the absorption of Vitamin B12, and a protective mucus layer for the stomach wall. The result of mixing food with gastric juices is an acidic slurry called chyme.
The Avian Gizzard (Gastric Mill)
In birds, the stomach expansion is called the gizzard. Its highly muscular walls contract, often utilizing small stones, to grind food; this process is known as the gastric mill.
Small Intestine: Absorption and Secretions
In humans, the small intestine has three parts: the duodenum, jejunum, and ileum. The liver and pancreas ducts empty into the duodenum at the ampulla of Vater. The liver secretes bile salts, which accumulate in the gallbladder. Bile salts emulsify fats, preparing lipids for enzymatic attack. The pancreas secretes pancreatic juice, a blend of enzymes including pancreatic amylase, lipase, and trypsin. The resulting slurry, where all nutrients are digested, is called chyle.
Uptake (absorption) occurs when nutrients pass from the intestine into the blood. Folds filled with blood vessels, called intestinal villi, facilitate absorption. These expansions are lined by an epithelium composed of enterocytes.
Large Intestine, Cecum, and Intestinal Flora
The small intestine joins the large intestine at the ileocecal valve. The large intestine begins with the cecum, which features an expansion called the appendix. Following the cecum is the colon, which absorbs water and compacts feces. The intestinal flora (a number of bacteria) acts here, aiding in the digestion of cellulose.
Biological Nutrition and Internal Environment
Autotrophic vs. Heterotrophic Nutrition
Nutrition is the set of processes by which living organisms exchange matter and energy with the outside environment. There are two types:
- Autotrophic (e.g., plants, bacteria): Organisms that produce their own nutrients.
- Heterotrophic (e.g., animals): Organisms that must obtain nutrients from the environment.
The nutrition of multicellular organisms is complex, involving various specialized systems, as nutrients are not directly accessible to all cells.
Key Biological Definitions
- Ureters: Tubes through which urine (filtered blood products) passes from the kidneys and empties into the bladder.
- Internal Environment: In multicellular animals, this consists of the interstitial fluid that surrounds the cells. This fluid facilitates the exchange of necessary substances and the expulsion of metabolic waste.
- Radula: The chitinous ‘tongue’ of snails, used for scraping leaves and stems during feeding.
- Gizzard: An expansion of the stomach in birds, featuring highly muscular walls that contract.
- Deglutition (Swallowing): The process of moving food into the esophagus. It involves closing the nostrils by lifting the soft palate (preventing food from moving upward) and the epiglottis closing the larynx.
Comparative Respiratory Systems
Branchial Respiration (Gills)
Gills are membranous expansions highly irrigated with blood vessels. They include:
- External Gills: Found in newts, salamanders, tadpoles, and some frogs. Advantage: They do not require ventilation. Disadvantage: They are fragile expansions that protrude from the animal’s body.
- Internal Gills:
- In cartilaginous fish, water enters through openings called spiracles and exits through five pairs of slits, where O₂ and CO₂ exchange occurs.
- Bony fish have four pairs of gill lamellae, which are highly vascularized structures attached to the gill arches and protected by a cavity closed by the operculum. When water enters the mouth, the operculum valve closes, preventing water from entering the digestive tract.
Tracheal Respiration in Arthropods
Arthropods utilize a network of tubes called tracheae that run throughout the animal’s body. These end in thin tubes called tracheoles, which are filled with liquid where gas exchange occurs directly with the cells. Air enters through external openings called stigmata (or spiracles) located along the body.