Digestive System

The digestive system is composed of the alimentary canal, along with glands such as the liver, salivary glands, and pancreas that produce substances needed in digestion. Digestion starts with the ingestion and chewing of food mixed with saliva. The food passes down the esophagus into the stomach, where the gastric and intestinal juices continue the process. Thereafter, the mixture of food and secretions, called chyme, is pushed down the alimentary canal by peristalsis, rhythmic contractions of the smooth muscle of the gastrointestinal system. The contractions are initiated by the parasympathetic nervous system and can be inhibited by the sympathetic nervous system. In ruminants, the stomach has multiple sections, and chyme is passed back and forth several times between the stomach sections and the mouth for rechewing and redigestion. Absorption of nutrients from chyme occurs mainly in the small intestine; unabsorbed food and secretions and waste substances from the liver pass to the large intestines and are expelled as feces. Water and water-soluble substances travel via the bloodstream from the intestines to the kidneys, which absorb all the constituents of the blood plasma except its proteins. The kidneys return most of the water and salts to the body, while excreting other salts and waste products, along with excess water, as urine.

Endocrine System

The endocrine system involves internal secretions related to the function of the endocrine glands such as the thyroid, adrenal, pituitary. Hormonal secretions from these glands pass directly into the blood stream. An important part of this system, the pituitary, lies at the base of the brain. This master gland secretes a variety of hormones, including hormones that stimulate the thyroid gland and control its secretion of thyroxine, which dictates the rate at which all cells utilize oxygen; control the secretion in the adrenal gland of hormones that influence the metabolism of carbohydrates, sodium, and potassium and control the rate at which substances are exchanged between blood and tissue fluid; control the secretion in the ovaries of estrogen and progesterone and the creation in the testicles of testosterone; control the rate of development of the skeleton and large interior organs through its effect on the metabolism of proteins and carbohydrates; and inhibit insulin—a lack of insulin causes diabetes mellitus.

The posterior lobe of the pituitary secretes vasopressin, which acts on the kidney to control the volume of urine; a lack of vasopressin causes

Thomas Henry Huxley's research in comparative anatomy helped establish the inductive method as the primary mode of scientific research. (Library of Congress)

diabetes insipidus, which results in the passing of large volumes of urine. The posterior lobe also elaborates oxytocin, which causes contraction of smooth muscle in the intestines and small arteries and is used to bring about contractions of the uterus in birth. Other glands in the endocrine system are the pancreas, which secretes insulin, and the parathyroid, which secretes a hormone that regulates the quantity of calcium and phosphorus in the blood.

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