The endocrine system acts with nervous system to coordinate the body's activities. Both systems enable cells to communicate with others by using chemical messengers. The endocrine system uses chemical messengers called hormones that are transported by the circulatory system (blood). They act on target cells that may be anywhere in the body. The endocrine system is slower than the nervous system because hormones must travel through the circulatory system to reach their target. Target cells have receptors that are specific to the signaling molecules. The binding of hormones to the receptors on or within the target cell produces a response by the target cell.
The chemical messengers used by the nervous system are neurotransmitters. Neurotransmitters travel across a narrow space (the synaptic cleft) and bind to receptors on the target cell. The nervous system conducts signals much quicker than the endocrine system. Endocrine Vs Exocrine glands
Endocrine glands do not have ducts. Exocrine glands have ducts that carry their secretions to specific locations. Two Kinds of Hormones
Peptide hormones are composed of amino acids.
A peptide hormone binds to a cell-surface receptor, it does not enter the cell. The resulting complex activates an enzyme that catalyzes the synthesis of cyclic AMP from ATP. Cyclic AMP activates other enzymes that are inactive.
Cyclic AMP is a second messenger; the hormone is the first messenger. Other second messengers have been discovered. Steroid Hormones
Steroid hormones enter the cell and bind to receptors in the cytoplasm. The hormone-receptor complex enters the nucleus where it binds with chromatin and activates specific genes. Genes (DNA) contain information to produce protein as diagrammed below. When genes are active, protein is produced.
Steroid hormones act more slowly than peptide hormones because of the time required to produce new proteins as opposed to activating proteins that are already present. Hypothalamus
The hypothalamus is part of the brain. It maintains homeostasis (constant internal conditions) by regulating the internal environment (examples: heart rate, body temperature, water balance, and the secretions of the pituitary gland). Pituitary Gland
The pituitary contains two lobes. Hormones released by the posterior lobe are synthesized by neurons in the hypothalamus. Unlike the posterior lobe, the anterior lobe produces the hormones that it releases. Refer to the diagram below as you read about the hypothalamus, pituitary, and each of the glands they control.
The posterior pituitary contains axons of neurons that extend from the hypothalamus. Hormones are stored in and released from axon endings in the posterior lobe of the pituitary. Oxytocin
Oxytocin stimulates uterine contractions of labor that are needed to move the child out through the birth canal. The hormone stimulates the release of milk from the mammary glands by causing surrounding cells to contract. After birth, stimulation of the breast by the infant feeding stimulates the posterior pituitary to produce oxyticin. Antidiuretic Hormone (ADH)
Antidiuretic hormone increases the permeability of the distal convoluted tubule and collecting duct of the kidney nephron resulting in less water in the urine. The urine becomes more concentrated as water is conserved. The secretion of ADH is controlled by a negative feedback mechanism as follows: concentrated blood (too little water) _ hypothalamus _ ADH _ kidney _ reabsorbs water, makes blood more dilute Below: Within the kidney, fluid and dissolved substances are filtered from the blood and pass through tubules where some of the water and dissolved substances are reabsorbed. The remaining liquid and wastes form urine. Details of this process are discussed in the chapter on the excretory system.
The presence of too much blood in the circulatory system stimulates the heart to...