Endocrine System: Hormones and Glands

• Endocrine glands, or ductless glands, secrete hormones to perform communication and control functions
• Target cells are the cells acted on by hormones
• Hormone secretion is controlled by feedback loops

Negative Feedback

• Mechanisms reverse changes in a physiological system
• Most hormonal secretion is regulated this way
• Examples include insulin, thyroid-stimulating hormone (TSH), and adrenocorticotropic hormone (ACTH)

Positive Feedback

• Mechanisms amplify physiological changes

• Oxytocin is an example

• The hypothalamus produces antidiuretic hormone (ADH), oxytocin, and regulatory hormones

• The pituitary gland is located in the sella turcica, attached to the hypothalamus via the infundibulum, and is also known as the hypophysis

• The pituitary gland is divided into the anterior and posterior pituitary glands

Anterior Pituitary Gland (Adenohypophysis)

• Thyroid-stimulating hormone (TSH) targets the thyroid gland, stimulating it
• Adrenocorticotropic hormone (ACTH) targets the adrenal cortex, stimulating its growth and hormone production
• Follicle-stimulating hormone (FSH) targets ovarian follicles in females, causing maturation, and seminiferous tubules in males, aiding spermatogenesis
• Luteinizing hormone (LH) targets ovaries/testes, causing ovulation in females and testosterone secretion in males
• Growth hormone (GH) targets the musculoskeletal system for growth and development
• Prolactin (lactogenic hormone) targets the mammary gland (breast) for milk production

Posterior Pituitary Gland (Neurohypophysis)

• Hormones are produced by the hypothalamus but released through the posterior pituitary gland

• The pars intermedia secretes melanocyte-stimulating hormone (MSH)

• Oxytocin targets the uterus and breast, aiding in childbirth and milk flow

• Antidiuretic hormone (ADH) targets the kidneys, regulating body fluid in response to electrolyte concentration or blood pressure

• Diabetes insipidus results from ADH underproduction

• The thyroid gland is located in the neck and is the largest endocrine gland, bilobed and connected by an isthmus

• Iodine is required for the production of major thyroid hormones

• The thyroid gland is the only endocrine gland that stores its hormone product extracellularly

• Functioning is controlled by TSH secreted by the anterior pituitary gland

• The thyroid gland controls cellular metabolism of almost all body cells

• Triiodothyronine (T3) and tetra-iodothyronine (T4), or thyroxine, target all body cells, the skeletal system, and the nervous system to control cellular metabolism, and growth and development

• Calcitonin targets the intestine, bone, and kidneys to lower blood calcium levels

• The parathyroid gland is located in the neck, behind the thyroid gland, as two pairs

• Parathyroid hormone (PTH) is antagonistic to calcitonin; it targets osteoclasts, renal tubules, and the intestine to increase blood calcium levels

Adrenal Glands

• Paired glands located above the kidneys and also called suprarenal glands
• Composed of two glands in one: the adrenal cortex and adrenal medulla
• Adrenal cortex is under the control of ACTH, secreted by anterior pituitary gland
• Adrenal cortex has 3 zones, secreting different hormones:
  • The zona glomerulosa (outer zone) produces mineralocorticoids, like aldosterone that target the renal tubules to save Na and H2O
  • The zona fasciculata (middle zone) produces glucocorticoids, like cortisol that targets the liver and immune system to increase blood glucose levels, be anti-inflammatory, and anti-immunity
  • The zona reticulate (inner zone) produces androgens, that target the reproductive system affecting libido
• The adrenal medulla is located internal to the adrenal cortex
• Adrenal medulla is part of the sympathetic nervous system
• Increased epinephrine secretion is the first endocrine response to stress
• Epinephrine (adrenaline) helps the body resist stress, intensifying and prolonging the effects of sympathetic stimulation
• Norepinephrine (noradrenaline) works with epinephrine to enhance the "fight or flight" response

Pancreas

• Located in the abdominopelvic cavity in the loop between the stomach and small intestine; is a mixed gland with exocrine and endocrine functions
• Exocrine function: secretes digestive enzymes
• Endocrine function: the endocrine part is the islets of Langerhans
• Types of cells in the pancreatic islets:
  • Alpha cells secrete glucagon, which increases blood glucose levels
  • Beta cells secrete insulin, which lowers blood glucose levels
  • Delta cells secrete somatostatin, which inhibits the release of other hormones

Female Sex Glands: The Ovaries

• The ovaries contain ovarian follicles that secrete estrogen, and the corpus luteum that secretes progesterone, relaxin, and a small amount of estrogen
• Effects of estrogen:
  • Development and maturation of breasts and external genitalia
  • Development of adult female body contours
  • Initiates the menstrual cycle
• Effects of progesterone:
  • Prepares the endometrium for pregnancy
  • Promotes the proliferation of milk-secreting ducts in the breast
• Primary targets of relaxin: the pubic symphysis, uterus, and mammary glands
• A decline in reproductive hormone levels marks menopause

Male Sex Gland

• The interstitial cells of testes produce androgens like testosterone
• Effects of testosterone:
  • Maturation of external genitalia
  • Growth of facial hair
  • Voice changes at puberty
  • Development of musculature and body contours typical of the male

Thymus

• Located in the anterior mediastinum (middle part of the thoracic cavity between the lungs)
• Thymus is essential before birth and during early childhood and degenerates gradually after puberty
• Degeneration results in decreasing size and fatty tissue replacement as a replacement
• The thymus is the site for the development of lymphocytes
• The hormone secretions, thymosin and thymopoietin, are essential for the development of lymphoid organs

Pineal Gland

• Small structure attached to the posterior end of the thalamus by a stalk
• Receives fibers from optic radiation, known as the "third eye"
• Stimulated by darkness and inhibited by light
• Plays a role in the circadian rhythm
• Secretes melatonin, which induces sleep
• Degenerates after middle age

Kidney

• When the oxygen content of the blood becomes too low, the kidney releases erythropoietin
• Erythropoietin stimulates red blood cell production

Respiratory System

• Respiratory tract/organs of the respiratory system

• Divisions of the respiratory tract:

  • Upper respiratory tract: nose, nasal cavity, paranasal sinuses, & nasopharynx
  • Lower respiratory tract: trachea, bronchial tree, & lungs

• Zones of the respiratory system:

  • Conducting zone: nose, pharynx, larynx, trachea, bronchi, & up to terminal bronchioles
  • Respiratory zone: respiratory bronchioles, alveolar ducts, alveolar sacs, & alveoli

• Respiratory mucosa is lined by pseudostratified columnar ciliated epithelium

• Cilia beat in one direction, moving mucus upward to the pharynx for removal

Nose

• Nasal septum separates the interior into two cavities
• Mucous membrane lines the nose
• Bony hard palate (maxillae and palatine bones) forms the floor of the nasal cavity
• Frontal, maxillary, sphenoidal, and ethmoidal sinuses drain into the nose
• The nose warms and moistens inhaled air and contains sense organs of smell

Pharynx (Throat)

• Shared by digestive and respiratory systems and a passageway for food, liquids and air
• Air distribution in the pharynx occurs through nasopharynx, oropharynx, and laryngopharynx
• Openings into the pharynx include the nasal cavities, mouth, esophagus, larynx, and auditory tubes
• Pharyngeal tonsils and openings of auditory tubes are open into the nasopharynx, palatine tonsils into the oropharynx
• About 12.5 cm (5 inches) long

Larynx (Voice Box)

• Several pieces of cartilage form the framework composing of:
  • largest thyroid cartilage (Adam's apple)
  • partially covers the opening into the larynx by the Epiglottis
  • Unpaired cartilages: thyroid, cricoid, & epiglottis
  • Paired cartilages: arytenoids, corniculate, & cuneiform
• Vocal cords and vestibular folds stretch across the interior, allowing air distribution to move to and from lungs & voice production

Trachea (Windpipe)

• A tube about 4.5 inches long extending from the larynx into the thoracic cavity
• C-shaped rings of cartilage hold the trachea open
• Passageway for air to move to and from lungs

Bronchi, Bronchioles, and Alveoli

• The trachea branches into right and left bronchi at T5 (thoracic vertebra 5)
• The right bronchus is wider and shorter, so foreign bodies from the trachea tend to enter the right bronchus
• Each bronchus branches into smaller tubes, eventually leading to bronchioles
• Bronchioles end in clusters of microscopic alveolar sacs, which are made up of alveoli Function:
• Bronchi and bronchioles: air distribution; passageway for air to move to and from alveoli
• Alveoli: gas exchange between air and blood
• Type I alveolar cells make up the alveolar wall
• Type II alveolar cells produce surfactant, reducing surface tension

Lungs and Pleura

• The lungs are large enough to fill the chest cavity, except for the middle space occupied by the heart and large blood vessels
• Apex: narrow upper part of each lung, under the collarbone
• Base: broad lower part of each lung; rests on the diaphragm
• The right lung has 3 lobes, while the left lung has 2 lobes
• Pleura: Moist, smooth, slippery membrane lining the chest cavity and covering the lungs; reduces friction during breathing

Function:

• Pulmonary ventilation (breathing) with the diaphragm contraction that increases thoracic cavity size, causing pressure to drop and air to flow into the lungs

Gas Exchange

• External respiration (lungs) where carbon dioxide moves out of lung capillary blood into alveolar air and out of the body in expired air
• Oxygen moves from alveoli into lung capillaries
• Internal respiration (tissues) with oxygen that moves out of tissue capillary blood into tissue cells
• Carbon dioxide moves from tissue cells into tissue capillary blood

Respiratory Control Centers

• Medulla oblongata controls the basic respiratory rhythm
• The pons works with the medulla to regulate the rate of breathing
• Chemoreceptors detect changes in CO2, O2, and pH to adjust breathing
• Mechanoreceptors prevent overinflation of the lungs and respond to physical irritants
• Higher centers in the cerebrum and hypothalamus also influence respiration