Gland Physiology - Anatomy

Tropic hormones stimulate the release of other hormones from other glands.

Adrenocorticotropic hormone (ACTH) stimulates the release of glucocorticoids (namely cortisol) from the adrenal cortex. Thyroid-stimulating hormone (TSH) stimulates the release of thyroid hormones (T3 and T4) from the thyroid gland. Gonadotropin-releasing hormone (GnRH) stimulates the release of luteinizing hormone (LH) and follicle-simulating hormone (FSH) from the anterior pituitary. LH stimulates the release of testosterone (and other sex hormones) from the testes in men, and the release of estrogen and progesterone (and testosterone) from the ovaries in women.

Glucagon is released from the pancreas, and acts directly on cells in the liver and skeletal muscle to stimulate the breakdown of glycogen to increase blood sugar while fasting.

Steroid hormones are released from the parent cell by simple diffusion. Thyroid hormones are also released via simple diffusion. Peptide and catecholamines are the hormones released by exocytosis. No hormones are released via apoptosis, which is a controlled, deliberate form of cell death.

The kidneys contain many other functions in the body other than endocrine function. One of the primary functions of the kidneys is urine production. The kidney's endocrine function is the synthesis and secretion of erythropoietin and renin. The pineal, parathyroid, adrenals, pituitary, and thyroid tissues are the tissues that are primarily endocrine in function.

The lacrimal glands are responsible for producing tears. The eccrine glands produce sweat. The mammary glands produce milk. The salivary glands produce saliva.

Exocrine glands are released through ducts and typically release enzymes, while endocrine glands are not released through ducts and produce hormones.

The ovaries secrete estrogen, progesterone, and testosterone.

Insulin is made in the beta cells of the pancreas in response to ATP from glucose metabolism. Insulin inhibits glucagon release by alpha cells of the pancreas in a negative feedback mechanism to maintain constant blood glucose levels.

Insulin has several anabolic effects, including increased glucose transport in skeletal muscle and adipose tissue, increased glycogen synthesis and storage, increased triglyceride storage, increased protein synthesis in muscles, and increased cellular uptake of potassium and amino acids.

Glycogen is made by pancreatic alpha cells and is secreted in response to hypoglycemia, resulting in glycogenolysis and gluconeogenesis to increase circulating blood glucose levels.

Pancreatic amylase is responsible for starch digestion in the duodenum. Salivary amylase also aids in this process, and is introduced early in digestion in the mouth.

Proteases like trypsin are responsible for cleaving proteins. Carboxypeptidase is also involved in specific protein digestion. Lipases are involved in lipid and fat digestion. All of these enzymes are introduced in the duodenum of the small intestine, where the majority of chemical digestion occurs.

Bile is composed of bile salts, phospholipids, cholesterol, bilirubin, water and ions. Bile functions in the emulsification and absorption of lipids and fat-soluble vitamins, as well as cholesterol excretion. Bile also has antimicrobial activity via membrane disruption. In the duodenum, bile will orient hydrophobic regions around a lipid micelle and provide hydrophilic interaction with the surrounding environment. The micelle can then be transported to the lacteals for introduction to the lymphatic system and absorption.

Bile is not involved in protein digestion; this is the function of proteases, such as trypsin.

The road back to homeostasis starts with cells in the kidneys detecting decreased blood volume. The kidneys then secrete renin. Renin activates angiotensinogen (produced by the liver) in the blood. The renin-activated peptide is called angiotensin I and when it passes through the pulmonary circuit, it is converted to angiotensin II by angiotensin converting enzyme (ACE). Angiotensin II is a vasoconstrictor and it causes the adrenal glands to secrete aldosterone. Aldosterone acts directly on the kidneys to conserve sodium, which stimulates the release of antidiuretic hormone from the posterior pituitary, and thereby increase the blood volume and pressure.

The pancreas can secrete glucagon or insulin to either increase or decrease blood sugar, respectively, to achieve normal blood glucose levels, which are between . The other glands listed do not secrete hormones that directly affect the blood sugar. Note that cortisol, released from the adrenal cortex, increases blood sugar levels.

ADH, also known as vasopressin, stimulates water reabsorption in the kidneys. When the body is dehydrated it makes sense to reduce the amount of water it releases. This hormone is released from the posterior pituitary gland, but is made in the hypothalamus. FSH is involved in reproduction; it stimulates follicle maturation in women and spermatogenesis in males. Glucagon stimulates conversion of glycogen to glucose in the liver and somatostatin suppresses secretion of glucagon and insulin. Calcitonin decreases the blood calcium levels by storing it in bones.

The pituitary gland is under the control of the hypothalamus. The hypothalamus determines which and how much of each hormone the pituitary will release by secreting releasing and/or inhibiting factors. The pituitary gland is connected to the base of the hypothalamus by the pituitary stalk.

The JG cells are the cells present within the afferent arteriole of the renal glomerulus and have baroreceptors (pressure receptors) that detect changes in blood pressure entering the kidney. When blood pressure is too low, the JG cells release the hormone renin which triggers the RAS response (renin-angiotensin system).

The macula densa are osmoreceptor cells that detect the salt concentration within the renal lumen at the level of the ascending loop of the nephron. These cells are in close proximity to the JG cells to encourage the the JG cells to release renin if the salt concentration within the renal lumen becomes too low.

Aldosterone is a hormone produced by the the glomerulosa layer of the adrenal cortex. This hormone helps with salt reabsorption at the distal convoluted tubule of the kidney.

Erythropoietin is a hormone released by renal fibroblasts that helps to trigger the bone marrow to produce more erythrocytes (red blood cells) in response to anemia.

Intravascular hemolysis is defined as the destruction and lysis of red blood cells in circulation. The lysis of red blood cells causes the release large amounts of hemoglobin which is broken down into unconjugated bilirubin. This large amount of unconjugated bilirubin often saturates the ability for the liver to conjugate the bilirubin, which subsequently leads to hyperbilirubinemia in the patient. Hyperbilirubinemia is clinically manifested as jaundice (yellow discoloration of tissues).

The pancreas executes both exocrine and endocrine functions. One particular endocrine function is to release insulin from the beta cells of the pancreatic islets. Insulin functions to lower blood glucose and increase storage of glycogen.

The pituitary gland can be further divided into the anterior and posterior pituitary gland. The anterior pituitary gland creates and secretes its hormones, but the posterior pituitary gland does not create its hormones. Instead, the hormones secreted by the posterior pituitary gland are created in the hypothalamus, and are then sent to the pituitary gland. The two hormones made in the hypothalamus are antidiuretic hormone (ADH) and oxytocin.

Adrenocorticotropic hormone (ACTH), prolactin, and thyroid-stimulating hormone (TSH) are synthesized and secreted from the anterior pituitary.

The anterior pituitary produces six hormones: follicle-stimulating hormone, luteinizing hormone, prolactin, growth hormone, adrenocorticotropic hormone, and thyroid-stimulating hormone.

The posterior pituitary releases only two hormones: anti-diuretic hormone (vasopressin) and oxytocin.

The anterior pituitary gland releases seven hormones. Four of these are tropic hormones, meaning that they act on other glands to stimulate other hormone activity. These four are follicle-stimulating hormone, luteinizing hormone, adrenocorticotropic hormone, and thyroid-stimulation hormone. The other three secretions are prolactin, endorphins, and growth hormone.

The hormones of the anterior pituitary can be remembered using the mnemonic "FLAT PEG."

Oxytocin is released from the posterior pituitary, along with antidiuretic hormone (vasopressin).

Oxytocin is a hormone found in both males and females, but serves a more widely recognized role in females. It is released from the posterior pituitary gland, along with antidiuretic hormone (vasopressin) hormone. Oxytocin is responsible for stimulating contractions during childbirth and is one of only a few hormones to be controlled via positive feedback.