Hormones

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Definitions

Endocrine gland: A gland that secretes hormones directly into the blood. Endocrine glands have no ducts.

Exocrine gland: A gland that secretes molecules into a duct that carries the molecules to where they are used.

Hormone: A molecule released into the blood which acts as a chemical messenger.

Target tissue: A group of cells that have receptors embedded in the plasma membrane that are complementary in shape to specific hormone molecules. Only these cells will respond to the specific hormone.

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First messenger and second messenger

The first messenger is the hormone that transmits a message around the body, e.g. adrenaline. The second messenger, e.g. cAMP, transmits a signal inside the cell. The cAMP acts by activating enzymes.

Adrenaline has different effects on different tissues because different tissues have different types of adrenaline receptors causing cAMP concentration to increase or decrease. cAMP may activate different enzymes in different target cells. Also the second messenger may be different, causing different effects.

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Adrenal glands

The adrenal glands have two distinct regions - the crotex region and the medulla region. The adrenal medulla releases adrenaline, which:

  • Relaxes smooth muscle in the bronchioles
  • Increases the stroke volume of the heart
  • Increases heart rate
  • Causes general vasoconstriction - raising blood pressure
  • Stimulates conversion of glycogen to glucose (glycogenolysis)
  • Dilates the pupils
  • Increases mental awareness
  • Inhibits the action of the gut
  • Causes body hair to erect

The adrenal cortex releases corticosteroid hormones which are made from cholesterol.

  • Mineralocorticoids help control the concentrations of Na and K in the blood
  • Glucocorticoids help control the metabolism of carbohydrates and proteins in the liver
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Histology

The exocrine cells of the pancreas secrete digestive enzymes into the pancreatic duct, which transports them to the small intestine. These cells make up the majority of the pancreas.

The endocrine cells are found in the Islets of Langerhan and consist of alpha and beta cells. The alpha cells manufacture and secrete glucagon, whereas beta cells manufacture and secrete insulin. They are involved in the regulation of blood glucose levels. 

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Regulating blood glucose concentration

If blood glucose concentration drops too low:

  • Detected by alpha cells which inhibit insulin production
  • They secrete glucagon into the blood
  • Glucagon binds to receptors on hepatocytes and causes: glycogenolysis (conversion of glycogen to glucose), more fatty acids are used in respiration, gluconeogenesis (converstion of amino acids and fats to glucose)
  • More glucose is released into the blood stream, it leaves cells by facilitated diffusion

If blood glucose concentration rises too high:

  • Detected by beta cells which inhibit glucagon production
  • They secrete insulin into the blood
  • Insulin binds to receptors on hepatocytes, in the liver
  • This activates adenyl cyclase in the cell
  • This converts ATP to cAMP
  • The cAMP activates a series of enzyme catalysed reactions within the cell: more glucose channels (transport proteins) are placed in the cell surface membrane, more glucose enters the cell, glycogenesis (glucose in the cell is converted to glycogen), more glucose is converted to fats, more glucose is used in respiration
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Glycogen and glucagon

Glycogen:

  • Type of compound: carbohydrate or polysaccharide
  • Role of compound: storage, to provide glucose, can undergo glycogenolysis
  • Site of production: Liver, hepatocytes

Glucagon:

  • Type of compound: hormone or polypeptide or protein
  • Role of compound: binds to cell receptor, causes conversion of glycogen to glucose, stimulates glycogenolysis, increases blood glucose concentration
  • Site of production: pancreas or Islets of Langerhans or alpha cells
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Control insulin secretion

1. The cell membranes of the beta cells contain Ca2+ and K+ ion channels.
2. The K ion channels are normally open, and the Ca ion channels are normally shut. K ions diffuse out of the cell, making the inside more negative.
3. When glucose concentration outside of the cells is high, more glucose molecules diffuse into the cell.
4. The glucose is quickly metabolised to ATP.
5. The extra ATP causes the K ion channels to close.
6. The K ions can no longer diffuse out, so the cells become more positive inside.
7. This change in potential difference opens to Ca ion channels.
8. Ca2+ ions enter the cell and cause the secretion of insulin by making the vesicles containing insulin to move to the cell surface membrane and fuse with it, releasing insuling by exocytosis.

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Diabetes

Type I diabetes: The body is unable to produce (enough) insulin, does not secrete insulin, produces ineffective insulin. The insulin-producing cells (beta cells) are destroyed by the body's own immune system. This is an auto-immune disease. It can be genetic. It can be triggered by a virus.

Type II diabetes: Body can produce insulin but insulin receptors lose the ability to detect and respond to insulin. Treatment - monitoring and controlling diet. Late onset (more prevalent over 40). Risk increased by:

  • Increasing age
  • Family history/ genetic/ hereditary
  • Being male
  • Being African/ Afro-Caribbean/ Asian/ Hispanic/ Oceanic
  • Being obese/ overweight/ having fat around abdomen
  • High/ frequent, intake of sugar/ highly processed food/ high GI food
  • Lack of physcial activity/ sedentary lifestyle
  • High blood pressure
  • Excessive alcohol intake
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Treating diabetes mellitus

Genetically modified bacteria

  • Exact copy of human insulin: faster acting, more effective
  • Less chance of developing tolerance
  • Less change of rejection
  • Cheaper
  • More adaptable to demand
  • Less likely to have more objections

Stem cells:

  • Could be used to produce new beta cells
  • Scientists have found stem cells in the pancreas of adult mice
  • Undifferentiated
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Control of heart rate

Action potentials sent down the accelerator nerve to the heart from the cardiovascular centre of the medulla oblongata so the sinoatrial node cause the heart rate to increase. As the SAN controls the frequency of the waves of depolarisation in the heart these impulses will speed up the heart rate. An increase may be required because of:

  • A drop in pH detected by chemoreceptors in the carotid arteries, the aorta and the brain (when we exercise we produce carbon dioxide, this dissolves in water in the blood and forms carbonic acid, reducing the pH)

Action potentials sent down the vagus nerve decrease the heart rate. This may be because of:

  • Blood pressure rising which is detected by baroreceptors

The presence of adrenaline increases the heart rate to prepare the body for activity.

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