Chapter 1 - pt 1

Hormone

Chemical messengers secreted by endocrine glands

• transported in the blood

• influence the activity of their target tissues effectors

Target tissue

are specific cells, tissues, or organs that have unique receptors for particular hormones, acting like locks that only specific hormone "keys" can fit, triggering responses like metabolism changes in the liver/muscle (insulin) or calcium regulation (PTH on bone/kidneys).

Difference between endocrine and exocrine

Endocrine

• They don’t have ducts

• Hormones are secreted into the blood

Exocrine

• They have ducts

• Chemical produced by the gland

Compare the nervous system with the endocrine system

• Both are communication systems

• Similarities

  • Shared brain structures

    • Hypothalamus-regulates nervous system function (body temperature) and endocrine function (pituitary gland)

  • Shared chemical messengers

    • Norepinephrine- secreted by neurons and the adrenal medulla (endocrine gland)

  • Shared regulatory processes

    • Work together to regulate body processes

  • Shared signaling pathways

    • Neurotransmitters and hormones can affect target cells

Contrast the nervous system with the endocrine system

• Mode of transport

  • Nervous: directly signals to target cell by action potential and neurotransmitters

  • Endocrine: transmits signals in the bloodstream throughout the body

• Speed of response

  • Nervous: responds faster (milliseconds)

  • Endocrine: responds slower (seconds)

• Duration of response

  • Nervous: short response

  • Endocrine: longer lasting

• Modulation of signal intensity

  • Nervous: frequency modulated signals- depends on how often a signal is sent during a certain period of time

  • Endocrine: amplitude modulated signals- amount of signal (hormone concentration) produced

Define the four major categories of physical processes regulated by the endocrine system

• Growth and development

• Metabolism

• Blood composition

• Reproduction

Describe the four classes of chemical messengers

• Autocrine

  • secreted by cells in a local area; affects the activity of the same cell from which it was secreted

• Paracrine

  • produced by manny tissues and secreted into extracellular fluid; has a localized effect on nearby tissues

• Neurotransmitter

  • produced by neurons; secreted into synaptic cleft by presynaptic nerve terminal; travels short distances; affect postsynaptic cells

• Endocrine

  • secreted into the blood by specialized cells travels some distance to target tissues; results in coordinated regulation of cell function; called hormones

List and describe the three stimulatory on hormone secretion and give examples of each

• Humoral stimuli

  • fluids in the body

  • molecule in the bloodstream directly stimulates hormone release

  • molecules bond to receptors on the hormone secreting cell

  • Glucose, Ca²⁺, Na⁺

• Neural stimuli

  • neurotransmitter stimulate the hormone producing cell

  • neurotransmitters can also be inhibitory

  • some neurons are secreted

  • input from the sympathetic division of the autonomic nervous system on the adrenal medulla

• Hormonal stimuli

  • hormones stimulate the secretion of other hormones

  • includes neuropeptides

    • neurotransmitters released into the blood supply

    • releasing hormones released by hypothalamus

  • some hormones prevent secretion

    • inhibiting hormones released by the hypothalamus

    • negative feedback mechanisms (thyroid hormone)

    • Ex: tropic hormones-secreted by the anterior pituitary gland

Describe the three main patterns of hormone secretion

• Chronic

  • results in relatively constant blood levels of the hormone

  • thyroid hormone

• Acute

  • hormones concentration changes suddenly and irregularly

  • levels differ with each stimulus

  • epinephrine release in response to stress or physical exercise

• Episodic

  • hormone is secreted at fairly predictable intervals and concentrations

  • reproductive hormones

List and describe the two classes of hormones

• Lipid-soluble hormones

  • Hydrophobic

  • Ex: steroids (all cholesterol based), testosterone, aldosterone

• Water-soluble hormones

  • Hydrophilic

  • Proteins

    • thyroid stimulating hormone, growth hormone

  • Peptides

    • insulin, thyrotropin releasing hormone

Binding proteins

• Help to protect hormones (small hormones) from hydrolytic enzymes in the blood

  • increase the half life of the hormone

• Increase water solubility of hormone (lipid soluble hormones)

• Reversible binding

  • can act as a reservoir of hormone

Bound hormone

Hormone that is attached to the binding protein

• hormones bind specific binding proteins

Free hormone

Hormone detached from the binding protein

• some hormones are “always free” and some are “sometimes free”

Effect of binding proteins on circulating hormone levels

• Increase water solubility of hormone (lipid-soluble hormones)

• Reversible binding

  • can act as a reservoir of hormone

Explain the influence of the chemical nature of a hormone on its transport in the blood, its removal from circulation, and its life span

In the blood

• Two mechanisms

  • Negative feedback mechanism

    • regulates the levels of most hormones

    • hormone inhibits its grown secretion (self-limiting)

  • Positive feedback mechanism

    • hormone stimulates its own secretion

Removal from circulation

• Hormones are destroyed in circulation by enzymes at their target cells

• Limits the length of hormone activity

• Breakdown products are excreted in urine or bile

• Elimination of lipid-soluble hormones

  • conjugation-allows for lipid soluble hormones to be broken down

    • specific enzymes in the liver attach water soluble to the hormone

    • prevents the hormone from reentering bloodstream

    • excreted by kidneys (urine) and liver (bile)

• Elimination of water soluble hormones

  • broken dow by hydrolytic enzymes (proteases) in the bloodstream and kidney eliminates breakdown products

  • target cells degrade them and may recycle the amino acids

Hormone half-life

• Amount of time it takes for 50% of circulating hormone to be removed from circulation and excreted

• Larger, more complex hormones are more stable

• Smaller, simpler hormones are less stable

Describe the major mechanisms that maintain blood hormone levels

• Attached carbohydrates (glycoproteins)

• Modified terminal end to protect from protease activity

• Binding proteins