The Endocrine System Flashcards
Endocrine system
Influences metabolic activity by means of hormones transported in the blood
It acts with the nervous system to coordinate and integrate body activities
Responses slower but longer lasting than nervous system repsonses
Hormones
Chemical messengers secreted by ductless glands directly into the bloodstream to have an effect elsewhere in the body at its target tissue
Endocrinology
Study of hormones and endocrine organs
What does the endocrine system control and integrate?
Reproduction
Growth and development
Maintenance of electrolyte, water and nutrient balance of blood
Regulation of cellular metabolism and energy balance
Mobilisation of body defenses
Neurotransmitters
Released by axon terminals into synaptic junctions and act locally to control never cell functions
Endocrine hormones
Released into the blood and affect target cells at another location in the body
Neuroendocrine hormones
Secreted by neurons into the blood and affect target cells at another location
Paracrines
Secreted by cells into the extra cellular fluid and affect neighbouring target cells
Autocrines
Secreted by cells into the extracellular fluid and affect the function of the same cells that produced them
Cytokines
Peptides secreted into the extracellular fluid and can function as paracrines, autocrines, or endocrine hormones
Exocrine glands
Produces non-hormonal substances, such as swear and saliva, and have ducts that carry these substances to a membrane surface
Endocrine glands
Known as ductless glands
Procures hormones and lack ducts
Release their hormones into the surrounding tissue fluid and typically have a rich vascular and lymphatic drainage that receives their hormones
How are the hormone producing cells in endocrine glands arranged?
Arranged in cords and branching networks, which maximises contact between them and the surrounding capillaries
What are included in the endocrine glands?
Pituitary
Thyroid
Parathyroid
Adrenal
Pineal glands
What is considered a neuroendocrine organ?
Hypothalamus, along with its neural functions as it produces and releases hormones
What contains endocrine tissue?
Pancreas
Gonads (ovaries, testes)
Placenta
Water solubility in hormones
Affects:
how the hormone is transported in the blood
How long it lasts before it is degraded
What receptors can act upon it
What are hormones classified as?
Amino acid based or steroids
Amino acid based hormones
They are water soluble and cannot cross the plasma membrane
Molecular size varies widely from simple amino acid derivates to peptides to proteins
What is considered as simple amino acid derivates?
Biogenic amines (epinephrine and thyroxine)
Steroid hormones
Synthesised from cholesterol
Lipid soluble and can cross the plasma membrane
Which hormones are steroid based?
Gonadal
Adrenocortical
Eicosanoids
Include leukotrienes and prostaglandins
Nearly all cell membranes release these biologically active lipids made from arachidonic acid
Leukotrienes
Signalling chemicals that mediate inflammation and some allergic reactions
Prostaglandins
Have multiple targets and effects, ranging from raising blood pressure and increasing the expulsive uterine contractions of birth to enhancing blood clotting, pain and inflammation
What do eicosanoids act as?
Paracrines and autocrines
Because the effects of them are typically highly localised, affecting only nearby cells, influencing distant targets
Target cells
Cells that have receptors for the specific hormone
Hormones alter target cell activity
Water soluble hormones
All amino acid based hormones except thyroid hormone
Act on plasma membrane receptors
Act via G protein second messengers
Cannot enter cell
Lipid soluble hormones
Steroid and thyroid hormones
Act on intracellular receptors that directly activate genes
Can enter cell
What changes can hormones produce?
Alters plasma membrane permeability or membrane potential, or both by opening or closing ion channels
Stimulates synthesis of enzymes and other proteins within the cell
Activates or deactivates enzymes
Induces secretory activity
Stimulates mitosis
Second messenger systems
Amino acid based hormones except thyroid hormone, exert second messenger systems
There are two types: Cyclic AMP and PIP2-Calcium
Parathyroid gland
Tiny, yellow-brown glands embedded in posterior aspect of thyroid
Contains parathyroid cells that secrete parathyroid hormone (PTH), or parathormone
Parathyroid hormone
Most important hormone in Ca2+ homeostasis
Secreted in response to low blood levels of Ca2+
Inhibited by rising levels of Ca2+
Target organs are skeleton, kidneys and intestine
Parathyroid gland location
Low blood Ca2+
Hypocalcemia
Hyperparathyroidism
Due to parathyroid gland tumour
Calcium leaches from bones, causing them to soften and deform
Elevated Ca2+ depresses nervous system and contributes to formation of kidney stones
Hypoparathyroidism
Following gland trauma or removal can cause hypocalcaemia
Results in tetany, respiratory paralysis, and death
Adrenal gland
Paired pyramid shaped organs at top of kidneys
Also referees as suprarenal glands
Adrenal cortex
Three layers of cortical cells that synthesise and secrete different corticosteroids
Zona glomerulosa- mineralocorticoids
Zona fasciculata- glucocorticoids
Zona reticularis- gonadocorticoids
Adrenal medulla
Neural tissue that is part of sympathetic nervous system
Mineralocorticoids
Aldosterone is the most potent mineralocorticoid
Stimulates sodium ions reabsorption by kidneys
Results in increased blood volume and blood pressure
Stimulates potassium ion elimination by kidneys
Aldosteronism
Hypersecretion usually due to adrenal tumours
Results in 2 major Problems:
Hypertension due to excessive sodium ions reabsorption
Exertion of potassium ions, leading to abnormal non responsive neurons and muscle
Glucocorticoids
Keep blood glucose levels relatively constant
Maintain blood pressure by increasing action of vasoconstrictors
Help us resist stressors
Gonadocorticoids (adrenal sex hormones)
Weak androgens (male sex hormones) converted to testosterone in tissue cells, some to oestrogens
Cyclic AMP signaling mechanism
- Hormone binds to receptor
- Hormone binding causes the receptor to change shape, allowing it to bind a nearby inactive G protein
- G prtoein activates adenylate cyclase
- Adenylate cyclase converts ATP to cyclic AMP
- Cyclic AMP activates protein kinases
PIP2- Calcium signlaing mechanism
Involves a G protein and a membrane bound effector called phospholipase C
Phospholipase C spilts a plasma memebrane phospholipid called PIP2 into 2 second messengers- diacylglycerol and inositol trisphophate
DAG activates a protein kinase enzyme, which triggers responses within the target cell
IP3 causes Ca2+ release from intracellular storage sites
How do calcium ions act as second messengers in cells?
Calcium ions (Ca²⁺) alter enzyme activity and channels or bind to regulatory proteins like calmodulin. When calcium binds to calmodulin, it activates enzymes that amplify the cellular response.
What happens when PIP₂ is hydrolyzed?
PIP₂ is broken down into DAG and IP₃ by phospholipase C.
DAG activates protein kinase C.
IP₃ stimulates calcium release from the endoplasmic reticulum, leading to enzyme activation via calmodulin.
How do lipid-soluble hormones activate gene transcription?
The hormone diffuses through the plasma membrane and binds an intracellular receptor.
The receptor-hormone complex enters the nucleus.
It binds to a specific DNA region (HRE).
This initiates transcription of the gene to mRNA.
The mRNA directs protein synthesis.
Which hormones use intracellular receptors?
Lipid-soluble hormones such as steroid hormones (e.g., estrogen, testosterone) and thyroid hormones.
What is the difference between positive and negative feedback in hormonal control?
Negative feedback prevents hormone overactivity by inhibiting hormone release (e.g., thyroid hormone regulation).
Positive feedback amplifies hormone release in response to a stimulus (e.g., oxytocin during childbirth).
Describe the negative feedback mechanism of thyroid hormones.
The hypothalamus releases TRH.
TRH stimulates the anterior pituitary to release TSH.
TSH stimulates the thyroid gland to produce thyroid hormones.
Increased thyroid hormone levels inhibit TRH and TSH release, maintaining homeostasis.
How does positive feedback regulate oxytocin release during childbirth?
The fetus and pituitary gland release oxytocin.
Oxytocin stimulates uterine contractions.
Contractions stimulate the placenta to release prostaglandins.
Prostaglandins enhance contractions, further increasing oxytocin release.
What molecular pathways are activated by oxytocin?
Oxytocin binds to its receptor, activating the Gq protein.
This leads to PLC activation, converting PIP₂ into DAG and IP₃.
IP₃ triggers Ca²⁺ release, activating calmodulin and MLCK.
This results in contraction and MAPK cascade activation.
What are the three types of endocrine gland stimuli?
Humoral stimuli
Neural stimuli
Hormonal stimuli
