What is homeostasis and why is it important? What are hormones and how do they work? Flashcards
What is homeostasis?
The presence of a stable (not static) environment
What is the stimulus?
Increase or decrease in a variable
What is the receptor?
Detects the change in the variable
What is the control centre?
Compares the change to the set point and sends appropriate signals to effector organs
What are the effectors?
Organs and/or tissues which respond with various affects which may be localised or throughout the body to return the variable to set point
What is set point?
The value we would expect the variable to be at
What is normal range?
The range which the value fluctuates between as it takes time to return the variable to the set point
What does each individual have?
A normal range within which their levels of a given variable fluctuate
What does the population have?
A reference range based on the breadth of individual normal ranges within a population
What does the population range tend to be?
Wider than normal fluctuations within an individual. Therefore each individuals normal range is more narrow than the population range
What may moving outside the individuals normal range lead to?
Symptoms of a disorder, even when hormone levels are within the population reference range
What do most individuals have?
A set point and normal range that is within the population reference range
What is involved in synaptic (neural) control systems?
Action potentials in axons and neurotransmitters release at synapse
How is targeting achieved by neural control systems?
By specific wiring
What is the speed of transmission in neural control systems?
Fast to minimise response delays
What is the neural control system good for?
Brief responses
What is the chemical signal of the neural control system?
Neurotransmitters
What is the distribution of effects for the neural control system?
Limited to a very specific area, target cells must have appropriate receptors
What is involved in the endocrine (hormonal) control system?
Hormones released into the blood
How is targeting achieved by hormonal control systems?
The presence of specific receptors on target cells
What is the speed of transmission in hormonal control system?
Relatively slow but long lasting action (prolonged effect)
What is the hormonal control system good for?
Widespread and sustained responses
Where is transmission in the neural control system?
Across synapses
Where is transmission in the hormonal control system?
Through the bloodstream
What is the chemical signal of the hormonal control system?
Hormones
What is the distribution of effects for the hormonal control system?
To target cells mainly in other distant tissues and organs and must have appropriate receptors
What does the endocrine system consist of?
Endocrine gland cells which contain vesicles of hormones (chemical messengers) that are carried in the bloodstream to the target cells upon which they act
Where are target cells?
In other distant tissues or organs and they must have appropriate receptors
What does the hypothalamus do?
Links the nervous system to the endocrine system and controls the secretion of many endocrine glands
What are some major endocrine glands?
Hypothalamus, pituitary gland, thyroid gland, adrenal glands, pancreas (pancreatic islets) and parathyroid glands
What are some variables maintained homeostatic ally through hormones?
Blood sugar concentration, growth and repair, basal metabolic rate, blood calcium concentration and many more
What is involved in the homeostasis of one variable?
Often many hormones
What are hormones?
Chemical messengers produced in one location and transported via the bloodstream to a second location (target cells) where they cause a response in the cells
What is paracrine communication?
Through the extracellular fluid with paracrine as chemical signals which only effect cells in the immediate vicinity
What is autocrine communication?
Through the extracellular fluid with autocrines as chemical signals which come back and act on the cell which released it
What is meant by specificity of hormone action?
A hormone can only affect cells with specific receptors for that hormone. Each receptor is a protein which may be in the target cell membrane or inside the target cell
What are the classifications of hormones?
Water soluble and lipid soluble
What is the chemical classification of water soluble hormones?
Peptides (most abundant) and catecholamines
What is the chemical classification of lipid soluble hormones?
Steroids and thyroid hormones
What is the storage of water soluble hormones?
They are stored until required and then released by exocytosis
What is the storage of lipid-soluble hormones?
Steroids are made from cholesterol as required (not stored) and thyroid hormones are made in the thyroid cells and stored until required
What is the transport of water soluble hormones?
Dissolved in the blood
What is the transport of lipid soluble hormones?
Bound to carrier proteins
Where are the receptors of water soluble hormones?
On the cell surface because they cannot cross the cell membrane
Where are the receptors of lipid soluble hormones?
Intracellular receptors in the cytoplasm or nucleus because they can diffuse across the cell membrane
What is the mechanism of action of water soluble hormones?
Through second messengers
What is the mechanism of action of lipid soluble hormones?
By altering gene transcription
What is the speed of communication of water soluble hormones?
Milliseconds to minutes (neural is faster)
What is the speed of communication of lipid soluble hormones?
Hours to days
What is the first step in cellular response of water soluble hormones?
Water soluble hormone binds to cell surface receptor
What happens after the water soluble hormone binds to cell surface receptor?
Hormone binding allows the association of G-protein
What happens after association of G-protein?
Second messenger production/reduction
What happens after second messenger production/reduction?
Amplification of the cellular response (FAST)
What is the first step in cellular response of lipid soluble hormones?
Lipid soluble hormone dissociates from carrier protein
What happens after lipid soluble hormone dissociated from carrier protein>
Hormones diffuse across the cell membrane
What happens after hormones diffuse across the cell membrane?
Hormone binds to intracellular receptor in cytoplasm or nucleus
What happens after hormone binds to intracellular receptor in cytoplasm or nucleus?
Hormone-receptor complex acts as a specific transcription factor
What happens after the hormone-receptor complex acts as a specific transcription factor?
Target gene is activated
What happens after the target gene is activated?
new mRNA is generated
What happens after new mRNA is generated?
New protein is generated by translation of mRNA
What happens after a new protein is generated by translation of mRNA?
New protein mediates cell specific response (slow process)
What is the common mechanism for maintenance of hormone levels?
negative feedback
What is involved in negative feedback?
Reduce change until stimulus is removed or directly inhibit further use
What is the other mechanism for maintenance of hormone levels?
Positive feedback
What is included in positive feedback?
Amplification of change until a desired outcome is achieved
Why do hormone levels need to be maintained?
To maintain the effects of hormones
What does the amount of hormones in the blood depend on?
Rate of hormone secretion and rate of removal from blood
What is removal of hormones controlled by?
Enzymes in the blood or in target cells
What is secretion of hormones usually controlled by?
A negative feedback loop but sometimes positive feedback is used by a few hormones at specific times
What is the goal of hormones?
To maintains homeostasis
What can too much or too little hormone lead to?
Endocrine disorders
What is the pancreas?
An endocrine and exocrine gland
What part of the pancreas is an exocrine gland?
Cells of the pancreatic acini which secrete digestive enzymes
What part of the pancreas is an endocrine gland?
Pancreatic islets which make up approximately 1% of the mass of the pancreas
What do the pancreatic islets contain?
Beta cells which secrete insulin and alpha cells that secrete glucagon
What happens if blood glucose concentration is too high for too long?
Diabetes develops
What happens if blood glucose concentration is too low for too long?
Hypoglycemia occurs
What must the brain be supplied with and why?
Glucose at all times as it is the only fuel that the brain uses
What happens to blood glucose concentration throughout the day?
It changes as we use fuel continuously but we eat intermittently
What are the two metabolic states we move between?
Fed state and fasting state
What happens in the fed state?
Cellular uptake of nutrients & anabolic metabolism (synthesis of glycogen, protein and fat)
What happens in the fasting state?
Mobilisation of nutrients and catabolic metabolism (breakdown of glycogen, protein and fat)
What hormones maintain blood glucose concentration?
Insulin and glucagon
What is the range for blood glucose concentration?
70-100 mgdL-1
What happens if blood glucose concentration is increased?
Insulin is secreted from the beta cells to return the value to the reference range (decrease it)
What happens if the blood glucose concentration decreases?
Glucagon is secreted from the alpha cells to return the value to the reference range (increase it)
What is new glucose synthesis called?
Gluconeogenesis
What happens in gluconeogenesis?
fatty acids and amino acids are slowly converted into glucose
What is the quick release of glucose called?
Glycogenolysis
What happens in glycogenolysis?
Stored glycogen is broken down into glucose in a fast process
What is glucose storage?
The formation of glycogen
What happens first when blood glucose concentration increases?
Insulin is secreted from the pancreatic islet beta cells into the bloodstream. The beta cells are the receptor and control centre
Where does the insulin in the bloodstream go to?
The liver, muscle and adipose cells
What does insulin do at the liver?
Stops glucose output to increase the net glucose uptake (glycogen and fat synthesis)
What does insulin do at the muscle and adipose cells?
Increase glucose uptake, increase amino acid uptake, glycogen and protein synthesis as well as fat synthesis
What is the result of insulins effects on the liver, muscle and adipose cells?
Decreased blood glucose concentration
What is insulin?
A peptide hormone which targets cells that have membrane receptors
What does insulin do?
Increases fuel storage. It is the only hormone that can decrease the blood glucose concentration
What does glucagon do?
Increases fuel release
What is glucagon?
A peptide hormone that targets cells which have membrane receptors
What happens when blood glucose concentration decreases?
The pancreatic islet alpha cells secrete glucagon into the bloodstream. The alpha cells are the receptor and control centre
What is the target cells of glucagon?
Liver cells
What does glucagon do to the liver cells?
Increase the breakdown of glycogen (glycogenolysis), increase glucose synthesis (gluconeogenesis) and increase ketone synthesis which can be used as an energy source for non-brain tissue
What is the result of glucagon’s effects on the liver cells?
Increase blood glucose and blood ketones concentration
What can also increase blood glucose concentration?
Growth hormone, adrenaline and cortisol
