Homeostasis and Endocrine System

Isabelle Chartrand
Mind Map by , created about 6 years ago

Module 1 - Introductory Human Physiology

Isabelle Chartrand
Created by Isabelle Chartrand about 6 years ago
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Homeostasis and Endocrine System
1 Definition: Integrative science that studies the functions of complex living organisms at levels ranging from molecules and cells to organs and systems.
1.1 Objective: Develop a cohesive picture of how organs systems coordinate to maintain life in a constantly changing environment.
2 Classification
2.1 Differentiated cells are cells specialized for a specific function.
2.2 Tissues are groups of cells which carry out related functions.
2.2.1 Epithelium tissue
2.2.2 Muscle tissue
2.2.3 Nervous tissue
2.2.4 Connective tissue
2.3 Organs are functional units formed by different tissues.
2.4 Organ systems include several organs that act in an integrated manner to perform a specific function.
2.4.1 Cardiovascular system
2.4.2 Respiratory system
2.4.3 Digestive system
2.4.4 Endocrine system Coordinates and integrates cellular activity within the whole body by chemical (hormones) delivered by blood Specific missions are to: Regulate sodium and water balance Regulate calcium balance Regulate energy balance Coordinate processes that cope with stressful environments Coordinate growth and development Coordinate processes associated with reproduction and lactation Hormones are secreted into the blood stream and circulate through the body at very low concentrations Rate of production mediated by + and - feedbacks Rate of delivery depends on perfusion and blood flow and follows carriers Types of hormones Peptide hormones Consist of 3 or more amino acids Soluble in blood Synthesized, packaged and transported as other secretory proteins Initially synthesized as preprohormones In rough endoplasmic reticulum form the prohormones In the Golgi, packaged and trimmed into active hormones and stored in vesicles Secreted in burst in response to stimuli Degraded by liver and cleared by kidney Half-life of a few minutes Bind to cell surface receptors which activate a second messenger Provides amplification: one hormone molecule can generate thousand of copies of second messenger and thereby affect many copies of responsive target molecules in the cell Provides memory: once activated, the second messenger stays on for several seconds to minutes Provides complex regulation: multiple pathways can be initiated by binding one hormone to a single receptor type Rapid change in function/metabolism (seconds) Steroid hormones Derived from cholesterol Insoluble in blood Made in the adrenal cortex, gonads and in placenta of pregnant women Synthesized on demand and are not stored Circulate bound to protein carriers Half-life extended by protein carrier (60-90 minutes) Bind to intracellular receptors to activate transcription Synthesis of new proteins = slow response (30 minutes or more) Amine hormones Derivatives of amino acids Some are soluble in blood Thyroid hormones Not soluble in blood Circulate bound to protein carriers Half-life extended by protein carrier (several days) Bind to intracellular receptors to activate transcription Synthesis of new proteins = slow response (30 minutes or more) Catecholamines Epinephrine = hormone Norepinephrine = hormone and neurotransmitter Duration of the response depends on how long the hormone is available Adaptation or desensitization of receptors: diminishes the cell's response when the level or hormone is chronically elevated Rebound: increased sensitivity of the target cell to a hormone due to its prolonged absence (target cell increases its receptors numbers and is therefore hyperactive with return of hormone) Hormone secretion is governed by three types of regulatory stimuli (neural, hormonal and nutrient) Stimuli activates reflex loop in which the endocrine cell is the effector Other interactions include: Feed forward: primes a target tissue (ex: smell of food cause secretion of gastrin by the stomach, which stimulates HCl secretion from the fundic stomach Synergy: net effect of several hormones acting on a target tissue causes a bigger response than simple addition of each independant hormonal effect Reinforcement: actions of a single hormone on multiple tissues converge to regulate a process (such as glucose production) Permissiveness: actions of a tropic hormone sensitize the target tissue to a second hormone Inactivation of the hormone signalling Cellular inactivation involves the termination of the receptor's response Systemic inactivation involves homeostatic control mechanisms such as negative feedback which removes the initial stimulus Disorders of the endocrine system Hyposecretion: insufficiency of hormone Hyper-secretion: excess of hormone Resistance: abnormal target organ response due to receptor and/or second messenger dysfunction despite adequate hormone levels Primary pathology: target endocrine gland itself is misbehaving Secondary pathology: problem occurs in regulating gland (ex: pituitary) Tertiary pathology: due to the most distal regulator (hypothalamus) Measurements of hormone levels Requires very high sensitivity and high specificity - because hormones are present at very low concentrations in body fluids Radio-immune assay and enzyme-linked immunosorbent assay measure the immuno-reactivity of the hormone in the body fluid Bioassay measures the biological activity to a challenge dose of hormone or substate such as glucose Certain hormones are secreted at regular intervals, so randomly taken sample may have assayed either a peak or a trough
2.4.5 Immune system
2.4.6 Integument system
2.4.7 Musculoskeletal system
2.4.8 Nervous system
2.4.9 Reproductive system
2.4.10 Urinary system
2.5 60% of body weight = WATER
2.5.1 Fluid compartments 2/3 ICF: Intracellular fluid is the cytoplasm within cell. High concentrations of K+ Low concentrations of Na+ Low concentrations of free Ca++ Concentration of phosphates and proteins higher than ECF Reducing environment 1/3 ECF: Extracellular fluid surrounds the cell and serves as buffer. 1/4 IVF: Intravascular fluid (plasma, lymph, cerebral spinal fluid). IVF = higher protein content 3/4 ISF: Interstitial fluid bathes the outside of the cell. Capillaries that separate IVF and ISF are leaky = composition of IVF and ISF essentially identical. Oxidizing environment Low concentrations of K+ High concentrations of Na+ High concentrations of free Ca++ Site of exchange where nutrients are delivered and cellular wastes removed. Composition of ICF and ECF differ due to the hydrophobic nature of cell membrane which prevents free exchange of ions and proteins. Self-regulatory mechanisms allows us to adapt to a changing environment. Steady state Passive leak of K+ across the plasma membrane, allowing K+ ions to move from the inside of cells to the outside. Leak matched by pumping K+ back into the cell via the Na+ K+ ATPase Requires energy (ATP) Each cycle of ATPase = 2 K+ exchanged for 3 Na+ and one molecule of ATP is hydrolyzed to ADP. When K+ is pumped into cells, Na+ is pumped out. Uneven distribution of Na+ and K+ across the plasma membrane = chemical gradient Also establishes a charge gradient = inside of the cell more negative relative to the outside of the cell. Intracellular and extracellular concentrations of Na+ and K+ are NOT EQUAL. Condition in which the amount or concentration of a substance is constant within a compartment and does not change with time. Input and output are EQUAL. Equilibrium No energy expenditure required Opposing forces are balanced Will occur if there is sufficient time for exchange and no barrier to movement from one compartment to the other. Homeostasis Maintenance of the ECF as a steady state. Composition of ECF changes with time, but certain factors must be kept within a narrow range for optimal functioning of cells, tissues and organs. Oxygen Carbon dioxyde Glucose Osmotic pressure Concentrations of H+, Ca++, K+, Mg+ Temperature Uncorrected deviations may lead to disease or death. Homeostatic regulation Local controls Autocrine (self-to-self) Paracrine (between neighbours) Cytokine is secreted in both cases Reflex controls Involves endocrine and nervous systems Responds to changes that are more widespread or systemic in nature Three basic components Input stimulus Integrator of the stimulus Integration center = BRAIN 1. Evaluates the incoming signal 2. Compares it with a set point (desired value) 3. Decides on the appropriate response Response (effector) Made at a distance from the target cell or tissue 4. Carries out the response to bring situation back to within normal limits Negative feedback loops Response removes the stimulus Allows the system to resist deviation of a given parameter from a preset range Most common form of homeostatic control Positive feedback loops Response reinforces the stimulus Used to elicit a change during development or maturation Finite loop: negative feedback will reduce or terminate these responses Feed-forward control Enables the body to anticipate a change and start a reflex loop Example: sight or smell starts mouth to water Tonic control Regulated by modulation (up/down) rather than by on/off switches Regulator of blood flow to the organs Antagonistic control Modulates the activity of an organ system by two separate regulators that act in opposition Example: sympathetic and parasympathetic neurons control of heart rate Circadian rhythms Allows control systems to fluctuate in a predictable, timed manner over a 24 hours cycle as their set points change. Govern many biological functions Blood pressure Body temperature Metabolic processes Arise from hypothalamus which is programmed by either the light-dark or the day-night cycle by input from our retina or our sleep(rest)/activity periods When the circadian clock is altered (ex: jet lag), temperature rhythm and the secretion of various hormones are also altered. It is possible to reset the set points Chemical or neurotransmitter is secreted in the bloodstream Receptor on the target cell recognize the chemical and binds to cause the target cell to have change in its activity Endocrine hormone is secreted from endocrine cell and reaches the site of action via the blood circulation Neuroendocrine hormone is secreted from neuron into the blood to act at a distance Physiopathology: input > output OR input < output in ECF There is a hierarchy of importance in the maintenance of life: brain and heart are ALWAYS prioritized Tonicity is used to compare two fluid compartments Refers to the concentration of non-penetrating solutes only Hypertonic cell: If a cell has a higher number of non-penetrating solutes Hypotonic cell: If a cell has a lower number of non-penetrating solutes Isotonic cell: cell and solution have the same number of non-penetrating solutes
2.5.2 Mass balance in the body refers to a steady state in which the total amount of substance = intake of X + production of X - output of X
2.5.3 Mass flow is mass balance over time.
2.5.4 Solute and water flow To maintain normal body function: importance of gradients and movements of solutes and water across barriers. Biological membranes = bilayers of lipids Restrict the movement of water soluble molecules (ex: ions and glucose) from entering cell) Lipid soluble molecules cross membranes easily Diffusion is the passive movement of material down their concentration gradient Simple diffusion: membrane permeable (lipid soluble) molecules cross membranes Occurs from an area of higher to one of lower concentration Do not require energy expenditure Continues until equilibrium is reached Occurs rapidly over short distance and slowly over long distance Directly related to temperature: molecules move faster at higher temperature Inversely related to the size of the molecule (larger molecules move slower) Facilitated diffusion: membrane impermeable (hydrophilic) molecules have restricted entry and may not enter cell at all Do not require energy expenditure Occur with the aid of a transport protein Transporters Transporters protein bind to specific molecules Carry molecules across the membrane by changing conformation Never form a direct connection between ICF and ECF Co-transporters Bind two or more molecules Symporters: move molecules in the same direction across the membrane Antiporters: move molecules in opposite direction across the membrane Channels Create a water filled passageway that connects ICF and ECF Open and closed state of the channel is determined by a part of the channel that acts as a gate Ligands gated (chemically) Voltage gated (electrical state of the cell) Mechanically gated (tension) Process exhibit specificity, competition and saturation Glucose enters most cells by facilitated diffusion Active transport moves a molecule against its concentration gradient Requires energy output (ATP) Active transporters are called pumps Exhibit specificity, competition and saturation Secondary active transport couples two processes Primary active transport Uses ATP as energy Transporters are enzymes with ATPase activity Generate an intracellular ion gradient, usually Na+ Allow entry of a solute by either a co-transporter or single channel Facilitated transport Molecules are moved across the whole cell
2.5.5 Osmosis is the movement of water across membranes Facilitated diffusion Water channel is called aquaporin Water is never actively transported Water flows from compartments with dilute solution (where concentration of water is high) to concentrated solution (where concentration of water is low) Occurs until the concentrations of both water and solute are equal in both compartments Non-penetrating solutes (ex: Na+) determine the movement of water Base for maintaining cell size at rest and protect against cell shrinkage or swelling Penetrating solutes (ex: urea, steroid hormones) diffuse freely so no net movement of water will occur
2.5.6 Osmolarity is the number of particles per liter of solution Isosmotic solution: both compartments A and B have the same number of solute particles Hyperosmotic solution: when solution A has a higher number of solute particles than solution B, solution A is hyperosmotic Hyposmotic solution: when solution A has a lower number of solute particles than solution B, solution A is hyposmotic Osmolarity of the human body is about 300 mOsM

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