Body Organization, Tissues, and Homeostasis

Body Organization and Tissue Types

• Body organization progresses from cells to tissues, then to organs, and finally to organ systems
• The four main tissue types are epithelial, connective, nervous, and muscle
• Epithelial tissue acts as a barrier and aids in absorption
• Connective tissue provides support
• Nervous tissue enables communication
• Muscle tissue facilitates movement

Homeostasis

• Homeostasis is maintaining a stable internal environment, not necessarily an equilibrium
• Claude Bernard introduced the concept of a constant internal environment
• Walter Cannon coined the term "homeostasis," to describe stable conditions achieved with variability
• Examples of homeostatically controlled variables include body temperature, blood glucose, pH, fluid volume, and O2/CO2 levels
• Negative feedback is the most common regulatory method, opposing changes to maintain set points
• Positive feedback is rare and amplifies changes, like oxytocin's role in childbirth

Membrane Transport

• Passive transport does not require energy and includes diffusion and facilitated diffusion
• Active transport requires ATP, such as the Na+/K+ pump
• In simple diffusion, solutes move from areas of high concentration to low concentration
• Facilitated diffusion uses carrier or channel proteins, like glucose transporters
• Primary active transport directly uses ATP
• Secondary active transport uses ion gradients to move substances
• Osmosis involves water moving toward higher solute concentration
• Tonicity describes a solution's effect on cell volume
• Isotonic solutions cause no change in cell volume
• Hypotonic solutions cause cells to swell and potentially burst
• Hypertonic solutions cause cells to shrink
• Membranes are composed of a phospholipid bilayer with hydrophilic heads and hydrophobic tails
• Integral and peripheral proteins in the membrane facilitate transport and signaling

Nervous System

• A neuron consists of dendrites (input), a soma (cell body), an axon (output), and an axon terminal (neurotransmitter release)
• Myelination by Schwann cells (PNS) or oligodendrocytes (CNS) increases signal speed
• An action potential involves a series of steps:
• Depolarization occurs as Na+ channels open, and Na+ enters the cell, making it more positive
• At the peak, Na+ channels inactivate, and K+ channels open
• Repolarization happens as K+ exits, and the cell returns to its resting state
• Hyperpolarization occurs as K+ channels close slowly, making the cell more negative than at rest
• The Na+/K+ pump restores resting potential at -70mV
• During the absolute refractory period, no new action potential is possible because Na+ channels are inactivated
• During the relative refractory period, a stronger stimulus is needed to trigger an action potential because some Na+ channels are recovering
• Electrical synapses feature direct ion flow through gap junctions and are fast
• Chemical synapses use neurotransmitters in the synaptic cleft and are slower
• Excitatory neurotransmitters (EPSP) cause Na+ entry and depolarization, such as glutamate
• Inhibitory neurotransmitters (IPSP) cause Cl- entry or K+ exit and hyperpolarization, such as GABA
• Temporal summation involves rapid signals from one synapse adding up
• Spatial summation involves multiple signals from different synapses combining

Endocrine System

• Peptide hormones are hydrophilic, use surface receptors, and act via second messengers, like insulin and ADH
• Steroid hormones are lipophilic, use intracellular receptors, and act on DNA, like cortisol and testosterone
• Amine hormones are derived from tyrosine and can be hydrophilic (catecholamines) or lipophilic (thyroid hormones)
• The posterior pituitary stores hormones from the hypothalamus, including ADH (water retention) and oxytocin (uterine contractions, milk ejection)
• The anterior pituitary releases hormones that affect various targets:
• TSH stimulates the thyroid to produce T3/T4, regulating metabolism
• ACTH stimulates the adrenal cortex to produce cortisol, involved in stress response
• GH promotes growth in the liver and bones
• FSH/LH act on gonads to produce sex hormones
• Prolactin stimulates mammary glands for milk production
• T3 and T4 increase metabolic rate, ATP production, and heat generation
• Iodine is required for T3 and T4 synthesis
• Hypothyroidism results in low energy, weight gain, and cold intolerance (e.g., Hashimoto’s)
• Hyperthyroidism results in high metabolism, weight loss, and heat intolerance (e.g., Graves’ Disease)
• The adrenal medulla releases epinephrine for the fight-or-flight response
• The adrenal cortex releases cortisol (stress hormone, increases glucose availability) and aldosterone (salt/water balance, raises blood pressure)
• Insulin lowers blood glucose levels and is produced by beta cells in the pancreas
• Glucagon raises blood glucose levels and is produced by alpha cells in the pancreas
• Type 1 diabetes is an autoimmune condition with no insulin production
• Type 2 diabetes involves insulin resistance (fewer receptors)
• PTH (parathyroid hormone) increases blood calcium by promoting bone breakdown, kidney reabsorption, and vitamin D activation
• Calcitonin lowers blood calcium by promoting bone storage

Key Concepts

• Focus on homeostasis and negative feedback mechanisms for temperature, glucose, and pH regulation
• Understand membrane transport processes like diffusion, osmosis, and active transport
• Learn the stages of the action potential and synaptic transmission
• Review endocrine system regulation, including hormone types and feedback loops
• Study the functions of the thyroid and adrenal glands in metabolism and stress response
• Grasp blood glucose regulation and the roles of insulin and glucagon
• Understand calcium homeostasis and the opposing functions of PTH and calcitonin

Exam Specifics

• Glucagon secretion decreases in response to increased blood glucose levels after a meal; insulin secretion increases
• Salt water balance causes increased salt retention in the body
• ADH is synthesized in the hypothalamus and stored in the posterior pituitary for release
• Graded potentials can be either excitatory or inhibitory
• Depolarization of an axon during an action potential is caused by the inward diffusion of Na+
• Myelination increases action potential conduction velocity by allowing the AP to jump down the axon
• Reducing the concentration gradient that drives Na+ influx would hyperpolarize the cell, decrease the depolarizing graded potential, and result in a smaller, slower action potential