Anatomy & Physiology: Homeostasis, Cell Physiology, and Endocrine System Study Guide

Homeostasis and Body Fluid Compartments

Definitions and Conditions

Homeostasis refers to the maintenance of a stable internal environment despite external changes. The body regulates electrical, chemical, and osmotic conditions to achieve equilibrium.

• Electrical disequilibrium: Imbalance of ions (Na+, K+, Cl-) across cell membranes.

• Chemical disequilibrium: Unequal distribution of solutes (e.g., more K+ inside cells, more Na+ outside).

• Osmotic equilibrium: Balance of water and solutes; number of particles is equal across compartments.

The body has two main fluid compartments:

• Intracellular fluid (ICF): Fluid inside cells, high in potassium and low in sodium.

• Extracellular fluid (ECF): Fluid outside cells, high in sodium and low in potassium.

The blood plasma acts as a buffer zone between the outside world and most cells of the body.

Body Cavities

Body cavities are spaces that hold organs:

• Cranial cavity

• Thoracic cavity (pleural sac, pericardial sac)

• Abdominal cavity

• Pelvic cavity

Cell Structure and Function

Major Organelles and Their Functions

• Plasma membrane: Controls movement of substances in and out of cells; selectively permeable.

• Nucleus: Contains genetic material (DNA); controls cell activities.

• Endoplasmic reticulum (ER):

  • Rough ER: Site for protein synthesis.

  • Smooth ER: Synthesizes lipids; stores calcium.

• Golgi apparatus: Modifies and packages proteins into vesicles for transport.

• Mitochondria: Produces ATP through cellular respiration.

• Lysosomes: Contain digestive enzymes; break down waste and cellular debris.

Tissues of the Human Body

Main Tissue Types

• Epithelial tissue: Secretion and absorption; lines body cavities and organs. Structure: can be simple (single layer) or stratified (multiple layers).

• Connective tissue: Supports, connects, and protects other tissues. Includes blood, lymph, bone, cartilage, adipose tissue.

• Muscle tissue: Responsible for movement; contains actin and myosin.

• Nervous tissue: Processes and transmits electrical signals for communication and control.

Transport Across Cell Membranes

Types of Molecules and Transport Mechanisms

• Small, non-polar molecules pass freely through the lipid bilayer.

• Polar or large molecules require transport proteins.

Concentration Gradient

A concentration gradient is the difference in concentration of a substance across a space. Molecules move "down" the gradient (from high to low concentration) passively, or "up" the gradient (from low to high) actively, which requires energy.

Types of Membrane Transport

• Simple diffusion: Passive movement of molecules down their concentration gradient through the lipid bilayer; no energy required.

• Facilitated diffusion: Passive movement using transport proteins; no energy required.

• Active transport: Movement against the concentration gradient; requires energy (usually ATP).

Channels vs. Carrier Proteins

• Channels: Form open pores for specific ions or water; can be gated (chemical, voltage, mechanical).

• Carrier proteins: Bind and transport specific molecules; can be uniport (one substrate), symport (two substrates in same direction), or antiport (two substrates in opposite directions).

Receptors and Ligands

• Receptor: Protein that binds a specific ligand and triggers a cellular response.

• Ligand: Molecule that binds to a receptor.

• Agonist: Activates the receptor.

• Antagonist: Blocks the receptor without activating it.

Feedback Mechanisms

Negative Feedback

Negative feedback counteracts a change, maintaining homeostasis.

• Example: Blood glucose regulation: high glucose triggers insulin release, lowering glucose levels.

Positive Feedback

Positive feedback amplifies a change, moving the system away from equilibrium.

• Example: Uterine contractions during childbirth: contractions stimulate more contractions.

Endocrine System: Hormones and Disorders

Major Hormones and Their Functions

• Thyroid hormones: T3, T4, Calcitonin

• TSH (Thyroid Stimulating Hormone): Stimulates release of T3 and T4

• Adrenal gland hormones: Cortisol (glucocorticoid), epinephrine (catecholamine)

Hypothalamic-Pituitary Portal System

A direct vascular connection between the hypothalamus and anterior pituitary gland allows for rapid hormone signaling.

• Anterior pituitary: Releases stimulating hormones based on signals from the hypothalamus.

• Posterior pituitary: Releases vasopressin (ADH) and oxytocin.

Cortisol

• Liver: Promotes gluconeogenesis (synthesis of new glucose).

• Adipose tissue: Promotes lipolysis (breakdown of fats).

• Muscle: Promotes breakdown of proteins.

CRH (Corticotropin-Releasing Hormone)

• Released by hypothalamus in response to stress.

• Stimulates anterior pituitary to release ACTH, which stimulates cortisol release from adrenal cortex.

Thyroid Disorders

• Hyperthyroidism: Symptoms include high blood glucose, increased metabolism, weight loss, sweating, rapid heartbeat.

• Hypothyroidism: Symptoms include fatigue, weakness, weight gain, sensitivity to cold.

• Goiter: Enlargement of the thyroid gland due to iodine deficiency.

Growth Hormone (GH)

• Released by anterior pituitary; stimulates growth and metabolism.

• GH-releasing hormone (GHRH) stimulates GH release; somatostatin inhibits it.

• GH stimulates release of insulin-like growth factors (IGFs) from the liver.

Calcium Homeostasis

• Calcitonin: Lowers blood calcium by increasing bone uptake and decreasing kidney reabsorption.

• Parathyroid hormone (PTH): Raises blood calcium by increasing bone breakdown and kidney reabsorption.

Other Themes in Physiology

Key Terms

• Molarity: Moles/liter of solution

• Osmolarity: Moles of particles/liter of solution

• Cell signaling: Communication between cells via chemical messengers

Important Equations

• Molarity: $\text{Molarity} = \frac{\text{moles of solute}}{\text{liters of solution}}$

• Osmolarity: $\text{Osmolarity} = \frac{\text{moles of particles}}{\text{liters of solution}}$

HTML Table: Comparison of Fluid Compartments

Compartment	Main Ions	Location
Intracellular Fluid (ICF)	K+, Mg2+	Inside cells
Extracellular Fluid (ECF)	Na+, Cl-	Outside cells (includes plasma and interstitial fluid)
Blood Plasma	Na+, Cl-, proteins	Fluid portion of blood
Interstitial Fluid	Na+, Cl-	Between cells, outside blood vessels

HTML Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2 and CO2 across cell membrane
Facilitated Diffusion	No	Down gradient	Glucose via GLUT transporter
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump

Additional info:

• Some context and definitions were inferred for clarity and completeness.

• Tables were constructed to summarize key comparisons and classifications.