Chapters 24–26: Metabolism, Urinary System, and Fluid/Acid-Base Balance – Study Guide

Chapter 24: Nutrition, Metabolism, and Energy Balance

Metabolic Pathways

Metabolism refers to all chemical reactions occurring in the body, including those that break down nutrients to produce energy and those that build complex molecules. The main metabolic pathways for energy production are glycolysis, the Krebs cycle, and the electron transport chain.

• Glycolysis: The breakdown of one glucose molecule (C6H12O6) into two molecules of pyruvic acid, producing a net gain of 2 ATP and 2 NADH.

• Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondrial matrix; each acetyl-CoA is oxidized, producing CO2, NADH, FADH2, and 1 ATP per cycle.

• Electron Transport Chain (ETC) & Oxidative Phosphorylation: NADH and FADH2 donate electrons to the ETC, generating a proton gradient used to produce ATP. Oxygen is the final electron acceptor, forming water.

• Aerobic Respiration: The complete oxidation of one glucose molecule yields approximately 30–32 ATP.

Example: Glycolysis breaks down glucose to yield 2 pyruvic acid and 2 ATP.

Carbohydrate Metabolism

• Glycolysis: Glucose → 2 Pyruvate + 2 ATP + 2 NADH

• Glycogenolysis: Breakdown of glycogen to glucose (mainly in liver and muscle).

• Gluconeogenesis: Formation of glucose from non-carbohydrate sources (e.g., amino acids, glycerol).

• Glycogenesis: Formation of glycogen from glucose for storage.

Comparison Table:

Process	Definition	Main Location
Glycolysis	Glucose breakdown to pyruvate	Cytoplasm
Glycogenolysis	Glycogen breakdown to glucose	Liver, muscle
Gluconeogenesis	Glucose synthesis from non-carbs	Liver
Glycogenesis	Glucose to glycogen storage	Liver, muscle

Lipid Metabolism

• Lipolysis: Breakdown of triglycerides into glycerol and fatty acids for energy.

• Lipogenesis: Synthesis of triglycerides from acetyl-CoA and fatty acids for storage.

Example: During fasting, lipolysis provides fatty acids for energy.

Metabolic Importance of the Liver

• Central organ for metabolism: processes carbohydrates, fats, and proteins.

• Stores glycogen, synthesizes plasma proteins, detoxifies substances, and produces bile.

Calories in Nutrients

• Carbohydrates: 4 kcal/g

• Proteins: 4 kcal/g

• Fats: 9 kcal/g

• Water: 0 kcal/g (no caloric value)

Macromolecules in the Body

• Carbohydrates: Glucose, glycogen

• Fats: Triglycerides, phospholipids

• Proteins: Enzymes, structural proteins (e.g., collagen)

Chapter 25: The Urinary System

Kidney Structure and Function

The kidneys filter blood, remove waste, and regulate fluid and electrolyte balance. Each kidney contains about 1 million nephrons, the functional units responsible for urine formation.

• Major Structures: Cortex, medulla, renal pelvis, calyces, renal artery and vein, ureter.

• Nephron Anatomy: Glomerulus, Bowman's capsule, proximal convoluted tubule (PCT), loop of Henle (descending and ascending limbs), distal convoluted tubule (DCT), collecting duct.

Types of Nephrons:

• Cortical Nephrons: 85% of nephrons; short loops of Henle; primarily in cortex; involved in most reabsorption and secretion.

• Juxtamedullary Nephrons: Long loops of Henle extend deep into medulla; important for concentrating urine.

Capillary Networks

• Glomerular Capillaries: Site of filtration; high pressure forces plasma into Bowman's capsule.

• Peritubular Capillaries: Surround PCT and DCT; involved in reabsorption and secretion.

• Vasa Recta: Specialized capillaries around the loop of Henle in juxtamedullary nephrons; maintain medullary osmotic gradient.

Bladder Anatomy

• Muscular sac for urine storage; lined by transitional epithelium.

• Trigone region: defined by openings of ureters and urethra.

Nephron Function: Filtration, Reabsorption, Secretion

• Filtration: Occurs in glomerulus/Bowman's capsule.

• Reabsorption: Movement of substances from filtrate back into blood.

• Secretion: Movement of substances from blood into filtrate.

Sites of Reabsorption and Secretion:

Segment	Main Function
PCT	Reabsorbs most water, Na+, glucose, amino acids
Descending Loop of Henle	Reabsorbs water (permeable to water, not solutes)
Ascending Loop of Henle	Reabsorbs Na+, Cl- (impermeable to water)
DCT	Reabsorbs Na+, Cl-; secretion of K+, H+
Collecting Duct	Variable water reabsorption (regulated by ADH)

Hormonal Regulation

• Renin: Released by juxtaglomerular cells in response to low blood pressure; initiates the renin-angiotensin-aldosterone system (RAAS).

• Angiotensin II: Vasoconstrictor; stimulates aldosterone and ADH release.

• Aldosterone: Increases Na+ reabsorption in DCT and collecting duct.

• Antidiuretic Hormone (ADH): Increases water reabsorption in collecting duct.

Chapter 26: Fluid, Electrolyte, and Acid-Base Balance

Body Fluid Compartments

• Intracellular Fluid (ICF): Fluid within cells (~2/3 of body water).

• Extracellular Fluid (ECF): Fluid outside cells (~1/3 of body water); includes plasma and interstitial fluid (IF).

• Interstitial Fluid (IF): Fluid between cells; part of ECF.

Major Cations and Anions

Compartment	Major Cation	Major Anion
ICF	K+	HPO42-, proteins
ECF	Na+	Cl-, HCO3-

Water Content in the Body

• Infants: ~73% water

• Adult males: ~60% water

• Adult females: ~50% water (higher fat content, lower water)

• Elderly: ~45% water

Basic Electrolytes

• Sodium (Na+), potassium (K+), calcium (Ca2+), chloride (Cl-), bicarbonate (HCO3-), phosphate (HPO42-).

Hormonal Regulation of Fluid Balance

• ADH (Antidiuretic Hormone): Released in response to increased plasma osmolality or decreased blood volume; promotes water reabsorption in kidneys.

• ANP (Atrial Natriuretic Peptide): Released by atria in response to increased blood volume; promotes Na+ and water excretion.

Water Gain and Loss

• Water Gain: Ingestion (liquids, food), metabolic water (from cellular respiration).

• Water Loss: Urine (~60%), insensible loss (skin, lungs), sweat, feces.

• Average Daily Intake/Loss: ~2.5 L/day

Acid-Base Balance

• Regulatory Organs: Lungs (CO2 excretion), kidneys (H+ excretion, HCO3- reabsorption).

• Chemical Buffers: Bicarbonate, phosphate, and protein buffer systems.

Acid-Base Disorders

Disorder	Primary Cause	Example
Respiratory Acidosis	CO2 retention (hypoventilation)	Emphysema
Respiratory Alkalosis	CO2 loss (hyperventilation)	Anxiety attack
Metabolic Acidosis	Low HCO3- or excess acid	Diabetic ketoacidosis, diarrhea
Metabolic Alkalosis	High HCO3- or loss of acid	Vomiting, antacid overuse

Osmolality

• Osmolality: Measure of solute concentration per kilogram of solvent.

• Changes in osmolality cause water to move between compartments, affecting cell volume and function.

Example: Increased ECF osmolality draws water out of cells, causing cell shrinkage.

Summary Table: Fluid Compartments and Electrolytes

Compartment	Volume (%)	Major Cation	Major Anion
ICF	~40%	K+	HPO42-, proteins
ECF (Plasma)	~5%	Na+	Cl-
ECF (IF)	~15%	Na+	Cl-

Additional info: The above content expands on the exam review outline, providing definitions, examples, and tables for clarity and completeness.