Unit 4 Review: The Urinary System and Fluid & Electrolyte Balance

Chapter 25: The Urinary System

Three Main Functions of the Kidney

The kidneys are vital organs responsible for maintaining homeostasis by regulating the composition and volume of blood.

• Excretion of metabolic wastes: Removes urea, creatinine, and other waste products from the blood.

• Regulation of blood volume and pressure: Adjusts water and electrolyte balance, influencing blood pressure.

• Regulation of blood pH: Controls acid-base balance by excreting hydrogen ions and reabsorbing bicarbonate.

• Additional info: The kidneys also produce hormones such as erythropoietin and activate vitamin D.

Functional Anatomy of the Urinary System

The urinary system consists of organs that produce, store, and eliminate urine.

• Kidneys: Filter blood and produce urine.

• Ureters: Transport urine from kidneys to bladder.

• Urinary bladder: Stores urine until excretion.

• Urethra: Conducts urine out of the body.

Functional Anatomy of the Kidney: Exterior and Interior

The kidney is a bean-shaped organ with distinct external and internal structures.

• Exterior: Covered by a fibrous capsule, surrounded by adipose tissue for protection.

• Interior: Consists of cortex (outer region), medulla (inner region with renal pyramids), and renal pelvis (collects urine).

• Renal hilum: Entry/exit site for blood vessels, nerves, and ureter.

Basic Path of Blood Through the Kidney

Blood flows through a series of vessels for filtration and reabsorption.

• Renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries → afferent arterioles → glomerulus (capillary bed) → efferent arterioles → peritubular capillaries/vasa recta → cortical radiate veins → arcuate veins → interlobar veins → renal vein.

• Additional info: The glomerulus is the site of filtration; peritubular capillaries and vasa recta are involved in reabsorption and secretion.

Nephrons: Anatomy and Physiology

Nephrons are the functional units of the kidney, responsible for urine formation.

• Renal corpuscle: Includes the glomerulus and Bowman's capsule; site of filtration.

• Renal tubule: Proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct.

• Physiology: Filtration, reabsorption, and secretion occur along the nephron.

Classes of Nephrons

There are two main types of nephrons, each with distinct roles.

• Cortical nephrons: Located mostly in the cortex; responsible for most urine formation.

• Juxtamedullary nephrons: Have long loops of Henle extending deep into the medulla; important for concentrating urine.

Nephron Capillary Beds

Nephrons are associated with specialized capillary networks.

• Glomerulus: Site of filtration.

• Peritubular capillaries: Surround cortical nephrons; involved in reabsorption and secretion.

• Vasa recta: Surround juxtamedullary nephrons; maintain osmotic gradient in the medulla.

Juxtaglomerular Complex (JGC)

The JGC regulates blood pressure and filtration rate.

• Macula densa: Detects sodium concentration in the DCT.

• Juxtaglomerular cells: Release renin in response to low blood pressure.

• Extraglomerular mesangial cells: Support communication between macula densa and juxtaglomerular cells.

Urine Production: Three Steps

Urine is formed through three main processes in the nephron.

• Glomerular filtration: Movement of water and solutes from blood into Bowman's capsule.

• Tubular reabsorption: Return of useful substances from filtrate to blood.

• Tubular secretion: Addition of waste products from blood to filtrate.

The Filtration Membrane

The filtration membrane allows selective passage of substances during glomerular filtration.

• Components: Fenestrated endothelium, basement membrane, and podocyte filtration slits.

• Function: Permits passage of water, ions, and small molecules; restricts proteins and cells.

Osmotic Gradient

The osmotic gradient in the kidney medulla enables concentration of urine.

• Countercurrent mechanism: Loop of Henle and vasa recta create and maintain gradient.

• Allows water reabsorption: Collecting ducts reabsorb water, concentrating urine.

The Remaining Parts of the Urinary System

Other components facilitate urine transport and elimination.

• Ureters: Muscular tubes that propel urine to the bladder via peristalsis.

• Urinary bladder: Distensible organ for urine storage.

• Urethra: Conducts urine out of the body; differs in length between males and females.

Chapter 26: Fluids & Electrolytes

Water Content in the Body

Water is the most abundant component of the human body, essential for physiological processes.

• Average adult: 50-60% water by weight.

• Distribution: Varies with age, sex, and body composition.

Fluid Compartments

Body fluids are distributed in distinct compartments.

• Intracellular fluid (ICF): Inside cells; about 2/3 of total body water.

• Extracellular fluid (ECF): Outside cells; includes plasma and interstitial fluid.

Solutes: Electrolytes and Non-Electrolytes; ECF vs ICF

Solutes are substances dissolved in body fluids, classified as electrolytes or non-electrolytes.

• Electrolytes: Dissociate into ions (e.g., Na+, K+, Cl-); conduct electricity.

• Non-electrolytes: Do not dissociate (e.g., glucose, urea); do not conduct electricity.

• ECF: High in Na+ and Cl-.

• ICF: High in K+ and phosphate ions.

Water Balance: Intake and Output

Water balance is achieved when intake equals output.

• Intake: Ingested fluids, food, and metabolic water.

• Output: Urine, feces, sweat, and insensible losses (e.g., evaporation).

• Formula: $\text{Water Balance} = \text{Intake} - \text{Output}$

Dehydration: Signs and Symptoms

Dehydration occurs when water output exceeds intake.

• Signs: Thirst, dry mouth, decreased skin turgor.

• Symptoms: Fatigue, confusion, decreased urine output.

Hypotonic Hydration

Hypotonic hydration results from excessive water intake or inadequate excretion.

• Causes: Overhydration, renal insufficiency.

• Effects: Cells swell, leading to cerebral edema and possible convulsions.

Edema

Edema is the accumulation of fluid in interstitial spaces.

• Causes: Increased capillary hydrostatic pressure, decreased plasma proteins, lymphatic obstruction.

• Effects: Swelling, impaired tissue function.

Electrolyte Balance

Electrolyte balance is crucial for nerve, muscle, and cellular function.

• Key electrolytes: Sodium, potassium, calcium, chloride.

• Regulation: Hormones such as aldosterone and antidiuretic hormone (ADH).

Chemical Buffer Systems

Chemical buffer systems maintain acid-base balance in body fluids.

• Bicarbonate buffer system: Most important in ECF.

• Phosphate buffer system: Important in ICF and urine.

• Protein buffer system: Proteins act as buffers in both ICF and plasma.

• Example equation: $\text{CO}_2 + \text{H}_2\text{O} \leftrightarrow \text{H}_2\text{CO}_3 \leftrightarrow \text{H}^+ + \text{HCO}_3^-$

Comparison Table: Fluid Compartments and Major Electrolytes

Compartment	Volume (% of body water)	Major Electrolytes
Intracellular Fluid (ICF)	~66%	K+, HPO42-, Mg2+
Extracellular Fluid (ECF)	~34%	Na+, Cl-, HCO3-
Plasma (subset of ECF)	~4%	Na+, Cl-, proteins
Interstitial Fluid (subset of ECF)	~20%	Na+, Cl-

Additional info: Table values are approximate and may vary by source.