Lecture 24: Urinary 1

Class Overview

  • Course: KNES 323: Integrative Physiology

  • Focus: Urinary System

  • Institution: University of Calgary, Faculty of Kinesiology, Sport Injury Prevention Research Centre

  • Date: Monday, October 27, 2025

Class Outline

  1. Introduction

    • Roles of the urinary system in maintaining homeostasis

  2. Kidney Overview

    • Structure and functions of the kidneys

  3. Nephron

    • Anatomy and physiology of the nephron

  4. Collecting Duct

    • Function and role in urine concentration

Introduction

  • The urinary system plays several vital roles in maintaining homeostasis:

    • Cleansing the blood: Removal of metabolic wastes.

    • Regulation of pH: Works with lungs and blood buffers.

    • Regulation of blood pressure: In cooperation with the heart and blood vessels.

    • Regulating solute concentration: Maintaining osmotic balance.

    • Erythropoietin production: Influences the concentration of red blood cells.

    • Vitamin D activation: Final synthesis step converting calcidiol to calcitriol.

  • Impacts of kidney failure:

    • Potentially devastating effects on homeostasis, may cause:

    • Weakness

    • Lethargy

    • Shortness of breath

    • Anemia

    • Widespread edema (swelling)

    • Metabolic acidosis

    • Rising potassium levels

    • Heart arrhythmias

  • Urinary storage and excretion:

    • Stores urine until disposal.

    • Controlled by the nervous system.

    • Loss of control leads to incontinence.

  • Think of kidneys as plasma regulators rather than mere urine producers:

    • Consider what happens if a particular function fails to understand the overarching significance in homeostasis and overall health quality.

Kidney Overview

  • Size: Roughly the size of a fist.

  • Blood supply: Receives approximately 20-25% of resting cardiac output.

  • Anatomical features:

    • Adrenal gland: Located on the superior surface of each kidney.

    • Adrenal cortex: Influences renal function via aldosterone for sodium reabsorption.

    • Adrenal medulla: Releases catecholamines (epinephrine and norepinephrine).

  • Internal anatomy:

    • Renal cortex: Outer region.

    • Renal medulla: Inner region.

    • Renal columns: Connective tissue extensions separating renal pyramids.

    • Renal pyramids: Bundles of collecting ducts transporting urine.

    • Renal hilum: Entry/exit site for renal structures.

    • Forms the renal pelvis from major and minor calyxes.

    • Renal arteries: Branch from the descending aorta.

    • Renal veins: Return cleansed blood to the inferior vena cava.

  • Nephrons: Functional units of the kidney responsible for cleansing blood and balancing circulatory content.

    • Afferent arterioles supply blood to the glomerulus.

    • Bowman's capsule: Surrounds glomerulus, receives filtrate.

    • Efferent arterioles: Form capillary networks around nephron tubules.

    • Retained solutes and water: Returned to circulatory system.

  • Identifying features:

    • Cortex appears lighter compared to medulla.

    • Renal corpuscles and tubules located in the cortex.

    • Cortical nephrons: Have short loops of Henle.

    • Juxtamedullary nephrons: Have long loops extending deep into medulla.

Nephron

  • Nephrons alter filtrate from blood before excreting as urine, achieving this through three main functions:

    1. Filtration: Retention of necessary substances, execusion of waste.

    2. Reabsorption: Essential solutes are taken back into blood.

    3. Secretion: Toxins and excess substances are expelled into the urine.

  • Functions also include:

    • Blood pressure maintenance via renin production.

    • Erythropoiesis promotion via hormone EPO.

    • Calcium absorption assistance via active vitamin D.

  • Filtration slits in Bowman's capsule allow:

    • 10-20% of plasma to filter through, preventing blood cells and large proteins from passing.

    • Substance filtration:

    • Substances < 4 nm can easily cross; those < 8 nm can pass with varying ease.

    • Charge considerations: Negatively charged pores repel similarly charged particles while allowing positive charges to pass.

  • Resulting filtrate characteristics:

    • No cells (RBCs) or large proteins (e.g., albumin).

    • Slight predominance of positive ions (e.g., Na+ and K+).

Functional Regions of the Nephron

  • Proximal Convoluted Tubule (PCT):

    • Reabsorption:

    • Water: 65% via osmosis

    • Na+: 65% via sodium-potassium pumps, symporters, antiporters

    • K+: 65% via diffusion

    • Glucose: 100% via symporters and facilitated diffusion

    • Amino acids: 100% via symporters and facilitated diffusion

    • Cl-: 50% via diffusion

    • HCO3-: 80-90% via facilitated diffusion

    • Urea: 50% via diffusion

    • Ca2+, Mg2+: Variable via diffusion

    • Secretion:

    • H+: Variable via antiporters

    • NH4+: Variable; increases during acidosis via antiporters

    • Urea: Variable via diffusion

    • Creatinine: Small amount

    • At the end, tubular fluid remains isotonic to blood: 300 mOsm/liter.

  • Loop of Henle:

    • Reabsorption:

    • Water: 15% via osmosis in descending limb

    • Na+: 20-30% via symporters in ascending limb

    • K+: 20-30% via symporters in ascending limb

    • Cl-: 35% via symporters in ascending limb

    • HCO3-: 10-20% via facilitated diffusion

    • Ca2+, Mg2+: Variable via diffusion

    • Secretion:

    • Urea: Variable (recycling from collecting duct)

    • At the end, tubular fluid is hypotonic: 100-150 mOsm/liter.

  • Renal Corpuscle:

    • Glomerular filtration rate (GFR): 105-125 mL/min; isotonic to blood.

    • Filtered substances: Water and all solutes in blood (except proteins), including ions, glucose, amino acids, creatinine, and uric acid.

  • Early Distal Convoluted Tubule (DCT):

    • Reabsorption:

    • Water: 10-15% via osmosis

    • Na+: 5% via symporters

    • Cl-: 5% via symporters

    • Ca2+: Variable, stimulated by parathyroid hormone

  • Late DCT and Collecting Duct:

    • Reabsorption:

    • Water: 5-9% (ADH stimulated)

    • Na+: 1-4% (aldosterone stimulated)

    • HCO3-: Variable depending on H+ secretion.

    • Urea: Variable (recycling to nephron loop)

    • Secretion:

    • K+: Variable to adjust for dietary intake

    • H+: Variable amounts to maintain acid-base homeostasis

    • Tubular fluid leaving collecting duct is dilute with low ADH and concentrated with high ADH levels.

  • Proximal Convoluted Tubule:

    • Filtrate enters PCT, lined with simple cuboidal cells with microvilli (brush border) for enhanced reabsorption of Na+, Cl-, glucose, etc.

  • Loop of Henle:

    • Composed of descending and ascending portions, differing in epithelium to regulate permeability.

  • Distal Convoluted Tubule:

    • Similar to PCT but shorter and less active; fewer microvilli and no brush border.

  • Juxtaglomerular Apparatus:

    • Where afferent/efferent arterioles enter/leave; monitors flow and Na+ concentration.

    • Contains macula densa (chemoreceptors) and juxtaglomerular cells (mechanoreceptors).

  • Regulatory Functions:

    • Higher filtrate osmolarity leads to afferent arteriole constriction; lowers GFR and reduces urine production.

    • Lower filtrate osmolarity leads to afferent arteriole relaxation; increases GFR and enhances fluid loss.

  • Renin-Angiotensin System:

    • Renin: cleaves angiotensinogen to produce angiotensin I.

    • Angiotensin I converted to II (via ACE) which increases blood pressure and stimulates aldosterone (Na+ reabsorption).

  • Natriuretic Hormones:

    • Stimulate Na+ excretion and inhibit aldosterone and ADH, leading to reduced water retention and lowered blood pressure.

Collecting Duct

  • Collecting ducts functionally distinct; collect filtrate from multiple nephrons.

  • Merge into about 30 terminal ducts, draining at papillae.

  • Cellular Structure:

    • Lined with simple squamous epithelium; have ADH receptors.

    • ADH effects: Insertion of aquaporin channels increases water recovery; absence leads to dilute urine.

  • Upcoming classes to cover:

    • Distinction between filtration, reabsorption, and secretion processes in urine production.