ch 36

Chapter 36: The Urinary System

Overview

The urinary system is essential for excreting metabolic wastes and maintaining homeostasis in animals. This chapter explores the structure and function of urinary systems across different animal groups, the process of urine formation, and the system's role in regulating internal environments.

Major Functions of Urinary Systems

Excretion and Homeostasis

• Excretion is the elimination of metabolic wastes from the body, primarily through urine.

• Urinary systems help maintain homeostasis by regulating water content, pH, and concentrations of ions and small organic molecules in body fluids.

• Urine is a watery fluid containing filtered waste products, excess ions, and other substances not needed by the body.

• The main nitrogenous wastes excreted are ammonia, urea, and uric acid, which result from protein and nucleic acid metabolism.

Nitrogenous Waste Types

• Ammonia (NH3): Highly toxic, excreted directly by aquatic animals.

• Urea: Less toxic, produced by mammals and some amphibians; requires energy to synthesize but conserves water.

• Uric Acid: Least toxic, excreted by birds and many reptiles as a paste or solid; conserves the most water.

Nitrogenous Waste	Example Animal	Water Requirement	Toxicity
Ammonia	Fish	High	High
Urea	Mammals	Moderate	Moderate
Uric Acid	Birds	Low	Low

Invertebrate Urinary Systems

Protonephridia in Flatworms

Flatworms use protonephridia, a network of tubules capped by ciliated flame cells, to filter interstitial fluid and excrete wastes.

Malpighian Tubules in Insects

Insects possess Malpighian tubules that extend from the intestine into the hemolymph, filtering wastes and returning useful substances to the body before excretion.

Nephridia in Annelids and Mollusks

Earthworms and similar animals use nephridia, tubular structures that filter coelomic fluid, reabsorb nutrients, and excrete wastes through nephridiopores.

Structure of the Mammalian Urinary System

Major Organs and Their Functions

• Kidneys: Filter blood, form urine, and regulate homeostasis.

• Ureters: Muscular tubes that transport urine from kidneys to bladder.

• Bladder: Stores urine until excretion.

• Urethra: Conducts urine from bladder to outside the body.

Kidney Structure

• Renal cortex: Outer layer where urine formation begins.

• Renal medulla: Inner region containing nephron loops and collecting ducts.

• Renal pelvis: Funnel-like chamber collecting urine for passage to ureter.

Nephron: The Functional Unit

• Each kidney contains about 1 million nephrons.

• Nephron parts:

  • Renal corpuscle: Includes the glomerulus (capillary knot) and glomerular capsule (collects filtrate).

  • Renal tubule: Proximal tubule (reabsorption), nephron loop (concentration), distal tubule (secretion and further reabsorption).

  • Collecting duct: Receives urine from multiple nephrons and channels it to the renal pelvis.

Urine Formation

Filtration

Filtration occurs in the glomerulus, where blood pressure forces water and small solutes through capillary walls into the glomerular capsule, forming filtrate.

• Glomerular capillaries are highly porous, and pressure is high due to narrow outgoing arterioles.

Reabsorption

Reabsorption returns most water, ions, and nutrients from the filtrate back to the blood, mainly in the proximal tubule, but also in the nephron loop and distal tubule.

• Occurs by diffusion and active transport.

• Distal tubule reabsorption is hormonally regulated.

Secretion

Secretion actively transports additional wastes and excess ions from the blood into the renal tubule, mainly in the proximal and distal tubules.

• Includes K+, H+, ammonia, drugs, and toxins.

Homeostatic Functions of Vertebrate Urinary Systems

Osmoregulation and Water Balance

• Osmoregulation is the maintenance of blood osmolarity (solute concentration) within strict limits.

• Kidneys adjust urine concentration based on hydration status, regulated by the hormone antidiuretic hormone (ADH).

• ADH increases water reabsorption in the distal tubule and collecting duct by increasing aquaporin channels.

Adaptations to Environment

• Mammals in dry environments have more nephrons with long loops of Henle, allowing greater urine concentration (e.g., desert rodents).

• Mammals in wet environments have more short-looped nephrons (e.g., beavers).

pH and Blood Pressure Regulation

• Kidneys maintain blood pH by reabsorbing bicarbonate (HCO3-) and secreting H+.

• Release of renin and angiotensin regulates blood pressure by increasing Na+ reabsorption, water retention, and vasoconstriction.

• Release of erythropoietin stimulates red blood cell production in response to low oxygen.

Osmoregulation in Fish

• Freshwater fish live in hypotonic environments; they excrete large amounts of dilute urine and actively uptake salts.

• Saltwater fish live in hypertonic environments; they drink seawater, excrete excess salt through gills, and produce small amounts of concentrated urine.

• Fish nephrons lack loops of Henle, so they cannot concentrate urine above blood osmolarity.

Summary Table: Nitrogenous Wastes

Waste	Animals	Water Needed	Toxicity	Energy Cost
Ammonia	Fish, aquatic invertebrates	High	High	Low
Urea	Mammals, amphibians	Moderate	Moderate	Moderate
Uric Acid	Birds, reptiles, insects	Low	Low	High

Key Equations:

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

• Renin-Angiotensin System (simplified): $\text{Renin} \rightarrow \text{Angiotensinogen} \rightarrow \text{Angiotensin I} \rightarrow \text{Angiotensin II}$

Additional info: This guide expands on the original notes with definitions, examples, and summary tables for clarity and completeness.