The Urinary System

Essentials of Human Anatomy & Physiology

Chapter 15: The Urinary System

Functions of the Urinary System

• Elimination of Waste Products
    - Nitrogenous Wastes
    - Toxins
    - Drugs

• Regulation of Homeostasis
    - Water Balance
    - Electrolyte Balance
    - Acid-Base Balance in Blood
    - Blood Pressure Regulation
    - Red Blood Cell Production
    - Activation of Vitamin D

Organs of the Urinary System

• Kidneys

• Ureters

• Urinary Bladder

• Urethra

• Hepatic Veins (cut)

• Inferior Vena Cava

• Adrenal Glands

• Renal Arteries

• Renal Hilus

• Renal Vein

• Aorta

• Uterus (part of female reproductive system)

Location of the Kidneys

• Positioned against the dorsal body wall

• Located at the level of T12 to L3

• Right Kidney: Crowded by the liver, slightly lower than the left kidney

• Attached to the ureters, renal blood vessels, and nerves at the renal hilus

• Adrenal Gland: Found on top of each kidney

Coverings of the Kidneys

• Renal Capsule: Surrounds each kidney

• Adipose Capsule: Provides protection and helps keep the kidney in place

Regions of the Kidney

• Renal Cortex: Outer region

• Renal Medulla: Inside the cortex

• Renal Pelvis: Inner collecting tube

Kidney Structures

• Medullary Pyramids: Triangular regions of tissue in the medulla

• Renal Columns: Extensions of cortex-like material inward

• Calyces: Cup-shaped structures that funnel urine towards the renal pelvis

Nephrons

• Definition: The structural and functional units of the kidneys

• Quantity: More than 1,000,000 per kidney

• Location: Found within the pyramids

• Function: Responsible for forming urine

• Main Structures:
    - Glomerulus
    - Renal Tubule

Glomerulus

• A specialized capillary bed

• Attached to arterioles on both sides to maintain high pressure

• Composed of:
    - Large Afferent Arteriole
    - Narrow Efferent Arteriole

Renal Tubule

• Components:
    - Glomerular (Bowman’s) Capsule
    - Proximal Convoluted Tubule
    - Loop of Henle
    - Distal Convoluted Tubule

Peritubular Capillaries

• Arise from the efferent arteriole of the glomerulus

• Characteristics:
    - Normal, low pressure capillaries
    - Attached to a venule
    - Cling close to the renal tubule
    - Reabsorb some substances from collecting tubes

Types of Nephrons

• Cortical Nephrons
    - Located entirely in the cortex
    - Includes most nephrons

• Juxtamedullary Nephrons
    - Found at the boundary of the cortex and medulla

Urine Formation Processes

• A. Filtration

• B. Reabsorption

• C. Secretion

Filtration

• A nonselective, passive process

• Water and solutes smaller than proteins are forced through capillary walls

• Blood cells cannot pass out to the capillaries

• Filtrate is collected in the glomerular capsule and leaves via the renal tubule

Reabsorption

• The peritubular capillaries reabsorb materials including:
    - Some water
    - Glucose
    - Amino acids
    - Ions

• Some reabsorption is passive; most is active

• Most Reabsorption: Occurs in the proximal convoluted tubule

Materials Not Reabsorbed

• Nitrogenous Waste Products:
    - Urea
    - Uric Acid
    - Creatinine

• Excess Water

Secretion: Reabsorption in Reverse

• Some materials move from the peritubular capillaries into the renal tubules including:
    - Hydrogen Ions
    - Potassium Ions
    - Creatinine

• Materials left in the renal tubule move toward the ureter

Formation of Urine

• Processes:
    - Filtration: Water and solutes smaller than proteins are forced through capillary walls and glomerular capsule into the renal tubule.
    - Reabsorption: Water, glucose, amino acids, and needed ions are transported out of the filtrate into tubular cells and then enter capillary blood.
    - Secretion: H extsuperscript{+}, K extsuperscript{+}, creatinine, and drugs are removed from peritubular blood and secreted by tubular cells into the filtrate.

Characteristics of Urine Used for Medical Diagnosis

• Color: Somewhat yellow due to pigment urochrome (from hemoglobin destruction) and solutes

• Sterile

• Slightly Aromatic

• Normal pH: Around 6

• Specific Gravity: Ranges from 1.001 to 1.035

Ureters

• Description: Slender tubes attaching the kidney to the bladder

• Continuous with the renal pelvis

• Enter the posterior aspect of the bladder

• Peristalsis aids gravity in urine transport

Urinary Bladder

• Smooth, collapsible, muscular sac

• Temporarily stores urine

• Components:
    - Ureteral Orifice
    - Trigone
    - Internal Urethral Orifice
    - External Urethral Sphincter
    - Internal Urethral Sphincter
    - Urogenital Diaphragm

Urinary Bladder Wall

• Composed of three layers of smooth muscle (Detrusor Muscle)

• Mucosa consists of transitional epithelium

• Walls are thick and folded in an empty bladder

• Bladder can expand significantly without increasing internal pressure

Urethra

• Description: Thin-walled tube that carries urine from the bladder to the outside by peristalsis

• Release of urine is controlled by two sphincters:
    - Internal Urethral Sphincter (involuntary)
    - External Urethral Sphincter (voluntary)

Urethra Gender Differences

• Length:
    - Females: 3-4 cm (1 inch)
    - Males: 20 cm (8 inches)

• Location:
    - Females: Along the wall of the vagina
    - Males: Through the prostate and penis

• Function:
    - Females: Only carries urine
    - Males: Carries urine and serves as a passageway for sperm cells

Micturition (Voiding)

• Both sphincter muscles must open to allow for urination

• The internal urethral sphincter is relaxed after the bladder is stretched

• Activation results from an impulse sent to the spinal cord and back via pelvic splanchnic nerves

• The external urethral sphincter must be voluntarily relaxed

Maintaining Water Balance

• Normal Amount of Water in the Human Body:
    - Young Adult Females: 50%
    - Young Adult Males: 60%
    - Babies: 75%
    - Old Age: 45%

• Water is necessary for many body functions, and levels must be maintained

The Link Between Water and Salt

• Changes in electrolyte balance cause water to move from one compartment to another

• Alters blood volume and blood pressure

• Can impair cell activity

Maintaining Water Balance (Intake and Output)

• Water Intake must equal Water Output:
    - Sources for Water Intake:
      - Ingested foods and fluids
      - Water produced from metabolic processes
    - Sources for Water Output:
      - Vaporization out of the lungs
      - Loss in perspiration
      - Losses in feces
      - Urine production

Average Water Intake/Output per Day

• Average Intake: 2500 ml
    - Metabolism: 250 ml (10%)
    - Foods: 750 ml (30%)
    - Beverages: 1500 ml (60%)

• Average Output: 1500 ml
    - Feces: 100 ml (4%)
    - Sweat: 200 ml (8%)
    - Insensible Losses via Skin and Lungs: 700 ml (28%)

• Urine Production: 1500 ml (60%)

Maintaining Water Balance

• Dilute Urine: Produced if water intake is excessive

• Concentrated Urine: Less is produced if large amounts of water are lost

• Proper concentrations of various electrolytes must be present

Regulation of Water and Electrolyte Reabsorption

• Regulation is primarily by hormones:
    - Antidiuretic Hormone (ADH): Prevents excessive water loss in urine
    - Aldosterone: Regulates sodium ion content of extracellular fluid
      - Triggered by the renin-angiotensin mechanism

• Active Monitoring: Conducted by cells in the kidneys and hypothalamus

Maintaining Acid-Base Balance in Blood

• Blood pH must remain between 7.35 and 7.45 to maintain homeostasis

• Alkalosis: pH above 7.45

• Acidosis: pH below 7.35

• Most ions originate as byproducts of cellular metabolism

Homeostatic Imbalances

• Cystitis: Bladder infection

• Urethritis: Urethral infection

• Incontinence: Inability to control urination

• Urinary Retention: Unable to expel urine; requires a urinary catheter if prolonged

• Pyelitis: Inflammation of renal pelvis and calyx

• Pyelonephritis: Inflammation of entire kidney

• Anuria: <50cc urine/day

• Renal Calculi: Kidney stones

• Renal Failure: Function at 10-15% of normal due to poisoning, injury, or illness.