Urinary System: Functions and Anatomy

Overview

• Urinary system: brief introduction focusing on anatomy, major functions, and selected diseases.
• Emphasis on how the urinary system maintains homeostasis and what happens when it malfunctions.

Major Functions of the Urinary System

• Excretion: removal of chemical or metabolic waste from the blood, producing urine.
• Primary role: cleans the blood; removes dissolved chemical wastes rather than solid wastes.
• Regulates blood volume and blood pressure by balancing sodium and potassium levels.
• Regulates blood pH by excreting or retaining acidic and basic substances.
• Works continuously; blood is filtered about 60 times per day.

Other Organ Systems Involved in Excretion

• Respiratory system:
  • Removes carbon dioxide, a metabolic waste from aerobic cellular respiration.
• Liver:
  • Converts old red blood cell components into bile.
  • Bile is used to emulsify fats but is itself a chemical waste product.
• Skin (sweat glands):
  • Sweat contains metabolic and chemical products the body wants to remove.
• Digestive system:
  • Removes solid waste from the body (not considered part of urinary excretion).

Blood pH and Urine pH

• Normal blood pH: 7.35–7.45, kept very constant.
• Kidneys help regulate blood pH by:
  • Excreting excess acids or bases in urine.
  • Retaining acids or bases when needed.
• Typical urine pH: around 6 (slightly acidic).
  • Can be more acidic if diet is high in acidic foods.
  • Acidic urine usually indicates kidneys are working to protect blood pH, not that something is wrong.

Metabolic Waste Products Removed by the Urinary System

• Urea:

  • Major nitrogenous waste in urine.
  • Produced from amino acid/protein metabolism.
  • Proteins are broken into amino acids; further breakdown produces ammonia.
  • Liver converts ammonia into urea; urea is still toxic and must be removed by kidneys.

• Uric acid:

  • Waste product from DNA metabolism.
  • All living cells contain DNA, so digestion of food includes digestion of DNA.
  • Uric acid is toxic and is removed from the blood by the kidneys.

Table: Key Metabolic Waste Products

Disease Example: Gout

• Gout:
  • Caused by high uric acid levels in the blood.
  • Uric acid comes from DNA metabolism.
• Pathology:
  • Excess uric acid crystallizes instead of staying dissolved.
  • Crystals deposit in joints causing pain, swelling, and inflammation.
• Commonly affected joints:
  • Base of the big toe (very common site).
  • Can also affect ankles, knees, fingers, and elbows.
• Clinical significance:
  • Very common condition; students are likely to know someone with gout.
  • Often associated with poor kidney (urinary system) function.
  • Indicates uric acid is not being removed efficiently from the blood.
• More common in men (exact reason not explained).

Anatomy of the Urinary System

Main Organs

• Kidneys (2):
  • Bean-shaped organs; where urine is produced from the blood.
  • All substances in urine were once in the blood.
• Ureters (2):
  • Tubes that carry urine from each kidney to the urinary bladder.
• Urinary bladder (1):
  • Hollow organ that stores urine.
• Urethra (1):
  • Tube that carries urine from bladder to the outside of the body.

Table: Urinary System Structures and Functions

Blood Supply to the Kidneys

• Aorta:
  • Major artery running down the torso; carries oxygenated blood.
• Vena cava:
  • Major vein returning deoxygenated blood to the heart.
• Renal artery:
  • Branches from the aorta.
  • Delivers blood to each kidney for filtration.
• Renal vein:
  • Drains cleaned blood from kidney to vena cava.
• Term “renal”:
  • Refers to kidneys (e.g., renal artery, renal vein, renal disease).

Kidney Location and Position

• Two kidneys, right and left, located in the abdominal area.
• Left kidney is slightly higher than the right kidney.
• Reason: liver occupies space on the right side and pushes the right kidney lower.
• Kidneys resemble kidney beans in shape.

Urine Flow Pathway

• Blood enters kidneys via renal arteries.
• Kidneys:
  • Clean the blood.
  • Remove toxic compounds, balance pH, balance sodium and potassium.
• Cleaned blood leaves via renal veins to return to heart.
• Urine (waste in solution) formed in kidneys flows:
  • From kidneys → through ureters → to urinary bladder.
  • Stored in bladder → leaves via urethra during urination.

Fun Facts and Sex Differences

• Entire blood supply filters through kidneys about 60 times a day.
  • Approximately 2.5 times per hour.
• Urethra length differs:
  • Males: longer urethra, passes through the penis.
  • Females: shorter urethra, exits close to the vaginal opening and anus.

Urinary Tract Infections (UTIs)

• More common in females.
• Cause:
  • Often from bacteria originating in feces.
• Anatomical reasons:
  • In females, urethral opening is close to the anus.
  • Shorter urethra provides shorter path for bacteria.
• In males, greater distance between anus and urethral opening; urethra is longer, lowering infection risk.

Control of Urination (Micturition)

Bladder Capacity and Stretch

• Urinary bladder capacity:
  • Typically about 400–500 mL (varies with body size).
  • Roughly equal to a standard 500 mL water bottle.
• Around 200–250 mL:
  • Stretch receptors in bladder wall are activated.

Sphincter Muscles

• Two sphincters at the exit of the bladder:
  • Internal urethral sphincter.
  • External urethral sphincter.
• Sphincter definition:
  • Circular muscle that can open or close a passage.
  • Previously discussed in digestive system (lower esophageal, pyloric, anal sphincters).

Involuntary vs Voluntary Control

• Internal sphincter:
  • Opens in response to bladder stretch (about 200–250 mL).
  • Action is involuntary.
  • Opening triggers the sensation of needing to urinate.
• External sphincter:
  • Under voluntary control in most older children and adults.
  • Allows a person to choose when to release urine if conditions are appropriate.

Potty Training and Nervous System Development

• Young children:
  • Have not yet gained full voluntary control of the external sphincter.
  • When internal sphincter opens, they urinate automatically.
• Potty training:
  • Requires nervous system maturity to control the external sphincter.
  • Usually not possible at age one.
  • Often achievable around age two and a half when nervous system has matured.
• If parents claim potty training at age one:
  • More likely the parent is closely observing and timing bathroom trips.
  • Not true full voluntary control by the child.

Kidney Failure and Dialysis

• Kidneys are essential for life.
• Without kidney function:
  • Accumulation of toxic compounds occurs.
  • Death likely within days to weeks, depending on individual factors.
• Dialysis:
  • Artificial method to clean the blood when kidneys fail.
  • Typically done in a dialysis center three times per week.
  • Process:
    • Blood removed a little at a time.
    • Passed through a machine that cleans it.
    • Returned to the body.
• Differences from normal kidney function:
  • Healthy kidneys clean blood continuously, about 60 times daily.
  • Dialysis cleans blood only three times a week.
  • Leads to cycles of toxin buildup and sudden removal.
  • Not ideal and associated with side effects, but can sustain life without kidney function.

Key Terms and Definitions

• Excretion:
  • Removal of chemical or metabolic waste from the body, especially from the blood.
• Urea:
  • Nitrogenous waste product from amino acid/protein metabolism.
• Uric acid:
  • Waste product from DNA metabolism.
• Renal:
  • Term referring to the kidneys.
• Kidney (renal) failure:
  • Inability of kidneys to perform normal cleaning and regulatory functions.
• Gout:
  • Painful joint condition caused by uric acid crystal deposition in joints.
• Ureter:
  • Tube connecting kidney to urinary bladder.
• Urinary bladder:
  • Organ that stores urine until excretion.
• Urethra:
  • Tube through which urine exits the body.
• Sphincter:
  • Ring-like muscle controlling opening and closing of a passage.
• Internal urethral sphincter:
  • Involuntary muscle at bladder outlet.
• External urethral sphincter:
  • Voluntary muscle controlling release of urine.

Action Items / Next Steps

• Study and be able to:
  • Identify kidneys, ureters, bladder, urethra, renal artery, and renal vein on diagrams.
  • State major function of each urinary structure.
• Review:
  • Blood pH range (7.35–7.45) and typical urine pH (around 6).
  • Roles of urea and uric acid and how they are produced.
• Prepare for exams:
  • Practice with textbook diagrams; same images may appear on Exam 4 and the final.
  • Answer “Check the Progress” questions 1 and 3 in the chapter; answers are in the textbook reading.