The Excretory System
Understanding how your body eliminates waste and maintains balance
Introduction
Energy Production and Waste Formation
Food and oxygen combine in the body's cells to produce energy through catabolism. During this process, the particles of food and oxygen aren't destroyed but rearranged into new combinations, creating various waste products.
When sugars and fats containing carbon, hydrogen, and oxygen combine with oxygen in cells, they produce carbon dioxide and water vapour as waste gases. These are efficiently removed through exhalation via the lungs.
Protein foods are more complex, containing carbon, hydrogen, oxygen, nitrogen, and other elements. When proteins combine with oxygen, they produce nitrogenous waste that cannot be converted to gas and exhaled.
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The Urinary System's Primary Functions
Urea Removal
The major function is removing urea from the bloodstream to prevent toxic accumulation in the body.
Fluid Balance
Maintains proper balance of water, salts, and acids in body fluids for optimal functioning.
Electrolyte Regulation
Adjusts amounts of sodium, potassium, and acids necessary for muscle and nerve cell function.
The kidneys adjust water and electrolyte amounts by secreting some substances into urine whilst holding back others in the bloodstream for bodily use. This sophisticated filtering system ensures that essential substances remain whilst waste products are efficiently eliminated.
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Kidneys as Endocrine Organs
Beyond waste removal, kidneys function as vital endocrine organs, secreting important hormones and substances that regulate various bodily functions.
Renin Production
Secretes renin, a crucial substance for controlling blood pressure throughout the body.
Erythropoietin
Produces erythropoietin, a hormone that regulates the production of red blood cells.
Vitamin D Activation
Secretes active form of vitamin D, necessary for calcium absorption from the intestine.
Hormone Processing
Degrades and extracts hormones such as insulin and parathyroid hormone from the bloodstream.
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Anatomy
The Kidneys: Structure and Location
Physical Characteristics
- Bean-shaped organs
- Fist-sized, weighing 4-6 ounces each
- Located retroperitoneally (behind abdominal cavity)
- Positioned on either side of vertebral column
- Situated in lumbar region of spine
Protective Features
Embedded in cushion of adipose tissue and surrounded by fibrous connective tissue for protection.
Internal Structure
Outer Cortex Region: Contains filtering units and blood vessels
Inner Medulla Region: Contains collecting structures
Hilum: Depression on medial border where blood vessels and nerves pass through
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The Ureters: Transport Pathways
1
Structure
Muscular tubes measuring 16-18 inches long, lined with mucous membrane
2
Function
Convey urine in peristaltic waves from kidneys to urinary bladder
3
Movement
Rhythmic contractions push urine downward continuously
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Urinary Bladder and Urethra
Urinary Bladder
A hollow, muscular, distensible sac located in the pelvic cavity that serves as a temporary reservoir for urine.
The Trigone: A triangular space at the base of the bladder where the ureters enter and the urethra leads out.
Urethra
Membranous tube through which urine is discharged from the urinary bladder. The process of expelling urine is called micturition.
Female Urethra: Approximately 1.5 inches long, lying anterior to the vagina and vaginal meatus.
Male Urethra: Approximately 8 inches long, extending downward through the prostate gland to the meatus at the tip of the penis.
Physiology
Blood Flow to the Kidneys
Blood enters each kidney from the aorta via the right and left renal arteries. After entering at the hilum, the renal artery branches into progressively smaller arteries throughout the kidney.
01
Renal Artery Entry
Blood enters kidney from aorta through renal artery at the hilum
02
Progressive Branching
Artery branches into smaller and smaller vessels throughout the cortex
03
Arteriole Formation
Smallest arteries become arterioles located throughout kidney cortex
04
Controlled Flow
Blood passes slowly but constantly through these tiny vessels
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Kidney's Blood Pressure Regulation
Blood flow through the kidney is so essential that the kidneys have their own special device for maintaining blood flow.
The Renin System
When blood pressure falls in kidney vessels and blood flow diminishes, the kidney produces renin and discharges it into the blood.
Pressure Drop Detected
Kidney senses decreased blood flow
Renin Released
Kidney produces and secretes renin
Arteriole Contraction
Renin stimulates vessel constriction
Pressure Restored
Blood pressure increases to normal
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The Glomeruli: Filtering Units
Each arteriole in the kidney cortex leads into a mass of tiny, coiled and intertwined capillaries shaped like a ball, called a glomerulus.
1M
Glomeruli Per Kidney
Each kidney contains approximately one million filtering units in the cortex region
2M
Total Glomeruli
Both kidneys together contain about two million microscopic filtering structures
These millions of glomeruli work continuously to filter blood and begin the process of urine formation, demonstrating the remarkable efficiency of the kidney's design.
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Filtration: The First Step
The kidneys produce urine through a sophisticated process of filtration. As blood passes through the many glomeruli, specific substances are filtered whilst others remain in the bloodstream.
What Passes Through
The thin walls of each glomerulus permit these substances to leave the bloodstream:
- Water
- Salts
- Sugar
- Urea
- Creatinine
- Uric acid
What Stays Behind
The glomerular walls prevent large substances from filtering:
- Proteins
- Blood cells
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Bowman's Capsule: Collection Point
Filtered materials are collected in a tiny, cup-shaped structure called Bowman's capsule, which surrounds each glomerulus like a catcher's mitt.
Water
Primary component of filtrate
Salts
Electrolytes pass through filter
Sugar
Glucose enters capsule
Waste Products
Urea and other nitrogenous wastes
This collected fluid, called filtrate, then moves into the renal tubule for further processing.
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Reabsorption
The Renal Tubule: Selective Recovery
Attached to each Bowman's capsule is a long, twisted tube called the renal tubule. This is where the critical process of reabsorption occurs.
1
Filtrate Entry
Water, sugar, salts, and wastes enter tubule from Bowman's capsule
2
Selective Reabsorption
Most water, all sugar, and some salts return to bloodstream through surrounding capillaries
3
Waste Retention
Only wastes, some water, salts, and acids remain in tubule
4
Urine Formation
Final urine composition: 95% water, 5% urea, creatinine, salts, and acids
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The Importance of Reabsorption
This reabsorption assures that the body retains essential substances such as sugar, water, and salts.
100%
Sugar Reabsorbed
All glucose is normally returned to the bloodstream
99%
Water Recovered
Most water is reabsorbed back into circulation
95%
Salt Retention
Majority of salts are reclaimed by the body
Without this selective reabsorption process, the body would rapidly lose essential nutrients and become dehydrated. The kidney's ability to distinguish between waste and valuable substances is crucial for survival.
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From Tubules to Bladder
Each renal tubule, now containing urine, ends in a larger collecting tubule. Thousands of these collecting tubules converge to form the pathway for urine to exit the kidney.
Collecting Tubules
Thousands of renal tubules merge into larger collecting tubules
Renal Pelvis
Collecting tubules lead to basin-like central area with cup-shaped calices
Ureter
Renal pelvis narrows into ureter carrying urine to bladder
Bladder Storage
Urine temporarily stored until ready for elimination
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The Micturition Process
The exit area of the bladder to the urethra is closed by sphincters that prevent urine from leaving the bladder involuntarily.
As the bladder fills with urine, pressure is placed on the base of the urethra. This pressure creates the sensation and desire to urinate.
When appropriate, the sphincters relax and urine is expelled through the urethra in the process called micturition or voiding. The external opening of the urethra is called the urethral or urinary meatus.
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Terminology
Essential Vocabulary: Structures
Arteriole
A small artery that branches from larger arteries
Bowman's Capsule
Cup-shaped capsule surrounding each glomerulus
Calyx/Calix
Cup-like collecting region of the renal pelvis (plural: calices)
Catheter
A tube for injecting or removing fluids
Cortex
Outer region; the renal cortex is the outer region of the kidney
Glomerulus
Tiny ball of capillaries in cortex of kidney (plural: glomeruli)
Hilum
Depression where blood vessels and nerves enter and leave organ
Medulla
Inner region; the renal medulla is the inner region of the kidney
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Essential Vocabulary: Processes and Substances
Filtration
Process whereby some substances pass through a filter. About 180 quarts filtered daily; 98-99% of water and salts returned. Only 1.5 quarts of urine excreted daily.
Reabsorption
Process of accepting again or taking back. Materials necessary to body are reabsorbed into blood from renal tubules.
Urea
Major nitrogenous waste product excreted in urine
Creatinine
Waste product of muscle metabolism; nitrogenous waste excreted in urine
Electrolyte
A chemical that carries an electrical charge in solution
Micturition
Urination; the act of voiding or expelling urine
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Essential Vocabulary: Organs and Hormones
Kidney
One of two bean-shaped organs located behind the abdominal cavity on either side of the backbone in the lumbar region.
Renal Pelvis
Central collecting region of the kidney.
Renal Tubules
Microscopic tubes in the kidney where urine is formed as water, sugar, and salts are reabsorbed into the bloodstream.
Ureter
One of the two tubes leading from the kidney to the bladder.
Urethra
Tube leading from the bladder to the outside of the body.
Urinary Bladder
Sac that holds urine.
Renin
A hormone synthesised, stored, and secreted by the kidney; raises blood pressure by influencing vasoconstriction.
Erythropoietin
A hormone secreted by the kidney to stimulate the production of red blood cells.
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Pathology
Kidney Diseases and Conditions
Glomerulonephritis
Inflammation of the kidney glomerulus, also known as Bright's disease. Affects the filtering units of the kidney.
Interstitial Nephritis
Inflammation of the renal interstitium, the connective tissue lying between the renal tubules.
Nephrolithiasis
Kidney stones or renal calculi. Hard deposits that form inside the kidneys.
Nephrotic Syndrome
Group of symptoms caused by excessive protein loss in the urine, also called nephrosis.
Polycystic Kidneys
Multiple fluid-filled sacs (cysts) within and upon the kidney.
Pyelonephritis
Inflammation of the renal pelvis, often caused by bacterial infection.
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Serious Kidney Conditions
Renal Cell Carcinoma
Cancerous tumour of the kidney occurring in adulthood. The most common type of kidney cancer in adults.
Renal Failure
Failure of the kidney to excrete urine. The final phase of chronic renal failure is end-stage renal disease (ESRD), requiring dialysis or transplantation.
Renal Hypertension
High blood pressure resulting from kidney disease. It is the most common type of secondary hypertension.
Wilms' Tumour
Malignant tumour of the kidney occurring in childhood. Also called nephroblastoma.
Bladder Cancer
Malignant tumour of the urinary bladder, often presenting with blood in the urine.
Diabetes and Kidney Function
Diabetes Insipidus
Inadequate secretion or resistance of the kidney to the action of antidiuretic hormone (ADH).
Major Symptoms:
- Polydipsia (excessive thirst)
- Polyuria (excessive urination)
Lack of ADH prevents water from being reabsorbed into the blood through the renal tubules. The term "insipidus" means tasteless, reflecting that the urine is very dilute and watery, not sweet as in diabetes mellitus.
Diabetes Mellitus
Inadequate secretion or improper utilisation of insulin.
Major Symptoms:
- Glycosuria (sugar in urine)
- Hyperglycaemia (high blood sugar)
- Polyuria (excessive urination)
- Polydipsia (excessive thirst)
Without insulin, sugar cannot enter body cells for energy. Sugar remains in blood and spills into urine when kidneys cannot reabsorb it. "Mellitus" means sweet, reflecting the urine's content.
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Laboratory Tests
Blood Urea Nitrogen (BUN) Test
This test measures the amount of urea in the blood, providing crucial information about kidney function.
1
Normal Condition
Urea level is low since urea is excreted in the urine continuously by healthy kidneys.
2
Kidney Disease
When the kidney is diseased or fails, urea accumulates in the blood.
3
Uraemia
Condition of elevated urea in the blood, which can lead to serious complications.
4
Severe Consequences
If untreated, uraemia can lead to unconsciousness and death.
Creatinine Clearance Test
This test measures the ability of the kidney to remove creatinine from the blood, providing a comprehensive assessment of kidney function.
01
Blood Sample Collection
A blood sample is drawn and creatinine concentration is measured
02
24-Hour Urine Collection
Amount of creatinine excreted in urine is measured over 24 hours
03
Comparison Analysis
Blood creatinine level is compared with urine creatinine amount
04
Function Assessment
If kidney isn't clearing creatinine well, blood levels will be disproportionately high compared to urine levels
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Imaging
X-Ray Procedures: CT Scans and IVP
CT Scans
Transverse x-ray views of the kidney taken with or without contrast material. Useful in diagnosing tumours, cysts, abscesses, and hydronephrosis. May be obtained in renal failure when contrast material should not be given.
Intravenous Pyelogram (IVP)
Contrast material injected within a vein travels to kidney where it's filtered into urine. X-rays show contrast filling kidneys, ureters, bladder, and urethra. Tests renal function and shows cysts, tumours, infections, hydronephrosis, and calculi. Also called excretory urogram.
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Additional X-Ray Procedures
Kidneys, Ureters, and Bladder (KUB)
X-ray record without contrast material demonstrating the size and location of kidneys in relation to other organs in the abdominopelvic region.
Renal Angiography
Contrast material injected into bloodstream with x-rays taken of kidney blood vessels. Helpful for diagnosing kidney tumours and outlining renal vessels in hypertensive patients.
Retrograde Pyelogram
Contrast material introduced directly into bladder and ureters through cystoscope. Used to determine presence of stones or obstructions. Indicated when poor renal function prevents IVP visualisation or when patient is allergic to intravenous contrast.
Voiding Cystourethrogram (VCUG)
Bladder filled with contrast material and x-rays taken of bladder and urethra as patient expels urine.
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Advanced Imaging Techniques
Ultrasonography
Uses sound waves to diagnose:
- Kidney size
- Tumours
- Hydronephrosis
- Polycystic kidney
- Ureteral and bladder obstruction
No information about renal function is obtained.
Radioisotope Studies
Radioactive substance injected into bloodstream in small amounts and taken up by kidneys.
Renal Scan: Shows size and shape of kidney
Renogram: Shows kidney function
Can indicate size of blood vessels, diagnose obstruction, and determine separate function of each kidney.
Magnetic Resonance Imaging (MRI)
Patient lies in cylindrical magnetic machine. Images made of pelvic and retroperitoneal regions using magnetic waves based on movement of protons.
Images taken in all three body planes: frontal, sagittal, and transverse. Useful for showing pelvic, retroperitoneal, and vascular anatomy.
Clinical Procedures
Cystoscopy and Dialysis
Cystoscopy
Visual examination of urinary bladder using a cystoscope. Hollow metal tube introduced through urinary meatus, passed through urethra into bladder. Light source, special lenses, and mirrors examine bladder mucosa for tumours, calculi, or inflammation. Catheter can be placed through cystoscope for urine samples or contrast material injection. Panendoscope provides wide-angle bladder view.
Dialysis
Waste materials like urea separated from bloodstream when kidneys cannot function. Haemodialysis: Artificial kidney machine receives waste-filled blood, filters it, and returns dialysed blood to patient's body. Peritoneal Dialysis: Fluid introduced into peritoneal cavity via catheter. Wastes pass from capillaries into fluid, then removed by catheter. Can be intermittent (IPD) or continuous ambulatory (CAPD).
Specialised Kidney Procedures
1
Extracorporeal Shock Wave Lithotripsy
Shock waves crush urinary tract stones into tiny fragments that can be passed in urine. Patient receives anaesthesia and is immersed in water tank. Shock waves generated electrically. Fluoroscopy positions patient so stone receives shock waves properly.
2
Renal Biopsy
Kidney biopsy performed during surgery (open) or through skin (percutaneous/closed). For percutaneous technique, patient lies prone. After local anaesthesia to skin and back muscles, biopsy needle inserted with fluoroscopy or ultrasonography guidance. Tissue obtained for microscopic examination by pathologist.
3
Renal Transplantation
Kidney transplanted into patient with renal failure from identical twin (isograft) or other individual (allograft). Best results when donor closely related to recipient. Better than 90% of kidneys survive for 1 year or longer.
4
Urinary Catheterisation
Flexible, tubular instrument passed through urethra into urinary bladder. Used for urine drainage, introducing irrigating solutions, or inflating bladder. Foley catheter is indwelling catheter held in place by balloon inflated with air or liquid.
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