Kidney

Urinary System Study Notes

Learning Objectives

  • Understand kidney structure and function in maintaining homeostasis.

  • Identify parts of the nephron and their roles in blood balance and urine formation.

  • Explain filtration, tubular reabsorption, and secretion in renal physiology.

  • Discuss urine volume regulation mechanisms.

  • Describe the structure and function of ureters, urinary bladder, and urethra.

Kidneys

Location of the Kidneys

  • Positioned under back muscles, behind the parietal peritoneum, just above the waistline.

  • Right kidney is typically lower than the left kidney.

Gross Structure of the Kidney

  1. Renal Cortex: The external layer of the kidney.

  2. Renal Medulla: The internal portion of the kidney.

Internal Structure of the Kidney

  1. Renal Pyramids: Triangular divisions within the medulla.

  2. Renal Columns: Cortical tissues that extend between renal pyramids.

  3. Renal Papilla: The narrow, innermost end of each pyramid.

  4. Renal Pelvis: The expanded top part of the ureter that is located inside the kidney.

  5. Renal Calyces: Divisions of the renal pelvis.

Microscopic Structure of the Kidney

  • Composed of over 1 million microscopic nephron units.

  • Nephrons have a unique shape beneficial for their function, mainly consisting of:

    • Renal Corpuscle: Composed of the Bowman capsule and glomerulus.

    • Renal Tubule: Includes several segments crucial for urine formation.

Renal Corpuscle

  • Bowman Capsule: A cup-shaped structure encompassing the glomerulus.

  • Glomerulus: A network of blood capillaries that filters blood and is enclosed by Bowman capsule.

  • Cortical Nephrons: Make up about 85% of total nephrons, primarily located in the renal cortex.

  • Juxtamedullary Nephrons: Located near the boundary between the cortex and medulla; essential for urine concentration.

Renal Tubule

  • Proximal Convoluted Tubule (PCT): The first segment of the tubule involved in reabsorption.

  • Henle Loop: An extension of the proximal tubule consisting of:

    • Descending Limb

    • Loop

    • Ascending Limb

  • Distal Convoluted Tubule (DCT): Continues from the ascending limb of Henle loop.

  • Collecting Duct (CD): A straight extension of the distal tubule that leads to urine drainage.

Kidney Functions

  • Excretion of Toxins: Removes nitrogenous wastes from the body.

  • Chemical Regulation: Controls various substances' levels in the blood.

  • Water Balance: Regulates hydration levels in the body.

  • Blood Pressure Regulation: Influences blood volume and pressure through various mechanisms.

  • Erythropoietin Secretion: Controls red blood cell production through the hormone EPO.

Formation of Urine

Filtration

  • Process: Occurs continuously in the renal corpuscles.

    • Glomerular blood pressure causes water and dissolved substances to filter from glomeruli into Bowman capsule.

  • Normal Glomerular Filtration Rate: About 125 ext{ mL/min}.

Reabsorption

  • Definition: Movement of substances from renal tubules back into blood in peritubular capillaries.

  • Involves: Reabsorption of water, nutrients, and ions.

    • Water reabsorbed by osmosis, primarily from proximal tubules.

    • Sodium reabsorption occurs at the Henle loop using countercurrent flow mechanisms.

  • Countercurrent Mechanisms: Help maintain hypertonicity in the medulla, enabling additional water reabsorption.

    • Sodium Reabsorption: Varies with diet; increased intake leads to more excretion in urine.

    • Chloride Ions: Passively absorbed due to the negative charge in blood.

Reabsorption of Glucose

  • Process: Glucose is absorbed from proximal tubules back into blood.

  • Renal Threshold: When blood concentration of glucose exceeds this, it remains in the urine, often indicating diabetes.

Secretion

  • Definition: Transfer of substances into urine via active transport from blood in peritubular capillaries to urine in distal and collecting ducts.

  • Substances Secretion: Includes hydrogen ions, potassium ions, urea, uric acid, creatinine, and certain medications. Ammonia is secreted by diffusion.

Control of Urine Volume

  1. Antidiuretic Hormone (ADH): Released by the posterior pituitary; promotes water reabsorption in collecting ducts, reducing urine volume.

  2. Aldosterone: Secreted by adrenal glands; promotes sodium and water reabsorption, reducing urine volume.

  3. Atrial Natriuretic Hormone (ANH): Promotes sodium and water loss through kidney tubules, increasing urine volume.

Renin-Angiotensin-Aldosterone System (RAAS)

  • Mechanism: Restores blood pressure and plasma volume.

    1. Low blood pressure or blood volume triggers the juxtaglomerular apparatus to release renin.

    2. Renin converts angiotensinogen into angiotensin I.

    3. Angiotensin-converting enzyme (ACE) transforms angiotensin I to angiotensin II, leading to vasoconstriction and aldosterone secretion.

    4. Aldosterone enhances water reabsorption in the kidneys.

Abnormal Urine Volume

  • Anuria: Complete absence of urine.

  • Oliguria: Reduced urine volume.

  • Polyuria: Excess urine production.

Ureters

Structure

  • Long, narrow tubes with expanded upper ends (renal pelvis) located within the kidney.

  • Lined with a mucous membrane and a muscular layer.

Function

  • Drains urine from the renal pelvis to the urinary bladder.

Urinary Bladder

Structure

  • Elastic muscular organ capable of considerable expansion.

  • Mucous membrane lining arranged in rugae (folds).

Functions

  • Storage of urine before voiding.

  • Voiding urine when appropriate.

Urethra

Structure

  • A narrow tube extending from the urinary bladder to the exterior.

  • Lined with a mucous membrane; opening is known as the urinary meatus.

Functions

  • Allows passage of urine from the bladder to the outside of the body.

  • In males, also serves as a pathway for reproductive fluid (semen).

Micturition

  • Defined as the passage of urine from the body, also called urination or voiding.

Regulatory Sphincters

  1. Internal Urethral Sphincter: Involuntary control.

  2. External Urethral Sphincter: Voluntarily controlled.

Emptying Reflex

  • Initiated by bladder wall stretch; causes bladder wall contraction, relaxation of internal sphincter, and external sphincter relaxation, leading to urination.

  • Enuresis: Involuntary urination in children.

Retention and Suppression

  • Urinary Retention: urine is produced but not voided.

  • Urinary Suppression: no urine production, despite normal bladder function.

Urinary Incontinence

  • Defined as involuntary urination.

    • Types:

    • Urge Incontinence: Smooth muscle overactivity in bladder wall.

    • Stress Incontinence: Due to weakened pelvic floor muscles.

    • Overflow Incontinence: Related to urinary retention and an overdistended bladder.

    • Reflex Incontinence: Lack of sensory warning or awareness, often post-stroke or spinal injury.

    • Nocturnal Enuresis: Bedwetting at night.

    • Neurogenic Bladder: Unpredictable voiding related to neurological conditions.

Urinalysis

  • Definition: Examination of urine's physical, chemical, and microscopic properties.

    • Characteristics Assessed: Color, turbidity, odor, specific gravity; providing diagnostic clues.

    • Chemical Analysis: Tests pH, urea concentration, and abnormal chemicals like proteins and glucose.

    • Microscopic Examination: Utilizes centrifugation to separate heavy cells for examination.

Renal and Urinary Disorders

Obstructive Disorders

  • Causes: Interfere with normal urine flow; can lead to hydronephrosis or other kidney damage.

    • Hydronephrosis: Enlargement of renal pelvis and calyces due to urine flow blockage.

    • Renal Calculi (Kidney Stones): Mineral chunks causing blockage and intense pain (renal colic).

    • Tumors: Such as renal cell carcinoma and bladder cancer; typically present with hematuria.

Glomerular Disorders

  • Result from damage to the glomerular-capsular membrane.

    • Nephrotic Syndrome: May present with proteinuria, hypoalbuminemia, and edema.

    • Acute Glomerulonephritis: Immune response delay post-streptococcal infection.

    • Chronic Glomerulonephritis: Slow inflammatory condition leading to renal failure.

Kidney Failure

  • Definition: Occurs when kidneys cease to function adequately.

    • Acute Renal Failure: Abrupt, often reversible dysfunction.

    • Chronic Renal Failure: Gradual nephron loss leading to uremia.

    • Polycystic Kidney Disease (PKD): Hereditary condition with fluid-filled cysts damaging kidney tissue.

Progression of Kidney Failure

  1. Stage 1: Healthy nephrons compensate for lost damaged nephrons.

  2. Stage 2 (Renal Insufficiency): Loss leads to uremia (elevated blood urea nitrogen levels, BUN).

  3. Stage 3 (Uremic Syndrome): Complete kidney failure; requires transplant or artificial kidney.

Questions?

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