Renal system

Renal System and Long-Term Blood Pressure Regulation

Anatomy of the Urinary System

Gross Anatomy of the Kidneys

  • Location: Located in the retroperitoneal space (posterior to the peritoneum)

  • Span: Extend from T12 to L3

  • Protection: Protected posteriorly by the floating ribs

Internal Kidney Anatomy

  • Renal Cortex:

    • Outer regions

    • Granular and reddish-brown appearance

  • Renal Medulla:

    • Inner regions composed of renal pyramids with striped appearance

  • Renal Columns (Medullary Pyramids): Separate renal pyramids

  • Calyces:

    • Major and minor calyces found in the renal pelvis

    • Collect urine from renal lobes (a pyramid and surrounding cortical tissue)

Structure and Function of the Ureters

  • Transport Role: Carry urine out of the kidneys to the bladder

  • Functionality: Capable of peristalsis

  • Backflow Prevention: Bladder filling compresses the distal end of the ureter, preventing backflow

Common Features of the Bladder

  • Trigone: Triangular area at the bottom of the bladder with corners being the paired ureters and the urethra

  • Inner Mucosa:

    • Transitional epithelium that folds into rugae

  • Middle Layer (Detrusor Muscle): Drives urination by compressing bladder

  • Internal Urethral Sphincter: Thick muscle near the urethra forming a ring of smooth muscle

  • External Urethral Sphincter:

    • Passes through a ring of skeletal muscle on its way out

    • External urethral orifice

Review: Spinal Reflexes in Urination (Micturition Reflex)

  • Stretch Receptors:

    • Located in the urinary bladder report fullness when stretched

    • Send nerve impulses to smooth muscle in the wall of urinary bladder

  • Spinal Cord Involvement: Nerve impulses contribute to the voiding process

Micturition and the Brain

  • Neural Control of Micturition:

    • Cerebral Cortex: Higher thought processes involved

    • Pons: Important for controlling micturition

    • Pelvic Nerve: Parasympathetic fibers control bladder contraction

    • Sensory and Motor Pathways:

    • Stretch receptors send impulses to detrusor muscle and sphincters

    • Internal sphincter controlled by autonomic nervous system; External by somatic motor fibers

Structure of the Urinary System in Females

  • Urethra Length: Only 3-5 cm in length; functions only in transport of urine

Structure of the Urinary System in Males

  • Urethra Length: Approximately 20 cm; functions in transport of both urine and semen

  • Complications:

    • Inflammation of the prostate can cause:

    • Compression of bladder

    • Narrowing of urethra

Micro-Anatomy of the Urinary System

Microscopic Anatomy of the Ureters

  • Deepest Layer (Mucosa):

    • Transitional epithelium that stretches with urine

  • Middle Layer (Muscularis): Trigger reflexive peristalsis when distended

  • Superficial Layer (Adventitia):

    • Fibrous connective tissue anchoring ureter

Kidney Vasculature

  • Renal Artery:

    • Enters at the hilus (hilum)

    • Branches into segmental arteries and interlobar arteries

  • Arcuate Arteries:

    • Branch from interlobar arteries

  • Cortical Radiate Arteries:

    • Lead to microscopic afferent arterioles

Nephrons

  • Definition: Nephron is the structural and functional unit of urine formation

  • Filtration:

    • Occurs in the renal corpuscle

    • Fluid and solute forced from glomerulus into the capsule's lumen

  • Blood Flow Order in Nephrons:

    1. Afferent arteriole

    2. Glomerulus

    3. Efferent arteriole

    4. Peritubular capillaries (cortex)

    5. Vasa recta (medulla)

    6. Interlobar vein

    7. Renal vein

    • Glomerular Capsule + Glomerulus = Renal Corpuscle

    • Renal tubules consist of numerous ducts and tubules.

Flow of Fluid Through the Nephron's Tubules

  • Pathway:

    • Proximal Convoluted Tubule (PCT)

    • Descending Tube (DT)

    • Loop of Henle (LH)

    • Ascending Tube (AT)

    • Distal Convoluted Tubule (DCT)

Nephron Classifications

  • Cortical Nephron:

    • Glomerulus closer to renal capsule

    • Shorter Loop of Henle; performs excretion and regulation

  • Juxtamedullary Nephron:

    • Glomerulus near junction of cortex and medulla

    • Longer Loop of Henle extending deep into the medulla; concentrates urine

Fluid Flow After Filtration Through Nephron

  • Collecting Duct (CD):

    • Contains filtrate from many nephrons

  • Principal Cells:

    • Maintain the body’s water, Na+, and K+ balance

  • Intercalated Cells:

    • Responsible for acid-base balance

Formation of Urine

Filtration Process

Filtration Layers

  1. Capillary Endothelium:

    • Fenestrated and very permeable

    • Allows passage of molecules smaller than a cell

  2. Basement Membrane:

    • Fused but less permeable

    • Blocks passage of all but small proteins

  3. Podocytes of Glomerular Capsule:

    • Surround capillaries; projections create filtration slits

    • Prevent passage of most molecules

Filtration Driven by Pressure Differences

  • GBHP (Glomerular Blood Hydrostatic Pressure):

    • Blood pressure in glomerulus driving filtration

  • CHP (Capsular Hydrostatic Pressure): Opposes filtration due to pressure in glomerular capsule

  • BCOP (Blood Colloid Osmotic Pressure): Opposes filtration due to osmotic pull of non-filtered proteins

  • NFP (Net Filtration Pressure): Calculated as
    NFP=GBHP−(CHP+BCOP)NFP = GBHP - (CHP + BCOP)

Glomerular Filtration Rate (GFR)

  • Definition: Total volume of filtrate formed by all glomeruli of both kidneys per minute

  • Typical Rates:

    • Males: 90-140 mL/min

    • Females: 80-125 mL/min

  • Relation between NFP and GFR: The magnitude of NFP is directly proportional to GFR

Reabsorption and Secretion

Around the Collecting Ducts and Loops

  • Reabsorption:

    • Substances forced out of glomerulus are selectively retained

  • Proximal Convoluted Tubule (PCT):

    • Active transport of ions (Na+, HCO3-, Ca2+, K+)

    • Active transport of glucose, proteins, vitamins

    • Passive reabsorption of water and some ions

  • Descending Loop:

    • Reabsorbs most remaining water

  • Ascending Loop:

    • Permeable to Na+, Cl-, K+; not permeable to water

Tubular Reabsorption and Secretion: The Countercurrent Multiplier

  • Countercurrent: Filtrate in ascending and descending limbs moves in opposite directions.

  • Multiplier:

    • Active reabsorption of NaCl in ascending limb

    • Urea pumped out of collecting duct, increasing medulla osmolarity

    • Water drawn from descending limb by osmosis; increases filtrate osmolarity

  • Definitions:

    • Osmolarity: Concentration of solute

    • Urea: Nitrogenous waste formed by protein breakdown in liver

Anatomy of the Juxtaglomerular Apparatus (JGA)

  • Location: Contact area of distal convoluted tubule and afferent arteriole

  • Macula Densa Cells:

    • Monitor Cl- and Na+ concentrations in filtrate

  • Granular Cells:

    • Respond to changes in blood pressure, releasing renin when blood pressure is low

Regulation of Fluid Pools

Volume & Osmolarity Sensors

  • Baroreceptors:

    • Located at carotid sinus, aortic arch, renal afferent arteriole, and juxtaglomerular apparatus

  • Hemodynamic Control: Response to changes in blood volume or pressure

Change in Plasma Osmolarity

  • Key Control Points:

    • Hypothalamus: Uses osmoreceptors to regulate thirst and ADH based on osmolarity changes

    • Kidneys: Ideate urine formation and regulate solute loss; principal cells in collecting duct are target for ADH

Acid-Base and Ion Balance

Ion Balance

  • Hyponatremia: Low plasma Na+ due to various conditions; leads to CNS dysfunction

  • Hypokalemia: Low plasma K+; results from vomiting, diarrhea; leads to muscle weakness

  • Hypernatremia: High plasma Na+; arises from dehydration, etc.; leads to CNS dysfunction

  • Hyperkalemia: High plasma K+; results from renal failure; can cause muscle fatigue and heart issues

Kidney Transport Mechanisms

  • Counter-Transport: Two molecules move oppositely across a membrane

  • Co-Transport: One molecule's movement is dependent on another's passive movement in the same direction

Physical Characteristics of Urine

General Observations

  • Color: Typically yellow-amber; varies with diet/hydration; dark indicates dehydration, red signals blood presence

  • Smell: Generally mild; strong odor indicates infection; sweet smell suggests diabetes

  • pH: Range of 4.6 - 8, averaging around 6.0

Specific Gravity

  • Defined as the ratio of substance density compared to distilled water

  • Normal range from 1.001 to 1.035

Turbidity

  • Normally clear or slightly cloudy; excess turbidity indicates suspended particles or infections

Long-Term Regulation of Blood Pressure

Hormonal Influence

  • Atrial Natriuretic Peptides (ANP):

    • Secreted by heart; maintains blood volume, Na+, and arterial blood pressure through Na+ excretion

  • Antidiuretic Hormone (ADH): Plays key role in water retention, influencing blood volume and pressure, acts on kidney mechanisms

  • Aldosterone: Increases sodium and water retention; impacts blood pressure

Renin-Angiotensin-Aldosterone System (RAAS)

  1. Initiation: Blood pressure falls (e.g., systolic below 100 mm Hg)

  2. Renin Release: Kidneys secrete renin, activating RAAS

  3. Angiotensinogen Split: Renin splits angiotensinogen into angiotensin I

  4. Conversion to Angiotensin II: ACE converts angiotensin I to angiotensin II, causing arterioles to constrict, thereby increasing blood pressure

  5. Effect of Angiotensin II: Triggers aldosterone and ADH release; increases Na+ retention, raises blood volume and pressure

Kidney Testing and Disease

Clinical Evaluation Through Testing

  • Blood Tests:

    • Serum Creatinine: Waste product from muscle use; healthy kidneys excrete, levels increase with dysfunction

    • GFR: Measures kidney efficiency in waste removal; normal is 90 or above; <60 indicates poor function

    • BUN: Byproduct of protein breakdown; normal levels between 7-20; rises with kidney function decrease

Testing for Obstruction and Inflammation

  • Ultrasound: Sound waves create images of kidney structure, can detect obstructions

  • CT Scan: Uses X-rays to check kidney structure; may use IV contrast dye

  • Biopsy: Examines kidney tissue for specific diseases and treatment responses

Urinalysis

  • Tests various substances in urine, using dipsticks for color change indicators

Renal Clearance Tests

  • Plasma Clearance: Estimates GFR through substances like creatinine

  • Inulin Clearance: Measures GFR after injecting inulin, as it is completely filtered by kidneys

Pharmacology

**Blood Pressure Medications: **Includes ACE Inhibitors, ARBs, and Diuretics to manage sodium and water retention, impacting blood volume and pressure

  • IV Fluids: Used in cases of toxic substance dilution

  • NSAIDs: Can disrupt renal vasodilation response and cause kidney issues

Effects of Aging on Kidney Function

  • Nephron Loss: Approximately 40% decrease in functional nephrons between age 20 and 70.

  • Filtration Decline: Progressive damage to filtration membrane; decreases GFR by around 50%

  • ADH Sensitivity Loss: Leads to excess Na+ loss in urine

  • Bladder Changes: Decrease in size leads to more frequent emptying and nocturia

  • Sphincter Control Loss: Leads to urinary incontinence; prostate enlargement can cause urinary retention in males

Clinical Disorders Related to Aging

  • Polycystic Kidney Disease (PKD):

    • Characterized by cysts in the kidneys and other organs; genetic in origin; symptoms include high blood pressure and abdominal issues; treatment is symptomatic with potential for dialysis or transplant.

Conclusion Check-In

  • Key Hormones in Regulation: ADH, ANP, Aldosterone

  • Creatinine Significance: Marker of kidney function; variations indicative of dysfunction

  • Urine Characteristics: Healthy urine color and pH ranges, indicative of overall health.