Comprehensive Renal Physiology and Urine Formation Study Guide

Introduction to the Renal System and Kidney Functions

• Definition of Excretion: Excretion is the process by which unwanted substances and metabolic wastes are eliminated from the body. During metabolic processes, waste materials and carbon dioxide are produced in tissues. Other substances requiring removal include residues of undigested food, heavy metals, drugs, toxic substances, and pathogenic organisms such as bacteria.

• Excretory Systems and Organs:

  • Digestive System: Excretes food residues, bacteria, and toxic substances in the form of feces.

  • Lungs: Remove carbon dioxide and water vapor.

  • Skin: Excretes water, salts, and some wastes; also removes heat from the body.

  • Liver: Excretes bile pigments, heavy metals, drugs, toxins, and bacteria through bile.

  • Renal/Urinary System: Holds the maximum excretory capacity and plays the primary role in homeostasis.

• Components of the Renal System:

  1. A pair of kidneys (produce urine).

  2. Ureters (transport urine to the bladder).

  3. Urinary bladder (stores urine until voided).

  4. Urethra (the path through which urine is voided from the bladder).

Primary Functions of the Kidney

• Role in Homeostasis: This is the primary function of the kidneys, accomplished through urine formation and the regulation of various body activities:

  • Excretion of Waste Products: Includes urea (amino acid metabolism), uric acid (nucleic acid metabolism), creatinine (muscle metabolism), bilirubin (hemoglobin degradation), and products of other substance metabolism. It also includes foreign chemicals like toxins, drugs, heavy metals, and pesticides.

  • Maintenance of Water Balance: Conserving water when it is decreased and excreting excess water.

  • Maintenance of Electrolyte Balance: Particularly sodium; kidneys retain sodium if osmolarity decreases and eliminate it if osmolarity increases.

  • Maintenance of Acid-Base Balance: Kidneys are the only organs capable of eliminating metabolic acids like sulfuric and phosphoric acids, preventing acidosis.

• Hemopoietic Function: Kidneys secrete erythropoietin (stimulating erythrocyte production) and thrombopoietin (stimulating thrombocyte production).

• Endocrine Function: Secretion of hormones including:

  • Erythropoietin

  • Thrombopoietin

  • Renin

  • 1,25-dihydroxycholecalciferol (calcitriol)

  • Prostaglandins

• Regulation of Blood Pressure: Regulates arterial blood pressure long-term by:

  • Regulating extracellular fluid (ECF) volume.

  • The renin-angiotensin mechanism.

• Regulation of Blood Calcium Level: Kidneys activate 1,25-dihydroxycholecalciferol into Vitamin D, which is necessary for intestinal calcium absorption.

Functional Anatomy of the Kidney

• Structure and Layers: The kidney is a compound tubular gland covered by a connective tissue capsule. The hilum is a depression on the medial border where the renal artery, renal veins, nerves, and ureter pass.

• Structural Layers:

  1. Outer Cortex: Dark and granular; contains renal corpuscles and convoluted tubules. Renal columns (Columns of Bertini) are cortical tissue penetrations into the medulla.

  2. Inner Medulla: Contains tubular and vascular structures in parallel lines. Divided into 8 to 18 medullary (Malpighian) pyramids. The apex of each pyramid projects into a minor calyx.

  3. Renal Sinus: Contains the renal pelvis (expanded ureter top), subdivisions (2-3 major calyces, ~8 minor calyces), nerve branches, arteries, veins, loose connective tissue, and fat.

• Tubular Structures: The kidney consists of uriniferous tubules:

  • Nephrons: Terminal/secretory tubules concerned with urine formation.

  • Collecting Ducts/Tubules: Transport urine from nephrons to the renal pelvis. They unite to form ducts of Bellini, which open into minor calyces through the papilla.

Anatomy and Classification of the Nephron

• Definition: The structural and functional unit of the kidney. Each kidney contains 1 to 1.3 million nephrons. After age 45-50, the number decreases at a rate of $0.8\%$ to $1\%$ per year.

• Major Parts of the Nephron:

  1. Renal Corpuscle (Malpighian Corpuscle): A spheroidal, flattened structure ($200\,\mu$ diameter) for filtration.

  2. Tubular Portion (Renal Tubule): Continuation of the Bowman capsule.

• Types of Nephrons:

  • Cortical (Superficial) Nephrons: $85\%$ of all nephrons. Corpuscles are in the outer cortex near the periphery. Loops of Henle are short, penetrating only the outer medulla. Supplied by peritubular capillaries.

  • Juxtamedullary Nephrons: $15\%$ of all nephrons. Corpuscles are in the inner cortex near the medulla. Loops of Henle are long, reaching the tip of the papilla. Supplied by vasa recta. Function in urine concentration.

Structure of the Renal Corpuscle

• Glomerulus: A tuft of capillaries enclosed by the Bowman capsule, interposed between the afferent arteriole (source) and the efferent arteriole (exit). The vascular system is purely arterial. The efferent arteriole has a smaller diameter than the afferent arteriole.

• Glomerular Histology: Single layer of endothelial cells with fenestrae (filtration pores) of $0.1\,\mu$ diameter.

• Bowman Capsule: A double-layered capsular structure ($200\,\mu$ diameter):

  • Inner Visceral Layer: Covers glomerular capillaries. Contains podocytes with cytoplasmic extensions called pedicles (feet). Pedicles leave cleft-like spaces called slit pores or filtration slits.

  • Outer Parietal Layer: Continues with the tubule wall.

  • The fused basement membranes of the capillary and visceral layer form the filtration separation.

Tubular Portion of the Nephron

1. Proximal Convoluted Tubule (PCT): Length $14\,mm$, diameter $55\,\mu$. Formed by cuboidal epithelial cells with "brush-bordered" hair-like projections for increased surface area.

2. Loop of Henle:

   • Descending Limb: Includes a thick descending segment (continuation of PCT, $6\,mm$ long, $55\,\mu$ diameter, brush-bordered) and a thin descending segment (flattened cells, no brush border).

   • Hairpin Bend: Flattened cells, no brush border.

   • Ascending Limb: Includes a thin ascending segment (flattened cells, $10-15\,mm$ total length for thin sections, $15\,\mu$ diameter) and a thick ascending segment ($9\,mm$ long, $30\,\mu$ diameter, cuboidal cells, no brush border).

   • Macula Densa: Terminal portion of the thick ascending segment running between afferent and efferent arterioles.

3. Distal Convoluted Tubule (DCT): Length $14.5-15\,mm$, diameter $22-50\,\mu$. Lined by cuboidal epithelial cells without brush borders (Intercalated cells or I cells).

4. Collecting Duct: Length $20-22\,mm$, diameter $40-200\,\mu$. Formed by Principal (P) cells and Intercalated (I) cells.

Juxtaglomerular Apparatus (JGA)

• Definition: A specialized organ situated near the glomerulus of each nephron.

• Structural Components:

  1. Macula Densa: Tightly packed cuboidal cells in the thick ascending segment. Acts as a sensor for sodium chloride.

  2. Extraglomerular Mesangial Cells: Also called agranular, lacis, or Goormaghtigh cells. Situated in the region between arterioles and macula densa.

  3. Juxtaglomerular (JG) Cells: Specialized smooth muscle cells (granular cells) in the wall of the afferent arteriole. They form a "polar cushion" or polkissen.

• Functions:

  • Secretion of Hormones: Renin (peptide with 340 amino acids) and Prostaglandins.

  • Renin Secretion Stimuli: Fall in arterial BP, reduction in ECF volume, increased sympathetic activity, or decreased NaCl load in macula densa.

The Renin-Angiotensin System

1. Angiotensinogen ($\alpha_1$-globulin) is converted to Angiotensin I (decapeptide) by Renin.

2. Angiotensin I is converted to Angiotensin II (octapeptide) by Angiotensin-Converting Enzyme (ACE) primarily in the lungs.

3. Angiotensin II (half-life $1-2\,minutes$) is degraded to Angiotensin III (heptapeptide) and then Angiotensin IV (hexapeptide) by angiotensinases.

• Actions of Angiotensin II:

  • Vasoconstriction (increases BP).

  • Stimulates Aldosterone secretion (sodium retention).

  • Regulates GFR (constricts efferent arteriole, contracts mesangial cells).

  • Stimulates thirst, ADH secretion, and CRH/ACTH secretion.

Renal Circulation

• Blood Flow: $1,300\,mL/minute$ ($26\%$ cardiac output).

• Arterial Path: Renal Artery $\rightarrow$ Segmental $\rightarrow$ Interlobar $\rightarrow$ Arcuate $\rightarrow$ Interlobular $\rightarrow$ Afferent Arteriole $\rightarrow$ Glomerular Capillaries.

• Portal System: Renal circulation uses two capillary sets: Glomerular capillaries (high pressure, $60-70\,mm\,Hg$) and Peritubular capillaries (low pressure, $8-10\,mm\,Hg$).

• Vasa Recta: Specialized straight capillaries for juxtamedullary nephrons.

• Autoregulation: Maintains GFR when mean arterial BP is between $60\,mm\,Hg$ and $180\,mm\,Hg$.

  • Myogenic Response: Stretch-induced contraction of afferent arteriole via calcium influx.

  • Tubuloglomerular Feedback: Macula densa-mediated control. High NaCl triggers adenosine to constrict the afferent arteriole.

Urine Formation: Glomerular Filtration

• Process: First step of urine formation. Plasma is filtered into the Bowman capsule, excluding plasma proteins due to their size.

• Glomerular Filtration Rate (GFR): Normal value is $125\,mL/minute$ or $180\,L/day$.

• Filtration Fraction: $\text{Filtration fraction} = \frac{\text{GFR}}{\text{Renal plasma flow}} \times 100 = \frac{125}{650} \times 100 \approx 19.2\%$.

• Pressures Determining Filtration:

  1. Glomerular Capillary Pressure: $60\,mm\,Hg$ (favors filtration).

  2. Colloidal Osmotic Pressure: $25\,mm\,Hg$ (opposes filtration).

  3. Hydrostatic Pressure in Bowman Capsule: $15\,mm\,Hg$ (opposes filtration).

  • Net Filtration Pressure: $60 - (25 + 15) = 20\,mm\,Hg$.

• Filtration Coefficient: $125\,mL / 20\,mm\,Hg = 6.25\,mL/mm\,Hg$.

Tubular Reabsorption and Secretion

• Reabsorption: $99\%$ of the filtrate is reabsorbed.

• Threshold Substances:

  • High-threshold: Not in urine normally (Glucose, amino acids, vitamins).

  • Low-threshold: Appear in urine normally (Urea, uric acid).

  • Non-threshold: Not reabsorbed at all (Creatinine).

• Transport Maximum ($T_{m}$): For Glucose ($T_{mG}$), it is $375\,mg/minute$ in males and $300\,mg/minute$ in females.

• Renal Threshold for Glucose: $180\,mg/dL$. Splay is the deviation between predicted and actual threshold due to varying nephron capacities.

• Water Reabsorption:

  • Obligatory: Secondary to sodium reabsorption in PCT ($7/8$ of filtrate).

  • Facultative: Occurs in DCT and collecting duct via ADH and Aquaporins (specifically AQP-2).

• Tubular Secretion:

  • Potassium (active in PCT, DCT, collecting duct).

  • Ammonia (PCT).

  • Hydrogen ions (PCT, DCT).

  • Urea (Loop of Henle).

  • Foreign substances: PAH, Penicillin, etc.

Concentration of Urine

• Osmolarity: Plasma is $300\,mOsm/L$; concentrated urine is $1,200\,mOsm/L$.

• Medullary Gradient: Gradual increase in medullary interstitial fluid osmolarity from $300$ up to $1,200\,mOsm/L$.

• Countercurrent Multiplier (Loop of Henle): Solute reabsorption (NaCl) in the ascending limb and addition of new solutes multiply medullary osmolarity.

• Countercurrent Exchanger (Vasa Recta): Maintains the gradient by removing water and retaining salt. The hairpin arrangement prevents salt washout.

• Urea Recirculation: Accounts for $50\%$ of hyperosmolarity in inner medulla; regulated by UT-A1 and UT-A3 transporters (activated by ADH).

Micturition Reflex and Bladder Control

• Anatomy: The bladder wall consists of the detrusor muscle. The trigone is the triangular area at the base. Male urethra ($20\,cm$) has prostatic, membranous, and spongy parts. Female urethra is $3.5-4\,cm$.

• Nerve Supply:

  • Parasympathetic (Pelvic Nerve/Nervus Erigens): S2, S3, S4. Contracts detrusor, relaxes internal sphincter (Nerve of Emptying).

  • Sympathetic (Hypogastric Nerve): L1, L2. Relaxes detrusor, constricts internal sphincter (Nerve of Filling).

  • Somatic (Pudendal Nerve): S2, S3, S4. Constricts external sphincter (Voluntary control).

• Cystometrogram: Graph of intravesical pressure vs. volume.

  • Segment I: Pressure rises to $10\,cm\,H_2O$ at $100\,mL$.

  • Segment II (Plateau): Constant pressure due to Law of Laplace: $P = \frac{T}{R}$.

  • Segment III: Pressure rises sharply after $400\,mL$.

• Abnormalities:

  • Atonic Bladder: Loss of tone, overflow incontinence (e.g., Tabes dorsalis).

  • Automatic Bladder: Hyperactive reflex, loss of voluntary control (spinal cord transection recovery).

  • Nocturnal Micturition (Enuresis): Bedwetting; normal below age 3, abnormal thereafter.