Anatomy & Physiology II

Chapter 25 - Urinary System

Introduction

This chapter covers the structure and function of the urinary system as presented in the textbook by OpenStax, as discussed by Dr. Parlos.

Glomerulus

• Overview: The glomerulus is a microscopic structure that is crucial for urine formation. It modifies the filtrate into urine.
• Cell Types:
    - Podocytes: These cells possess extensions known as pedicles, which contribute to the filtration process by forming filtration slits.
    - Fenestrae: These are small pores in endothelial cells that aid in filtration.
    - Mesangial Cells: Positioned within the glomerulus, they have supportive and regulatory functions. Refer to p.1214 for detailed diagrams and descriptions.

Juxtaglomerular Apparatus (JGA)

• Location: The JGA is found where the distal convoluted tubule (DCT) meets both the afferent and efferent arterioles of Bowman’s capsule.
• Components:
    - Macula Densa: This consists of a wall of cuboidal cells located between the JGA and DCT that monitors the fluid composition of the DCT.
    - Juxtaglomerular (JG) Cells: These are modified smooth muscle cells of the afferent arteriole that respond to ATP released by the macula densa.
      - Their contraction and relaxation regulate blood flow to the glomerulus.
      - High filtrate levels stimulate contraction, leading to a decreased Glomerular Filtration Rate (GFR), resulting in less urine production and fluid retention. Refer to p.1215-1216 for a visual representation.

Hormonal Regulation in the Urinary System

• Renin: Released by JG cells; regulates by the macula densa that cleaves angiotensinogen into angiotensin.
    - Angiotensin: Converts to Angiotensin I, and subsequently to Angiotensin II, a powerful vasoconstrictor.
      - Aldosterone: Stimulates the kidneys to absorb Na^+, leading to water retention and increased blood pressure (p.1216).

Urine Formation and Pressure Dynamics

• Hydrostatic Pressure: This pressure results from the fluid pressing against surfaces, facilitating movement of fluid out of capillaries into Bowman’s space.
• Osmotic Pressure: Opposes hydrostatic pressure and is created by solute concentration, drawing fluid into capillaries from the capsule (p.1218).

Glomerular Filtration Rate (GFR)

• Definition: GFR is the volume of filtrate formed by both kidneys per minute, occurring when glomerular hydrostatic pressure exceeds luminal (capsular) hydrostatic pressure.
• Pathophysiology: An imbalance may lead to systemic edema, which is the swelling of surrounding tissues (p.1219).

Net Filtration Pressure (NFP)

• Calculation: NFP is derived as follows:
    - $ext{NFP} = ext{GBHP} - ( ext{CHP} + ext{BCOP})$
    - Where GBHP is the glomerular blood hydrostatic pressure, CHP is the capsular hydrostatic pressure, and BCOP is the blood colloid osmotic pressure.
    - The average value of NFP is about 10 mmHg, which is crucial for determining filtration rates through the kidneys (p.1220).

Tubular Reabsorption

• Overview: Each day, 180 L of fluid passes through the nephrons; approximately 10% of this original fluid proceeds to the collecting tubules.
• Mechanisms of Reabsorption:
    - Movement of ions occurs through different means, including active transport, diffusion, facilitated diffusion, secondary active transport, and osmosis (p.1221).

Proximal Convoluted Tubule (PCT)

• Function: It is the initial site for urine formation involving modification through absorption and secretion.
• Reabsorption Process:
    - Substances that are reabsorbed return to circulation via peritubular and vasa recta capillaries.
    - The capillaries adjust pressure in the afferent arteriole by constricting or relaxing.
    - Obligatory Water Reabsorption: Na^+ is pumped, and water follows passively through osmosis into the interstitial space (p.1224).

Specific Ion Transport Mechanisms

• Na^+/K^+ Pump: This mechanism moves Na^+ into the interstitial space with approximately 67% of H2O, Na^+, and K^+ being reabsorbed.
• Reabsorption of Organic Substances: Nearly 100% of glucose, amino acids, and organic substances (vitamins) are reabsorbed.
    - Glycosuria: This condition occurs when excess glucose is present in the urine, commonly linked to diabetes mellitus (Type I or II) (p.1226).

Ion Transport Types

• Symport: Na^+ moves down its concentration gradient and cotransports glucose along with it.
• Active Transport: Utilizes the Na^+/K^+ pump.
• Cotransport: HCO3^- ions move alongside Na^+.
• Antiport: Exchanges Na^+ with H^+ ions.
• Osmotic Gradient: Created due to solute recovery with Aquaporins facilitating H2O movement out of the collecting duct (p.1226).

Bicarbonate (HCO3^-) Recovery

• Importance: Critical for acid-base balance in the body and acts as a significant buffer.
• Carbonic Anhydrase (CA): This brush border enzyme facilitates the creation of HCO3^- in other areas of the body (p.1226).

Loop of Henle

• Structure: Comprises the descending (DL) and ascending loops (AL), which have thick and thin portions.
• Function: It recovers Na^+, Cl^-, and H2O. The nature of nephron impacts recovery; cortical nephrons are located higher up while juxtamedullary nephrons extend deeper into the medulla (p.1226).

Countercurrent Multiplier System (CMS)

• Mechanism: This system is established by the nephron loop and vasa recta; the DL and AL flow in opposite directions creating a concentration gradient.
• Concentration Gradients: Ranging from 300 to 1200 to 100 mOsmol/kg, which demonstrates the efficiency of the countercurrent mechanism in concentrating urea and Na^+ in the medulla.
• Urea Dynamics: NH3, a byproduct from protein metabolism, is converted in the liver to urea, which is less toxic. Urea is also absorbed by the loops as it descends through the medulla (p.1227).

Distal Convoluted Tubule (DCT) and Collecting Duct

• DCT Functions: The DCT recovers about 10-15% of H2O (totaling 90-95%), as well as Na^+, Cl^-, and Ca^2+ (under the influence of parathyroid hormone, PTH).
• Collecting Duct Cells: There are two types:
    - Principal Cells: These possess channels for Na^+ and K^+.
    - Intercalated Cells: Important for maintaining acid-base homeostasis.
• Regulatory Mechanisms:
    - ADH: Stimulates the insertion of aquaporins, leading to increased H2O reabsorption.
    - Aldosterone: Regulates Na^+ recovery (p.1229).

Blood Flow Regulation

• Sympathetic Innervation: This mechanism constricts the afferent arteriole, redirecting blood flow to other organs under stress.
• Autoregulation: Renal autoregulation maintains a steady filtration rate regardless of fluctuations in overall activity (e.g., changes in blood pressure).
• Mechanisms of Autoregulation:
    - Myogenic Mechanism: Involves the smooth muscle in blood vessels regulating flow.
    - Tubuloglomerular Feedback: Involves the macula densa cells in the JGA monitoring Na^+ concentration; it constricts the afferent arteriole leading to decreased GFR (p.1230).

Endocrine Regulation of the Kidneys

• Renin-Angiotensin-Aldosterone System:
    - Renin: Produced by the kidneys, it activates the RAA system to increase blood volume and pressure.
    - Angiotensin I and II: Angiotensin I is converted to Angiotensin II, which has several direct effects, including constricting arteries and decreasing glomerular filtration rate, leading to water retention and increased thirst.
    - Aldosterone: Produced by the adrenal glands; it promotes Na^+ and water reabsorption in the distal tubules of the nephron contributing to increased blood volume.
    - Antidiuretic Hormone (ADH): Synthesized by the hypothalamus and secreted by the posterior pituitary, mediating aquaporin insertion for enhanced water reabsorption (p.1231).

Other Hormonal Influences

• Endothelin: This is a potent vasoconstrictor produced by endothelial and mesangial cells.
• Natriuretic Peptide: Promotes Na^+ excretion by kidneys and inhibits aldosterone and ADH release, decreasing water reabsorption.
• Parathyroid Hormone (PTH): Responds to low calcium levels, blocking the reabsorption of phosphate, thereby increasing calcium levels in the blood (p.1232).

Fluid Volume and Diuretics

• Diuretics: These are compounds that enhance urine volume. Common examples include caffeine and alcohol. They promote vasodilation in the nephron, increasing GFR while decreasing water absorption in the collecting duct, which results in increased urine output (p.1234).

Fluid Volume Regulation

• Measurement: Blood volume is monitored by baroreceptors located in the aorta and carotid sinuses.
• Ion Interactions: Sodium (Na^+) plays a vital role in blood osmolarity, causing water retention proportional to its concentration. Conversely, potassium (K^+) is secreted when Na^+ is absorbed. Chloride (Cl^−) plays a role in maintaining acid-base balance in extracellular spaces, mirroring Na^+ regulation. Calcium (Ca^2+) and phosphates (PO4^3-) are regulated by PTH during low calcium levels (p.1233).

Homeostasis and Kidney Functions

• Vitamin D Synthesis: The kidney aids in the activation of vitamin D, crucial for absorbing calcium (Ca^2+).
• Erythropoietin (EPO): The kidneys produce about 85% of circulating EPO, with the remainder sourced from the liver.
• Blood Pressure Regulation: This is facilitated through osmotic mechanisms and sodium concentration, where ADH stimulates the production of aquaporins for water conservation. The kidneys collaborate with the lungs, liver, and adrenal cortex via the renin-angiotensin-aldosterone system (RAAS) to recover electrolytes and maintain osmolarity and pH levels (p.1236).