Urinary System and Body Fluids

Urinary System Overview

  • The urinary system's anatomy, role, and function.
  • Potential disruptions of the urinary system and their impact on the human body.
  • The urinary system's role and its impact on other body systems.
  • Interrelationships of the urinary system related to common pathophysiology, utilizing nursing judgment.

Organs of the Urinary System

  • Kidneys (2)
  • Ureters (2)
  • Urinary bladder (1)
  • Urethra (1)
  • Inferior vena cava
  • Aorta
  • Renal artery
  • Renal vein
  • Diaphragm
  • Adrenal gland

Functions of the Kidneys

  • Regulate blood volume and electrolytes.
  • Regulate acid-base balance.
  • Excrete nitrogenous waste.
  • Regulate blood pressure.
  • Regulate RBC production via Erythropoietin.
  • Regulate blood glucose.

Kidney Structure

  • Renal Capsule
  • Renal cortex
  • Renal medulla
  • Nephron
  • Calyx
  • Renal pelvis
  • Renal papilla
  • Hilum
  • Renal vein
  • Renal pyramids
  • Renal artery
  • Ureter

Nephron Unit: Tubular Structures

  • Bowman's capsule
  • Proximal convoluted tubule
  • Loop of Henle (Descending and Ascending limbs)
  • Distal convoluted tubule
  • Collecting duct
  • Glomerulus

Three Steps of Urine Formation

  1. Glomerular filtration
    • Location: Glomerulus
    • 180 liters/24 hours
  2. Tubular reabsorption
    • Location: Along the entire length of tubule
    • 178.5 liters/24 hours
    • Influenced by Aldosterone (acting on the Distal Convoluted Tubule - DCT) and ADH (acting on the collecting duct)
  3. Tubular secretion
    • Involves the secretion of some solutes

Urine Formation: Glomerular Filtration

  • Water and dissolved solutes filter across glomeruli into Bowman’s capsule.
  • Large molecules (albumin and RBCs) are not filtered.
  • Unfiltered blood goes to peritubular capillaries.

Urine Formation: Tubular Reabsorption

  • Returns filtrate from the tubules to the blood of peritubular capillaries.
  • Most reabsorption takes place in the proximal tubule.

Urine Formation: Tubular Secretion

  • Moves very small amounts of select substances from the peritubular capillaries into the tubules.
  • Secreted substances: Potassium ions (K+), hydrogen ions (H+), uric acid, ammonium ions, and drugs.

Control of Fluid Balance

  • Factors influencing fluid balance:
    • Blood Pressure (B/P)
    • Osmoreceptors
    • Sodium (Na+)
    • Hormones: ADH, Aldosterone, Angiotensin II
    • Renin

Aldosterone

  • Works on the distal tubule.
  • The “salt-retaining” hormone.
  • Effects:
    • Na^+ and H_2O reabsorption (expands blood volume).
    • K^+ excretion.

Antidiuretic Hormone (ADH)

  • Released by the posterior pituitary in response to low blood volume or high concentration.
  • H_2O moves out of the collecting duct into tissue spaces and then into peritubular capillaries.

Angiotensin II and Renin

  • Decreased sodium content (thus BP) increases renin production (enzyme).
  • Renin participates in the production of angiotensin II.
  • Angiotensin II raises blood pressure through:
    • Vasoconstriction
    • Increased ADH production
    • Increased aldosterone production

Fluid Volume Regulation

  • Increased Na^+ concentration leads to increased blood osmolarity.
  • Increased blood osmolarity tends to increase water retention/intake.
  • Both lead to increased blood pressure and increased fluid volume.

Fluid Compartments

  • Extracellular compartment: water outside cells
    • Interstitial fluid
    • Plasma
    • Lymph
    • Transcellular fluid
    • Cations (positive): Na, K, Ca, Mg
  • Intracellular compartment: water inside cells
    • Anions (negative): HCO_3, Cl, P, Proteins

Urine Composition

  • 95% water, 5% dissolved solids and gases.
  • pH averages 6.0
  • Specific gravity measures the amount of dissolved substances.
  • Normal range: 1.002 (very dilute) to 1.040 (very concentrated).

pH Scale

  • Normal blood pH: 7.35-7.45
  • Acid: pH < 7.35 (Acidosis)
  • Base: pH > 7.45 (Alkalosis)
  • Urine pH: averages 6

pH Regulation Defense Lines

  • Buffers: First line of defense in regulating pH; prevent sharp changes in hydrogen ion concentration.
  • Lungs: Second line of defense; regulate CO_2
  • Kidneys: Third line of defense; regulate hydrogen ions and create new molecules like bicarbonate (HCO_3)