The Kidney and Urine Formation

• What is the major organ of the excretory system?

  Kidney
• What shape is the kidney?

  Bean-shaped
• What is the order of urine formation in the kidneys?

  1. Glomerulus (filters blood)
  2. Renal tubules:

  • Proximal Convoluted Tubule
  • Loop of Henle
  • Distal Convoluted Tubule

  1. Collecting Duct
  2. Calyx (12 minor calyces and 2-3 major calyces)
  3. Renal Pelvis
  4. Ureter (approx. 25 cm long)
  5. Urinary bladder
  6. Urethra
• What is the functional unit of the kidney?

  Nephron
• What is the primary responsibility of the nephron?

  Urine formation
• How many nephrons does each kidney contain approximately?

  1 to 1.5 million
• What are the two distinct areas of the kidney?

  • Renal cortex: lighter and outer portion
  • Renal medulla: darker and inner portion
• What are the five distinct areas of the nephron?

  1. Blood vessels on the Bowman’s capsule
  2. Glomerulus
  3. Peritubular capillaries
  4. Vasa recta
  5. Renal tubule
• What are the two types of nephrons and their functions?

  • Cortical Nephrons (85%): removal of waste products and reabsorption of nutrients
  • Juxtamedullary Nephrons: concentration of urine and maintenance of osmotic gradient
• What artery receives blood from the heart to the kidneys?

  Renal artery
• What percentage of blood from the heart does the kidney approximately receive?

  25%
• Describe the flow of blood through the nephron.

  1. Blood enters through the afferent arteriole
  2. Flows through the glomerulus
  3. Enters the efferent arteriole
  4. Passes into peritubular capillaries and vasa recta
  5. Returns to circulation via the renal vein
• What is the total renal blood flow?

  Approximately 1,200 mL/min
• What is the normal range for total renal plasma flow?

  600 to 700 mL/min
• What does the glomerulus consist of?

  A coil of approximately eight capillary lobes
• What is the purpose of Bowman’s capsule?

  To catch the filtrate filtered by the glomerulus
• What substances are non-filterable in the glomerulus?

  • Large molecules
  • Blood cells
• What is the molecular weight limit for substances to be filtered by the glomerulus?

  Less than 70,000 Daltons
• What is the specific gravity of the filtrate as it leaves the glomerulus?

  1. 010
• What are the three cellular layers that plasma filtrate must pass through?

  1. Capillary wall membrane (endothelial cells)
  2. Basement membrane (basal lamina)
  3. Visceral epithelium of the Bowman’s capsule
• How do endothelial cells of the capillary wall differ from other cells?

  They have pores (fenestrated)
• What is hydrostatic pressure?

  • A force that pushes fluid out of the blood capillaries
• What is oncotic pressure?

  • A force that resists hydrostatic pressure by pushing fluid into the blood capillaries
• What is the role of juxtaglomerular cells?

  • Found on the afferent arteriole
  • Produce hormone renin when blood pressure is low
  • Constrict or dilate the afferent arteriole
• What does the macula densa do?

  • Found on the distal convoluted tubule
  • Senses changes in blood pressure
  • Sends signals to JG cells to produce hormone
• What is the Renin-Angiotensin-Aldosterone System (RAAS)?

  • A hormone system essential for regulating blood pressure and fluid balance
  • Comprised of Renin, Angiotensin, and Aldosterone
• When is RAAS activated?

  When there is decreased blood pressure, sodium concentration, or water in the body
• What happens when blood pressure is high regarding RAAS?

  RAAS activation should be inhibited
• Describe the RAAS cascade of reaction.

  1. Renin produces angiotensin I
  2. Angiotensin I is converted to angiotensin II by ACE in the lungs
  3. Angiotensin II:

  • Dilates afferent arteriole and constricts efferent arteriole
  • Stimulates sodium reabsorption
  • Triggers aldosterone release
  • Triggers antidiuretic hormone production
• What is tubular reabsorption?

  • The process where filtered molecules return to circulation
  • Primarily occurs in the proximal convoluted tubule
• What is active transport in the context of tubular reabsorption?

  • Requires energy to transport substances from renal tubule to peritubular capillaries
  • Involves carrier proteins in renal tubular epithelial cells
• What substances are reabsorbed through active transport?

  • Glucose, amino acids, salts (in proximal convoluted tubule)
  • Chloride (in ascending loop of Henle)
  • Sodium (in proximal and distal convoluted tubules)
• What is passive transport in tubular reabsorption?

  • Movement of molecules from higher concentration to lower concentration
  • Occurs due to concentration or electrical potential gradients
• What is the renal threshold for glucose?

  160-180 mg/dL
• What happens when plasma concentration reaches the renal threshold?

  Glucose appears in the urine
• How does renal concentration begin in the loops of Henle?

  • Filtrate is exposed to the high osmotic gradient of the renal medulla
  • Important for sodium and water reabsorption
• What occurs in the descending loop of Henle?

  • Water is reabsorbed
  • Maintains osmotic gradient in the renal medulla
• What occurs in the ascending loop of Henle?

  • Salt is reabsorbed
  • Walls are impermeable to water
• Where does passive reabsorption of water take place?

  • In all parts of the nephron except the ascending loop of Henle
• What is the term for the plasma concentration at which active transport stops?

  Renal threshold