Chapter 25 Part B: The Urinary System - Human Anatomy and Physiology
Chapter 25: The Urinary System Part B
25.5 Step 2: Tubular Reabsorption
General Overview:
Definition: Tubular reabsorption is a process where most tubular contents are quickly reclaimed and returned to the blood.
Nature of Process: It is a selective transepithelial process.
Contents Reabsorbed:
Almost all organic nutrients are reabsorbed (e.g., glucose, amino acids).
Water and ion reabsorption are hormonally regulated and precisely adjusted.
Types of Tubular Reabsorption:
Active tubular reabsorption
Passive tubular reabsorption
Reabsorption Routes:
Two primary routes utilized for tubular reabsorption are:
Transcellular Route
Paracellular Route
25.5.1 Transcellular and Paracellular Routes
Transcellular Route:
Process Details:
Solute enters the apical membrane of tubule cells.
Travels through the cytosol of tubule cells.
Exits through the basolateral membrane of tubule cells.
Enters the blood through the endothelium of peritubular capillaries.
Paracellular Route:
Process Details:
Occurs between tubule cells.
Limited by tight junctions but may be leaky in proximal nephron.
Substances that move via this route:
Water
Ca²⁺
Mg²⁺
K⁺
Some Na⁺ in the PCT (Proximal Convoluted Tubule)
25.6 Tubular Reabsorption of Sodium
Transport across the Basolateral Membrane:
Dominance of Na⁺: Na⁺ is the most abundant cation in the filtrate.
Transport Mechanism:
Transport of Na⁺ is achieved through primary active transport via Na⁺-K⁺ ATPase pumps.
Na⁺ is pumped into the interstitial space.
This process results in Na⁺ being swept by bulk flow into peritubular capillaries.
Transport across the Apical Membrane:
Na⁺ enters tubule cell at the apical surface through either:
Secondary active transport (cotransport)
Facilitated diffusion through channels.
Electrochemical Gradient:
The active pumping of Na⁺ at the basolateral membrane leads to a strong electrochemical gradient.
Facilitates low intracellular Na⁺ levels to help Na⁺ diffusion inward.
K⁺ may leak out into interstitial fluid, maintaining a net negative charge inside cell that aids Na⁺ entry.
25.7 Tubular Reabsorption of Nutrients, Water, and Ions
Mechanisms of Reabsorption:
Na⁺ Reabsorption:
Via primary active transport, it generates energy for reabsorbing other substances.
Substances that are cotransported with Na⁺ include:
Glucose
Amino acids
Some ions
Vitamins.
Passive Tubular Reabsorption of Water:
Water reabsorption is driven by osmotic gradients created by Na⁺ and other solutes.
Mechanisms Involved:
Aquaporins facilitate water reabsorption by osmosis.
Two Types of Water Reabsorption:
Obligatory Water Reabsorption:
Aquaporins are permanently present in PCT.
Facultative Water Reabsorption:
Aquaporins are inserted in collecting ducts only in presence of ADH (Antidiuretic Hormone).
Passive Tubular Reabsorption of Solutes:
The reabsorption of water increases solute concentration in the filtrate.
Concentration gradients drive the entry of solutes into tubule cells and peritubular capillaries.
Solutes that follow water:
Fat-soluble substances
Some ions
Urea (noted as it creates complications for drug reabsorption).
25.8 Transport Maximum (Tm)
Definition of Transport Maximum:
Reflects the number of carriers in renal tubules available for a given solute.
When carriers become saturated, excess solute is excreted in urine.
Example Scenario:
In cases of hyperglycemia, high blood glucose levels exceed Tm, leading to glucose spilling over into urine.
25.9 Reabsorptive Capabilities of Renal Tubules and Collecting Ducts
Proximal Convoluted Tubule (PCT):
Site with the most reabsorption of nutrients (e.g., glucose, amino acids).
Reabsorption Percentage:
65% of Na⁺ and water.
Majority of ions, uric acid, and about half of urea (to be secreted later).
Nephron Loop:
Descending Limb:
Water Behavior: H₂O can leave, but solutes cannot.
Ascending Limb:
Water Behavior: H₂O cannot leave, but solutes can.
Process Details:
Thick segment possesses Na⁺-K⁺-2Cl⁻ symporters and Na⁺-H⁺ antiporters.
Some Na⁺ can enter the cell via paracellular route.
Distal Convoluted Tubule (DCT) and Collecting Duct:
Features hormonally regulated reabsorption:
ADH (Antidiuretic Hormone):
Released by posterior pituitary;
Causes insertion of aquaporins in collecting duct cells, increasing water reabsorption.
Aldosterone:
Targets principal cells in collecting duct and DCT;
Promotes synthesis of Na⁺ and K⁺ channels and Na⁺−K⁺ ATPases, leading to Na⁺ reabsorption (water follows).
Functions to increase blood pressure and decrease K⁺ levels.
Atrial Natriuretic Peptide (ANP):
Reduces blood Na⁺, causing decreased blood volume and pressure upon release from cardiac atrial cells.
Parathyroid Hormone:
Works on DCT to enhance Ca²⁺ reabsorption.
25.10 Summary of Tubular Reabsorption and Secretion
Tubular Secretion:
Considered as reabsorption in reverse predominantly occurring in PCT.
Substances secreted include:
K⁺
H⁺
NH₄⁺
Creatinine
Organic acids and bases.
Functions: Disposing of drugs bound to plasma proteins, eliminating undesirable substances, controlling blood pH by adjusting H⁺ or HCO₃⁻ in urine.
25.11 Regulation of Urine Concentration and Volume
Primary Functions of Kidneys:
Maintaining body fluid osmotic concentration near 300 mOsm.
Osmolality Definition:
Number of solute particles per kg of H₂O.
1 osmol equals 1 mole of particles in 1 kg of H₂O.
Body fluids use milliosmol to express smaller amounts (1 mOsm = 0.001 osmol).
25.12 Mechanisms: Countercurrent Flow
Countercurrent Mechanism:
Fluid in adjacent segments flows in opposite directions within the same tube with hairpin turn.
Types of Countercurrent Mechanisms:
Countercurrent Multiplier: Interaction of filtrate flow in ascending/descending limbs of nephron loops of juxtamedullary nephrons.
Countercurrent Exchanger: Blood flow in ascending/descending limbs of vasa recta.
25.12.1 Countercurrent Multiplier
Characteristics:
Involves nephron loop with filtrate flowing oppositely in descending and ascending limbs.
Key Differences:
Descending limb is permeable to H₂O but impermeable to solutes.
Ascending limb is impermeable to H₂O and selectively permeable to solutes, reabsorbing Na⁺ and Cl⁻ actively.
25.12.2 Countercurrent Exchanger
Mechanism Explanation:
Blood in vasa recta follows a countercurrent flow, allowing for exchanges of NaCl and water with surrounding interstitial fluid.
Blood remains isosmotic with surrounding fluid.
Key Functions:
Prevents rapid salt removal from the interstitial space.
Removes reabsorbed water, sustaining osmotic gradients.
25.13 Formation of Dilute or Concentrated Urine
Requirement for Functionality:
The medullary osmotic gradient must be established to form dilute (< 300 mOsm) or concentrated urine (~1200 mOsm).
Fluid Behavior:
Overhydration leads to production of large volumes (~100 mOsm).
Dehydration causes the production of small volumes (~1200 mOsm).
25.14 Urea Recycling**
Importance of Urea:
Contributes to the formation of the medullary gradient.
Urea enters the filtrate in ascending thin limb. It is reabsorbed in the cortical collecting duct, and highly concentrated urea leaves the collecting duct and moves back into the ascending thin limb, enhancing osmolality in the medulla.
25.15 Diuretics**
Influence on Urinary Output:
Chemicals that enhance urinary output include:
ADH inhibitors (e.g., alcohol).
Na⁺ reabsorption inhibitors (e.g., caffeine, antihypertensive drugs).
Loop diuretics that inhibit medullary gradient formation.
Osmotic diuretics - substances that remain unabsorbed, resulting in water retention in urine.
Concluding Remarks
The processes of tubular reabsorption and secretion are complex yet critically important for homeostasis, impacting fluid and electrolyte balance along with waste excretion.