Notes on Kidney Function and Structure • Kidney Basics: • Weight: ~0.5 kg. • Shape: Fist-shaped. • Function: Filter wastes from blood; hold up to 25% of the body’s blood at a time. • Urine Formation and Storage: • Filtration: Kidneys filter waste, conducting it to the bladder via ureters. • Storage: • Urinary sphincter muscle: Acts as a valve at the bladder’s base, allowing urine storage. • Urge Signals: • At ~200 mL: Bladder stretches slightly; nerves signal the brain. • At ~400 mL: Stretch receptors activate further; urgency increases. • At ~600 mL: Voluntary control is lost; sphincter relaxes, urine enters the urethra and is voided. • Kidney Anatomy (Referencing Figure 1): • Cortex: Outer layer of connective tissue encircling the kidney. • Medulla: Inner layer beneath the cortex. • Renal pelvis: Hollow chamber connecting the kidney to the ureter.

Notes on Nephrons and Urine Formation

• Nephrons:

• Functional units of the kidneys (~1 million per kidney).

• Structure:

• Afferent arterioles: Branches of the renal artery, supply blood to nephrons.

• Glomerulus:

• Capillary bed receiving blood from afferent arterioles.

• Blood exits through efferent arterioles (unlike other capillaries).

• Efferent arterioles lead to peritubular capillaries, which wrap around kidney tubules.

• Bowman’s capsule: Funnel-shaped structure surrounding the glomerulus. Located in the cortex.

• Function: Receives fluids from blood to process into urine.

• Tubules of the Nephron:

1. Proximal tubule: Thin tubule extending from the Bowman’s capsule.

2. Loop of Henle:

• Descends into the kidney’s medulla.

• Key role in concentrating urine.

3. Distal tubule: Last segment of the nephron.

4. Collecting ducts:

• Receive urine from multiple nephrons.

• Merge in the renal pelvis.

• Urine Pathway:

1. Bowman’s capsule → Proximal tubule → Loop of Henle → Distal tubule → Collecting ducts.

2. Collecting ducts merge in the kidney pelvis before being transported via ureters.

Notes on Urine Formation

Urine Formation involves three key functions:

1. Filtration:

• Movement of fluids from blood into the Bowman’s capsule.

• Process:

• Blood enters the glomerulus via the afferent arteriole (high-pressure filter).

• Pressure in the glomerulus: ~65 mm Hg (compared to ~25 mm Hg in regular capillaries).

• High pressure pushes dissolved solutes through glomerular walls into the Bowman’s capsule.

• Large molecules like plasma proteins, blood cells, and platelets cannot pass through.

• Comparison of Solutes:

• Fluids extracted from the glomerulus and Bowman’s capsule show only small molecules entering the nephron.

2. Reabsorption:

• Transfer of essential solutes and water from the nephron back into the blood.

3. Secretion:

• Movement of materials from the blood into the nephron for excretion.

Key Points:

• Each nephron has its own blood supply.

• Filtration is driven by high pressure in the glomerulus.

• Only small molecules move into the Bowman’s capsule; larger components remain in the blood.

Notes on Reabsorption in the Kidneys

Significance of Reabsorption:

• Kidney Filtration Rates:

• ~600 mL of fluid flows through the kidneys every minute.

• ~20% (~120 mL) is filtered into the nephrons.

• Without reabsorption:

• 120 mL of urine would form per minute.

• To maintain homeostasis, at least 1 L of fluids would need to be consumed every 10 minutes.

• Actual Urine Output:

• Only 1 mL of urine is formed per 120 mL of filtrate.

• ~119 mL of fluids and solutes are reabsorbed back into the blood.

Mechanism of Reabsorption:

• Types of Transport:

1. Active transport:

• Carrier molecules move positive ions (e.g., Na⁺) across nephron cell membranes.

• Requires energy supplied by numerous mitochondria.

2. Passive transport:

• Negative ions (e.g., Cl⁻, HCO₃⁻) follow Na⁺ due to charge attraction.

Limitations:

• Reabsorption continues until the threshold level of a substance is reached.

• Excess substances (e.g., NaCl) remain in the nephron and are excreted with urine.

Notes on Reabsorption of Solutes and Water in the Kidneys

Active Transport of Molecules:

• Glucose and Amino Acids:

• Attach to specific carrier molecules.

• Actively transported from the proximal tubule back into the blood.

• Limitations:

• Only a limited amount of solute can be reabsorbed.

• Excess glucose (e.g., in individuals with high blood glucose or high sugar intake) remains in the nephron and is excreted in urine.

Osmotic Gradient and Water Reabsorption:

• Solutes in the Nephron:

• Actively transported solutes create an osmotic gradient.

• This gradient draws water from the nephron into the blood.

• Proteins in Blood:

• Proteins not filtered into the nephron create a second osmotic force.

• These proteins draw water from the interstitial fluid into the bloodstream.

Concentration of Solutes in the Nephron:

• As water is reabsorbed:

• Remaining solutes (e.g., urea, uric acid) become more concentrated.

• Some urea and uric acid diffuse back into the blood, but less is reabsorbed than originally filtered.

Notes on Secretion in the Kidneys

Definition:

• Secretion is the movement of wastes and other substances from the blood into the nephron.

Substances Secreted:

• Nitrogen-containing wastes.

• Excess ions:

• H⁺ (hydrogen ions).

• K⁺ (potassium ions).

• Drugs:

• Examples include penicillin.

Mechanism:

• Occurs in the distal tubule.

• Lined with cells containing numerous mitochondria to supply energy for active transport.

• Similar to reabsorption in using active transport, but molecules are transported into the nephron rather than back into the blood.

Flashcards

Flashcards are a popular study tool used for memorization and learning.

Key Features:

• Format: Typically consist of two sides - one with a term (or question) and the other with a definition (or answer).

• Purpose: Facilitate active recall, spaced repetition, and self-testing, making them effective for learning vocabulary, concepts, or facts across various subjects.

• Customization: Can be created for any topic, allowing for personalized learning experiences.

Benefits:

• Flexibility: Can be used in various settings – at home, in class, or on-the-go.

• Engagement: Encourages active participation in the learning process rather than passive reading.

• Efficiency: Helps succinctly condense information, making it easier to review.

  ureters tubes that conduct urine from the kidneys to the bladder

  urethra tube that carries urine from the bladder to the exterior of the body

  cortex outer layer of the kidney medulla area inside of the cortex

  renal pelvis area where the kidney joins the ureter

  nephrons functional units of the kidneys

  afferent arterioles small branches that carry blood to the glomerulus

  glomerulus high-pressure capillary bed that is the site of filtration

  efferent arterioles small branches that carry blood away from the glomerulus to a capillary net

  peritubular capillaries network of small blood vessels that surround the nephron

  Bowman's capsule cuplike structure that surrounds the glomerulus

  proximal tubule section of the nephron joining the Bowman's capsule with the loop of Henle

  loop of Henle carries filtrate from the proximal tubule to the distal tubule

  distal tubule conducts urine from the loop of Henle to the collecting duct

  collecting duct tube that carries urine from nephrons to the pelvis of a kidney

  filtration process by which blood or body fluids pass through a selectively permeable membrane

  reabsorption transfer of glomerular filtrate from the nephron back into the capillaries

  secretion movement of materials, such as ammonia and some drugs, from the blood back into the distal tubule

  threshold level maximum amount of material that can be moved across the nephron

  interstitial fluid fluid that surrounds the body cells