Lecture 29 - Nephrons and Glomerular Filtration

Tubular reabsorption

The process in the nephron of the kidney where essential substances like water, glucose, ions, and amino acids are reabsorbed from the filtrate back into the bloodstream. This occurs mainly in the proximal convoluted tubule, nephron loop, distal convoluted tubule, and collecting duct, helping to maintain fluid and electrolyte balance.

Solvent drag

The process in which dissolved solutes are carried along with the movement of a solvent across a membrane. This occurs primarily in biological systems where water moves through a permeable membrane, such as in the kidneys and intestines, dragging small solutes with it.

Anatomy Context:

• In the renal system, solvent drag plays a role in reabsorbing water and solutes in the kidneys.

• In the intestinal epithelium, water movement through tight junctions can carry ions and nutrients along with it.

• In capillary exchange, fluid movement due to hydrostatic and osmotic pressures can facilitate the transport of small molecules.

Transport maximum

Maximum rate at which a substance can be reabsorbed or secreted by the renal tubules before the transport proteins become fully saturated. Once this limit is reached, any excess of the substance appears in the urine. An example is glucose, which has a Tm of approximately 375 mg/min in healthy kidneys.

Tubular secretion

The process in which the renal tubules actively transport substances, such as hydrogen ions (H⁺), potassium ions (K⁺), and certain drugs, from the blood in the peritubular capillaries into the filtrate within the nephron. This occurs primarily in the proximal tubule, distal tubule, and collecting duct of the kidney. It plays a crucial role in maintaining acid-base balance, electrolyte levels, and the elimination of waste products.

Diueresis

Refers to the increased production of urine by the kidneys. It occurs when the body excretes more urine than usual, often due to factors like increased fluid intake, medications (diuretics), or medical conditions such as diabetes.

Brief Anatomy Involvement:

• Kidneys: Filter blood, regulate fluid balance, and produce urine.

• Nephrons: Functional units of the kidneys responsible for filtering and reabsorbing substances.

• Ureters: Transport urine from the kidneys to the bladder.

• Bladder: Stores urine until excretion.

• Urethra: Expels urine from the body.

Describe the reabsorption routes in the PCT.

In the proximal convoluted tubule (PCT) of the nephron, reabsorption occurs through two main routes:

1. Transcellular Route (Through Cells)

   • Substances pass through the apical membrane, cytoplasm, and basolateral membrane of tubular epithelial cells before entering the bloodstream.

   • Involves active transport (e.g., sodium-potassium pump) and passive diffusion (e.g., water via aquaporins).

2. Paracellular Route (Between Cells)

   • Substances pass through tight junctions between epithelial cells.

   • Driven by concentration gradients and solvent drag (water movement pulling solutes along).

Explain how sodium chloride, glucose, and water are reabsorbed in
the PCT.

In the proximal convoluted tubule (PCT) of the nephron, sodium chloride (NaCl), glucose, and water are reabsorbed efficiently to maintain homeostasis.

1. Sodium Chloride (NaCl) Reabsorption:

• Active transport: Sodium ions (Na⁺) are actively transported out of the PCT cells into the interstitial fluid via the Na⁺/K⁺ ATPase pump.

• Passive transport: Chloride ions (Cl⁻) follow sodium passively due to the electrochemical gradient.

2. Glucose Reabsorption:

• Secondary active transport: Glucose is reabsorbed along with sodium via sodium-glucose cotransporters (SGLTs) on the apical membrane.

• Facilitated diffusion: Once inside the PCT cells, glucose moves into the bloodstream via GLUT transporters on the basolateral membrane.

3. Water Reabsorption:

• Osmosis: As NaCl and glucose are reabsorbed, they create an osmotic gradient that drives water movement.

• Aquaporins: Water moves through specialized protein channels (aquaporins) to follow solute reabsorption.

What are aquaporins? Where are they located?

Specialized membrane proteins that function as water channels, allowing water molecules to pass in and out of cells efficiently.

Location:

• Kidneys: Found in the nephron's collecting ducts, regulated by antidiuretic hormone (ADH) to control water reabsorption.

• Brain: Helps regulate cerebrospinal fluid balance.

• Lungs: Assists in maintaining hydration in respiratory tissues.

• Eye (Cornea & Lens): Helps with fluid transport and clarity.

• Other tissues: Found in skin, salivary glands, and intestines, aiding in fluid movement.

Describe the reabsorption that occurs in the nephron loop.

Plays a crucial role in water and salt reabsorption, helping to concentrate urine and maintain the body's fluid balance.

Brief Reabsorption Process:

1. Descending Limb:

   • Permeable to water but not to solutes.

   • Water is reabsorbed into the surrounding medullary interstitial fluid, concentrating the filtrate.

2. Ascending Limb:

   • Impermeable to water but actively transports sodium (Na⁺), potassium (K⁺), and chloride (Cl⁻) ions into the interstitial fluid.

   • This dilution of the filtrate helps create the medullary osmotic gradient, essential for urine concentration.

Describe the reabsorption that occurs in the DCT. How is this
reabsorption modified by aldosterone and ADH

A section of the nephron in the kidneys that plays a key role in the reabsorption of water, ions, and other substances. Here's a brief overview of the reabsorption that occurs in the DCT and how it is modified by aldosterone and antidiuretic hormone (ADH):

Reabsorption in the DCT:

• Sodium (Na⁺): A small amount of sodium is reabsorbed in the DCT, influencing fluid balance and blood pressure.

• Calcium (Ca²⁺): Calcium is also reabsorbed, regulated by hormones like parathyroid hormone (PTH).

• Chloride (Cl⁻): Reabsorption of chloride often occurs in conjunction with sodium.

• Bicarbonate (HCO₃⁻): Some bicarbonate is reabsorbed, helping to maintain the pH balance of the blood.

Modification by Aldosterone:

• Aldosterone increases sodium reabsorption in the DCT by activating sodium channels and sodium-potassium pumps. This promotes water retention because water follows sodium, increasing blood volume and blood pressure. It also causes potassium secretion into the urine.

Modification by ADH:

• ADH primarily affects the collecting duct, but it can influence the DCT as well. It enhances water reabsorption by making the DCT and collecting ducts more permeable to water. This allows water to be reabsorbed back into the bloodstream, reducing urine output and concentrating the urine.

Explain how the collecting duct accomplishes water conservation

Collecting duct in the kidneys plays a crucial role in water conservation by regulating the reabsorption of water into the bloodstream. Here's a brief explanation of how it works:

1. ADH (Antidiuretic Hormone): When the body is dehydrated or needs to conserve water, ADH is released. It binds to receptors on the cells of the collecting duct.

2. Aquaporins: ADH triggers the insertion of aquaporin water channels into the walls of the collecting duct. These channels allow water to be reabsorbed from the filtrate (pre-urine) back into the surrounding blood vessels.

3. Osmotic Gradient: The collecting duct runs through the kidney's medulla, where there is a high osmotic concentration. This gradient allows water to passively move out of the collecting duct and into the blood.

4. Result: The process conserves water by reducing urine volume and increasing the concentration of urine.

What hormone controls water loss?

antidiuretic hormone