Ninja Nerd- Renal

Glomerular Filtration Video 1/9

  • Definition of Renal Corpuscle:

    • Composed of two parts:

    1. Glomerulus: A tuft of capillaries, specifically designed for filtration.

      1. contains endothelial lining and G.B.M

    2. Bowman's Capsule: Also known as the glomerular capsule, collects filtrate from the glomerulus.

      1. contains parietal and visceral layer (podocytes)

Glomerulus Structure

  • Glarus: The tuft of capillary vessels in the glomerulus.

  • Afferent Arteriole:

    • The blood vessel that supplies blood to the glomerulus.

  • Efferent Arteriole:

    • The resulting vessel that drains blood from the glomerulus; one of the few instances in the body where a capillary bed is supplied by an arterial and drained by another arterial.

Capillary Types in the Glomerulus

  • Fenestrated Capillaries:

    • Characteristics:

    • Endothelial cells have numerous small pores (finestra) that allow for filtration.

    • Pores are approximately 50 to 100 nanometers in diameter.

    • Filtration Capacity:

    • Formed elements (e.g., red blood cells, white blood cells, platelets) cannot pass through due to size.

    • Small proteins, electrolytes (sodium, potassium, calcium, chloride), water, nutrients, and waste products can pass through.

Glomerular Basement Membrane (GBM)

  • Definition:

    • The critical filtration barrier of the glomerulus comprising three layers:

  1. Lamina Rara Interna:

  • The side nearest the endothelial cells.

  • Composed of proteoglycans (e.g., heparan sulfate), which are negatively charged (critical for filtration).

  1. Lamina Densa:

  • Consists of type IV collagen and laminins (lamins= proteins), dense structure.

  1. Lamina Rara Externa:

  • The outer side toward the podocytes, also rich in heparan sulfate.

Filtration Process

  • Charge Interaction:

    • Negatively charged particles (e.g., plasma proteins like albumin) are repelled by the negatively charged GBM, while positively charged particles pass through easily.

    • Neutral particles: May pass through but not as efficiently as positively charged particles.

    • Ex. positive substance: electrolytes Na, K, etc

  • Filtration Slits:

    • Formed by podocytes (foot cells) in the Bowman's capsule; spaces between foot processes are 25 to 30 nanometers wide.

    • Néphrine Protein:

    • Forms the slit diaphragm, only permitting particles around 7 to 9 nanometers to pass through.

      • Very important for controlling what enters fenestration pores, negatively charged GBM, and <25-30nm will make it through

      • Nephrine only allows <7-9 nm to pass

Bowman's Capsule

  • Layers of Bowman's Capsule:

    1. Parietal Layer:

    • Forms the capsule surrounding the glomerulus.

    1. Visceral Layer (Podocytes):

    • Formed by foot cells (podoctyes).

Filtration Outcome

  • Commonly Filtered Substances:

    • Water, glucose, amino acids, electrolytes (sodium, potassium, calcium, magnesium), urea, creatinine, vitamins.

  • Exclusions:

    • Plasma proteins (e.g., albumin, immunoglobulins) due to size and charge properties (repelled from GBM).

Key Cells Associated with Filtration

  • Mesangial Cells:

    • Function:

    • Can phagocytose trapped molecules in the slit diaphragm and regulate blood flow via contractile activity from A arteriole to glomerulus

  • Juxtaglomerular (JG) Cells:

    • Baroreceptors that produce renin, important for blood pressure regulation.

Net Filtration Pressure (NFP)

  • Definition:

    • The driving pressure of filtration in the glomerulus.

  • Formula for NFP:
    NFP=(Pglom−Posmotic−Pcapsular)NFP = (P_{glom} - P_{osmotic} - P_{capsular})

  • Pressures Involved:

    1. Glomerular Hydrostatic Pressure (P_{glom}):

      • Average of 55 mmHg, pushes fluids out from capillaries into Bowman's capsule.

    2. Colloid Osmotic Pressure (P_{osmotic}):

      • Average of 30 mmHg, exerted by plasma proteins (albumin) to retain fluid in the blood.

    3. Capsular Hydrostatic Pressure (P_{capsular}):

      • Average of 15 mmHg, back pressure of fluids in Bowman's capsule to capillary (adding too much water to funnel)

  • Calculation of Net Filtration Pressure:

    • Average NFP = 55 mmHg (hydrostatic) - 30 mmHg (osmotic) - 15 mmHg (capsular)

    • NFP = 10 mmHg.

    • NFP= PRESSURE PUSHINHG IN - PRESSURE PULLING IN

      • (GHP) - (COP + CHP)

      • NFP DIRECTLY RELATED TO GFR

      • NFP INC→ GFR INC

Glomerular Filtration Rate (GFR)

  • Definition:

    • The volume of plasma filtered per minute, ~125 mL/min.

    • 1200 ml passed through A. arteriole.

    • 625 ml is going to filtered (other 575 passes by)

    • 20% of the 625 is going to be filtered (125ml/min)

  • Factors Affecting GFR:

    • Net filtration pressure and filtration coefficient

    • (KF = surface area x permeability).

      • Increased surface area increases GFR; decreased area leads to lower GFR.

      • Permeability= alterations can further impact filtration rates (e.g., diabetic nephropathy affects thickness, glomerulonephritis affects porosity l/t proteinuria).

Clinical Correlations to GFR

  • High Blood Pressure: —> Increases glomerular hydrostatic pressure.

  • Low Blood Protein Levels (Hypoproteinemia):—> Decreases colloid osmotic pressure l/t more filtration into Bowman's capsule ( can’t hold onto water)

    • High protein (multiple myeloma) =protein retains fluid in the bloodstream—> increase in colloid osmotic pressure and reducing the amount of fluid filtered into Bowman's capsule.

  • Obstructions (e.g., Kidney Stones/ hydronephrosis): —> stone will push back into glomerulus which l/t Increased capsular hydrostatic pressure, reducing net filtration rate.

Proximal Convoluted Tubule Video 2/9

Glomerular Filtration Process

  • Location of Filtration

    • Glomerulus acts as the filtration site.

    • Filtering various substances from blood into the Bowman’s capsule.

  • Filtered Substances

    • Water

    • Electrolytes including:

    • Sodium (Na⁺)

    • Potassium (K⁺)

    • Chloride (Cl⁻)

    • Calcium (Ca²⁺)

    • Magnesium (Mg²⁺)

    • Nutrients such as:

    • Glucose

    • Amino acids

    • Vitamins

    • Lipids

    • Very small proteins like insulin and hemoglobin.

Osmolality and its Importance

  • Definition of Osmolality

    • Measures the volume of particles per kilogram of solvent.

    • Distinction from molality:

    • Molality is defined as moles of solute over kilograms of solvent.

    • Osmolality is relevant in assessing kidney functions:
      extOsmolality=racextnumberofparticlesextkgofsolventext{Osmolality} = rac{ ext{number of particles}}{ ext{kg of solvent}}

  • Normal Blood Osmolality Values

    • Typically around 300 milliosmoles per liter (mOsm/L).

    • Filtrate into the PCT remains at about 300 mOsm/L.

Tubular Processes: Secretion and Reabsorption

  • Definitions

    • Tubular Secretion: Movement of substances from blood into kidney tubules; requires ATP. because its active process

    • Tubular Reabsorption: Movement of substances from kidney tubules back into the blood; may be active or passive (depends on what molecule it is)

Tubular Reabsorption Mechanisms

  • Transport Processes

    • Sodium-Potassium ATPase Pumps:

    • Pump 3 Na⁺ out and 2 K⁺ into the cell against their concentration gradients using ATP.

    • Result: Low sodium concentration inside cells, high potassium concentration.

  • Specialized Transporters

    • Sodium-Glucose Co-transporter= Secondary active transport

    • Transports sodium (which moves in from the tubule) and glucose (which moves against its gradient).

    • Mechanism:

      • Sodium moves down its concentration gradient (passive).

      • Glucose moves against its gradient (active) using energy from sodium.

    • Type of Transport: Secondary Active Transport.

  • Amino Acids

    • Similar process with sodium-Amino acid co-transporters.

      • Na in cells is low

      • A.A are also transporting in (low out, high inside cells)

        • normally would need ATP to go from low to high concentration gradient. However, since Na is moving across concentration gradient, A.A can tag along

        • Example of secondary active transport

      • REMEMBER: 100% of glucose, A.A & lactate are reabsorbed from kidneys tubules to blood stream

  • Bicarbonate Reabsorption

    • Reaction involving CO₂ and water forming carbonic acid via Carbonic Anhydrase.

      • CO2 +H20 ←→ H2CO3 Because of Carbonic Anhydrase

    • Carbonic acid (H2CO3) dissociates into protons (H⁺) and bicarbonate (HCO₃⁻).

    • The bicarbonate (approx 90%) then enters the renal tubule cells, where it is further processed and ultimately reabsorbed into the bloodstream, contributing to the regulation of blood pH and maintaining acid-base balance.

    • Sodium-H⁺ Antiporter:

    • Sodium enters the cell while H⁺ is secreted.

    • Bicarb & H combine to form carbonic acid (H2CO3)

    • carbonic anhydrase converted H2CO3 into CO2 + H2O to be secreted

    • About 90% of bicarbonate is reabsorbed into the blood.

    • Ex of secondary active transport

Obligatory Water Reabsorption

  • Definition

    • Water follows sodium during reabsorption (obligatory), primarily by osmosis.

  • Reabsorption Rates

    • ~65% of filtered sodium and 65% of filtered water reabsorbed at the PCT.

Para-Cellular Transport

  • Definition: Transport occurs between cells to move into blood stream, essential for reabsorption of:

    • Calcium

    • Magnesium

    • Potassium

    • Chloride

    • Approximate reabsorption rates:

      • Potassium: 55%

      • Chloride: 50%

Chloride Transport Mechanisms

  • Use of Sodium-Chloride Co-transporter: (symporter)

    • Sodium and chloride enter the cell and subsequently pushed in the bloodstream.

Transport of Lipids

  • Lipids

    • Lipid-soluble substances pass directly through phospholipid bilayer membranes into blood

    • Urea partially reabsorbed.

Small Proteins Reabsorption

  • Mechanism for Proteins

    • Small proteins (e.g., insulin, hemoglobin) may be filtered and reabsorbed through receptor-mediated endocytosis.

      • Normally proteins aren’t filtered, but can be if small

    • Proteins engulfed by clathrin-coated vesicles, proteins are broken down in lysosomes to amino acids, and A.A reabsorbed/ pushed into the bloodstream.

Influence of Parathyroid Hormone (PTH)

  • PTH Effects

    • Binds receptors on PCT cells, affecting sodium-phosphate transporter.

    • PTH is Gs —> adenylate cyclase activation leads to an increase in cAMP levels,

    • which subsequently enhances the reabsorption of calcium and promotes the excretion of phosphate in the kidneys.

Glutamine and Ammonium Secretion

  • Glutamine: Undergoes deamination to produce 2 ammonium (NH₄⁺) and 2 bicarbonate (HCO₃⁻).

    • 2 bicarbonate ions in blood raise pH during metabolic acidosis.

      • Cl is then coming into the cells as exchange

    • Ammonium is actively secreted into kidney tubules using ATP.

      • Can dissociate into NH3 & H+ (ammonia & hydrogen)

      • H gets pushed out of the cells and Na enters as exchange

Tubular Secretion of Drugs

  • Active Secretion of Drugs

    • Substances like penicillin, cephalosporins, methotrexate, morphine, and organic bases & organic acids (e.g., uric acid), bile salts/ acids are actively secreted into the proximal convoluted tubule, requiring ATP.

Loop of Henle Video 3/9

Introduction to the Loop of Henle

  • The video focuses on the Loop of Henle, a key component of the nephron in the kidneys.

  • Viewers are encouraged to watch related videos on glomerular filtration and proximal convoluted tubule (PCT) before watching this one for better understanding.

Structure of the Loop of Henle

  • Comprises two main parts:

    • Descending limb: The part that descends into the renal medulla.

    • Ascending limb: The part that ascends back towards the cortex.

Nephrons

  • Definition of Nephron: The functional unit of the kidney, responsible for filtering blood and forming urine.

    • Composed of:

    • Renal Corpuscle:

      • Glomerulus: A network of capillaries.

      • Bowman's Capsule: Surrounds the glomerulus.

    • Proximal Convoluted Tubule (PCT): Reabsorbs nutrients, water, and electrolytes.

    • Loop of Henle: Divided into descending and ascending segments.

    • Distal Convoluted Tubule (DCT): Filters additional minerals and also adjusts pH and electrolyte concentrations.

  • Number of Nephrons:

    • ~ 1.2 million nephrons in each kidney so 2.4 million in two kidneys.

Overview of Functions

Filtration Process

  • Glomerular Filtration: Blood is filtered through the glomerulus, forming a filtrate that enters Bowman's capsule.

  • Tubular Reabsorption: Substances needed by the body are reabsorbed from the filtrate back into the blood.

  • Tubular Secretion: Additional substances are secreted into the filtrate from the blood.

Osmolality in the Loop of Henle

  • Initial Osmolality:

    • In the glomerulus, Bowman's capsule, and PCT, the osmolality is ~300 milliosmoles (mOsm/l). (isotonic with blood plasma)

  • Osmolality Changes in the Descending Limb:

    • Starts at 300 mOsm at the top and increases as it descends:

    • 500 mOsm at a shallow depth,

    • 700 mOsm at mid-depth,

    • 900 mOsm further down,

    • reaches peak at 1,200 mOsm at the bottom of the loop.

    • As the descend, the fluid becomes hypertonic (increased solute concentration).

Osmolality Definitions

  • Hypertonic: High solute (NaCl) concentration, low water content.

  • Hypotonic: Low solute (NaCl) concentration, high water content.

  • Isotonic: Equal solute and water concentrations.

Mechanisms in the Loop of Henle

Countercurrent Multiplier Mechanism

  • Function: Establishes a gradient in osmolality in the renal medulla, enhancing water reabsorption.

  • Process in Ascending Limb:

    • Sodium Potassium 2 Chloride (Na-K-2Cl) Cotransporter: Transports sodium, potassium, and chloride ions into the cells from the filtrate.

      • Therefore Na & Cl are pumping out

      • Some K is leaking out, some is staying in

        • K gets repelled because there are a lot of positive chargers

          • Ca & mag also move paracellularly

      • This is why increases osmolality as going down LOH. its getting saltier

    • Ascending limb is impermeable to water and actively pumps out sodium and chloride, thus increasing osmolality in the renal medulla.

  • Water Movement: Water moves out due to the osmotic gradient established via obligatory water movement, leading to further concentration of sodium and chloride outside.

  • Process in Descending Limb:

    • Aquaporin-1 channels allow water to re-enter the renal medullary interstitial space bc it wants to dilute it. via countercurrent multiplier mechanism

    • The descending limb is permeable to water but completely impermeable to solutes (NaCl and other ions).

  • Once turns the DLOH to ALOH, osmo decreases because solutes (Na & Cl) are leaky into medullary interstitial space

The Role of Vasa recta

  • Definition: A branch of the peritubular capillaries within the kidney; branch of the efferent arteriole

  • Function of Vasa Recta= Countercurrent Exchanger:

    • As blood flows down, it picks up sodium and chloride ions; simultaneously, water is lost to the renal medulla, maintaining osmotic balance (sluggish flow to prevent washout of salts).

    • When returning, it pushes sodium chloride out and reabsorbs water, maintaining the osmotic gradient of the renal medulla.

    • Significance: Prevents rapid removal of sodium chloride, ensuring the medullary interstitial gradient remains.

    • 2nd function is provides oxygen to tissue cells

Summary of Mechanisms at Work

  • Descending Limb:

    • Water is reabsorbed (permeable), solutes are retained (impermeable).

    • Aquaporin-1 channels facilitate water movement into the medullary interstitium.

  • Ascending Limb:

    • Sodium, potassium, and chloride ions are actively pumped out into the interstitium, making it hypertonic.

  • Countercurrent Multiplier Mechanism:

    • Water exits the descending limb while ions are secreted from the ascending limb, establishing gradients that allow for concentrated urine formation.

  • Vasa Recta:

    • Functions as a countercurrent exchanger, which aids in maintaining the medullary gradient and supplying nutrients and oxygen to renal tissues.

Distal Convoluted Tubule (DCT) Video 4/9

  • The distal convoluted tubule (DCT) is the part of the nephron following the loop of Henle.

  • Understanding the DCT requires prior knowledge of the proximal convoluted tubule and loop of Henle, so it's recommended to observe those sections first.

Overview of Mechanisms in the DCT

  • Two main processes occur in the DCT: tubular reabsorption and tubular secretion.

    • Tubular Reabsorption: The movement of substances from the kidney tubules back into the blood.

    • Tubular Secretion: The movement of specific solute molecules from the blood into kidney tubules.

Loop of Henle Connections

  • In the loop of Henle, two key segments exist: the descending limb and the ascending limb.

  • The ascending limb contains specialized transporters that pump sodium (Na⁺), potassium (K⁺), and chloride (Cl⁻) ions into the cells.

  • Basolateral membrane features channels to pump sodium out and chloride ions out, contributing to the medullary interstitium gradient.

  • This creates a highly concentrated interstitial fluid (salty), affecting osmolarity as follows:

    • Starts at 300 mOsm/L and can reach 1200 mOsm/L in the deepest renal regions.

Osmolarity and Reabsorption Percentages

  • Absorption percentages at various renal sites:

    • PCT: 65% sodium and 65% water absorption.

    • Descending Loop of Henle: 15% water absorption.

  • DCT= Remaining ~20% water entering the DCT:

    • 25% of remaining sodium is reabsorbed in the DCT.

  • Ultimately, 10% sodium remains after reabsorption processes.

  • This is part of the countercurrent multiplier mechanism and supported by the vasa recta to prevent major loss of NaCl

Distal Convoluted Tubule Structure

  • The DCT can be subdivided into early DCT and late DCT.

Early DCT (10% Na remaining)

  • Specialized transporters in the early DCT function:

    • Sodium-Potassium ATP pumps move 3 sodium out and 2 potassium in, requiring ATP -bc its pumping against concentration their gradient.

    • High sodium concentration in filtrate draws sodium ions into the cells via sodium-chloride transporters, bringing chloride ions along with it. because of NaCL symporter (same direction)

  • Reabsorption Rate:

    • About 5-6% of sodium is reabsorbed in the early DCT. (4-5% sodium remaining)

  • Calcium ions also present in this section, with parathyroid hormone stimulating reabsorption when blood calcium levels are low.

Hormonal Mechanism for Calcium Reabsorption

  • Parathyroid hormone (PTH) is released in response to low blood calcium levels:

    • It activates receptors in the DCT to stimulate a second messenger system.

    • G stimulatory protein and adenylate cyclase, converting ATP to cyclic AMP.

      • cAMP further activates protein kinase A (PKA). PKA stimulates calcium channels in the luminal membrane.

  • Calcium is transported from the tubular fluid into the cells and then into the bloodstream.

    • Some calcium binds to calbindin, while the majority re-enters the blood to increase blood calcium levels.

  • Secondary active transport occurs where sodium moves into the cells while calcium is forced out. (ATP dependent process)

Late Distal Convoluted Tubule (DCT)

  • Cells in this segment respond to aldosterone, a steroid hormone from the adrenal cortex:

    • Stimulated by factors including angiotensin II, low sodium, and high potassium levels; and small amounts of corticotropin-releasing hormone.

  • Aldosterone stimulates gene expression leading to the formation of proteins involved in sodium and potassium transport (Na/K):

    • Proteins allowing sodium influx (increased blood sodium and decreased potassium), promoting excretion into the urine.

  • ADH (antidiuretic hormone) MAKES LATE DCT permeable to water via aquaporin channels to follow Na, increasing reabsorption and thus blood volume and pressure.

Key Drug Interaction

  • Thiazide Diuretics: These medications inhibit the sodium-chloride co-transporters (symporter), affecting sodium and water reabsorption, leading to diuresis.

Intercalated Cells, and Collecting Ducts Video 5/9

Principal Cells: Principal cells are specialized epithelial cells that play a crucial role in balancing mineral and water levels in the body. They are responsible for regulating water reabsorption and sodium/potassium exchange.

  • Functions:

    • Maintain mineral balance.

    • Regulate water balance.

Intercalated A and B Cells

  • Function: Intercalated cells primarily function in maintaining acid-base balance in the body.; intercalted cells are found in late DCT and CD

  • Intercalated A Cells (for Acidosis)

    • Conditions Managed: These cells respond to conditions of acidosis, including respiratory and metabolic acidosis.

    • Mechanism:

    • High levels of CO₂ in blood enter the intercalated A cells and react with water in the presence of carbonic anhydrase to form carbonic acid (H₂CO₃).

    • Carbonic acid dissociates into bicarbonate (HCO₃⁻) and protons (H⁺).

    • Increased protons lead to lower pH (acidosis). The bicarbonate serves as a weak base, neutralizing the protons.

    • A specific channel protein facilitates the exchange of protons and potassium ions, requiring ATP:

      • Protons exit the cell, while potassium ions (K⁺) enter, which requires ATP.

      • This process is ATP-dependent, aimed at buffering the blood pH.

    • Ammonia (NH₃) secretion occurs in acidosis, where ammonia can bind to protons that were pumped out to form ammonium (NH₄⁺), a weak acid that contributes to urine acidity and aids in buffering.

    • Bicarbonate Transport: Bicarbonate ions exit through the basolateral membrane into the bloodstream, raising pH back to normal.

    • Chloride ions enter the cells to maintain ionic balance due to bicarbonate’s exit.

  • Intercalated B Cells (for Alkalosis (Basic conditions)

    • Conditions Managed: These cells respond to alkalosis, including metabolic and respiratory alkalosis.

    • Mechanism:

    • CO₂ enters the cell and combines with water to form carbonic acid via carbonic anhydrase.

    • Bicarbonate is transported out of the cell, contributing to the urine and decreasing bloodstream bicarbonate levels.

    • Chloride ions enter the cells to maintain charge balance as bicarbonate exits.

    • Protons are reabsorbed into the blood, assisting in lowering the pH to homeostatic levels, which requires ATP for transport processes.

Antidiuretic Hormone (ADH) and its Action

  • Function: ADH (also known as vasopressin) primarily regulates the kidney’s ability to reabsorb water and maintain blood volume and pressure.

  • Mechanism of Action:

    • Stimuli for Release: ADH is released when plasma osmolality increases or in response to angiotensin II, indicating a need to conserve water.

    • It binds to specific vasopressin receptors on principal cells in the collecting ducts and late distal tubules, activating the G-stimulatory protein, which in turn stimulates adenylate cyclase.

    • cAMP Production: The activation of adenylate cyclase converts ATP into cyclic AMP (cAMP), l/t activation of protein kinase A (PKA).

    • PKA promotes the fusion of vesicles containing aquaporin-2 channels via phosphorylation with the luminal membrane, increasing water permeability and allowing water to flow from the renal tubules, aquaporin 3 & 4 channels into the bloodstream.

    • As water reabsorbs into the bloodstream, blood volume and pressure increase while plasma osmolality normalizes, returning to isotonic levels (~300 mOsm).

  • 20% of water remaining depends on ADH; Calcium relies on PTH; Remaining 4-5% of Na depends on aldosterone

Collecting Duct Functions

  • Secretion Processes:

    • Collecting ducts can also secrete various substances: drugs, toxins, creatinine, ammonia, protons, and bicarbonate, contributing to the fine-tuning of blood chemistry.

Counter-Current Exchange Mechanism and Vasa Recta

  • Role of Vasa Recta: Specialized capillary network that runs parallel to nephron tubules.

    • It helps maintain the osmotic gradient in the medulla, essential for water reabsorption.

    • Prevents rapid NaCl removal (starts 300 osm→ ends in 325 osmo) while providing oxygen to renal tissues with its slow blood flow.

  • Importance: Contributes to urine concentration and the medullary interstitial gradient, facilitating obligatory water movement out of nephron structures into the bloodstream.

Urea Recycling

  • Urea aids in creating a hyperosmotic environment in the medullary interstitial space through urea recycling:

    • After water reabsorption in the collecting duct, urea concentration rises, leading to its passive diffusion into the interstitium and aiding in maintaining osmolarity (urea= solutes).

    • Contributes to the generation of concentrated urine & contribute to medullary gradient, vital for bodily homeostasis.

Filtration, Reabsorption, and Secretion Overview (Video 6/9)

Glomerular Filtration Process

  • Afferent Arteriole:

    • Brings blood to the glomerulus.

    • The glomerulus is a network of capillaries that performs filtration.

  • **Filtration Pressures: **

    • Glomerular hydrostatic pressure - pressure from systemic blood pressure pushing fluid out into the Bowman's capsule.

    • Osmotic pressure - exerted by proteins in the blood that pull water back into the bloodstream.

    • Capsular hydrostatic pressure - pressure exerted by the filtrate in the capsule pushing fluid back into the glomerulus.

    • Capsular osmotic pressure - typically zero as plasma proteins like albumin should not filter through.

  • Net Filtration Pressure (NFP):

    • The overall result of the pressures involved; approximately 10 mmHg.

  • Glomerular Filtration Rate (GFR):

    • Directly proportional to NFP.

    • Normal GFR is about 125 mL/minute.

Nephron Structure

  • Definition: Nephron = Glomerulus + Bowman's capsule + PCT + LOH + DCT.

  • Nephrons in Kidney: Approximately 1.2 million per kidney → 2.4 million in two kidneys (unless renal agenesis occurs).

Proximal Convoluted Tubule (PCT)

  • Major site of reabsorption.

  • Substances reabsorbed include:

    • Sodium: ~65% reabsorbed; water follows due to osmotic gradient.

    • Bicarbonate: ~85-90% reabsorbed; depending on body needs.

    • Potassium: ~60% reabsorbed.

    • Chloride: ~50-60% reabsorbed.

    • Calcium: ~60% reabsorbed.

    • Magnesium: Varied depending on sources, but questionable.

    • Urea: ~50% reabsorbed; contributes to osmotic gradient.

    • Glucose & Amino Acids: 100% reabsorbed under physiological conditions (dependent on sodium).

  • Transport Mechanisms:

    • Sodium-glucose co-transporters bring glucose into the blood.

    • Small proteins (e.g., albumin, insulin) reabsorbed by endocytosis.

    • Lipids passively diffuse through the membrane.

Tubular Secretion

  • Defined as the process of moving substances from blood into the filtrate.

  • Substances Secreted:

    • Drugs and metabolic wastes (e.g., ammonium, creatinine).

  • Requirement of ATP: Useful for pumping protons and other substances depending on ATP.

Loop of Henle

  • Osmolality Changes: As filtrate descends, osmolality increases from 300 mOsm/kg to 1200 mOsm/kg.

    • Descending limb allows water reabsorption; impermeable to solutes.

  • Ascending Limb Characteristics:

    • Sodium-potassium-chloride co-transporter responsible for moving solutes out (~25% sodium, ~30% potassium, ~30% chloride).

    • Contributes to medullary interstitial gradient facilitating countercurrent multiplication.

Distal Convoluted Tubule (DCT)

  • Two parts: Early and Late distal tubule.

  • Early DCT: Sodium and chloride reabsorption through specific transporters.

    • Hormonal Regulation: Influenced by PTH activating cAMP pathways affecting calcium reabsorption.

  • Late DCT:

    • Aldosterone: Promotes sodium reabsorption and potassium secretion through its action on sodium-potassium pumps.

      • Stimulated by low sodium or high potassium levels.

    • Antidiuretic hormone (ADH): Increases aquaporin expression in collecting duct to concentrate urine and increase blood volume.

Collecting Duct

  • Final adjustments to urine composition:

    • Urea Recycling: Contributions lead to concentrated urine and maintain osmotic gradient.

    • ADH enhances water reabsorption.

  • Metabolic Regulation: Intercalated cells function in acid-base balance during acidosis (A cells) and alkalosis (B cells).

Conclusion and Key Metrics

  • Remaining Filtrate Composition:

    • Water: ~20% (variable based on ADH).

    • Sodium: ~5%-10% (dependent on aldosterone).

    • Calcium: Reabsorption dependent on parathyroid hormone.

  • Overall, these mechanisms ensure fluid, electrolyte regulation, and acid-base balance in the body.

Note: Key hormone influences and physiological implications in renal function are highlighted throughout each nephron structure.