Nephron Function and Urine Formation Quiz

• Tubular reabsorption and secretion are key processes in urine formation.

Introduction

• Blood entering the glomerulus undergoes filtration.
• The filtrate proceeds to the proximal convoluted tubule via Bowman's space.
• The filtrate undergoes reabsorption and secretion until it becomes urine.
• Filtrate is only considered urine after reabsorption and secretion cease.

Tubular Reabsorption

• The body reabsorbs substances like water, electrolytes, and glucose to prevent losing 120-125 mL of water every minute.
• Tubular reabsorption returns filtered water and solutes from the tubules to the bloodstream; about 99% of filtered water is reabsorbed.
• The proximal convoluted tubule cells contribute the most to reabsorption.
• Reabsorption is selective, targeting essential substances.

Filtration, Reabsorption, and Excretion

• Glucose: 100% of the filtered load is reabsorbed.
• Bicarbonate: >99.9% reabsorbed.
• Sodium: 99.4% reabsorbed.
• Chloride: 99.1% reabsorbed.
• Potassium: 87.8% reabsorbed.
• Urea: 50% reabsorbed.
• Creatinine: 0% reabsorbed.

Active Transport

• Substances move across cell membranes against osmotic gradients, and it is always transcellular.
• Substances are forcibly transported against their concentration gradient.
• It moves substances from low to high concentration areas, and it requires carrier proteins and energy (ATP hydrolysis).

Passive Transport

• Movement occurs due to concentration or electrical potential differences, and can be transcellular or paracellular.
• Diffusion moves substances from high to low concentration, achieving equilibrium.
• Passive transport doesn't need carrier proteins or energy.

Pathways for Substance Movement

• Transcellular: Substances enter and exit the same cell to reach peritubular capillaries.
• Paracellular: Substances move through tight junctions between cells into blood vessels.

Bulk Flow/Ultrafiltration

• Substances move from the tubular cell into peritubular capillaries by crossing the interstitial fluid.

Brush Borders

• Fingerlike microvilli projections increase surface area for enhanced reabsorption.

Reabsorption of Sodium

• Sodium diffuses across the luminal membrane into the cell down an electrochemical gradient (passive transport). The luminal membrane faces the lumen.
• Across the basolateral membrane (facing interstitial fluid), sodium is transported against an electrochemical gradient by the Na+-K+ ATPase pump.

Sodium-Potassium ATPase Pump

• Uses ATP to release sodium extracellularly and intake potassium.
• It expels sodium from tubular cells into peritubular capillaries for reabsorption.

Reabsorption of Glucose

• Glucose is transported into the tubular cell cytoplasm by SGLT against its concentration gradient.
• SGLT relies on the sodium gradient created by the Na+-K+ ATPase pump for energy.
• Glucose uses GLUT transporters for facilitated diffusion to leave the cell.

Reabsorption of Proteins and Water

• Smaller proteins are reabsorbed in the proximal convoluted tubule of the kidney as they are essential.
• PCT reabsorbs proteins via pinocytosis and digests them into amino acids in vesicles.
• Proteins bind to the megalin cubilin complex before pinocytosis occurs
• The amino acids are recycled.

Water Reabsorption

• Water reabsorption depends on aquaporins and tight junctions in tubular cells.
• Aquaporins allow water to pass intracellularly or through tight junctions.
• Water permeability varies across nephron segments, as PCT/DLOH have abundant aquaporins, promoting high water reabsorption.
• ALOH’s low surface area and less permeable tight junctions result in low water reabsorption.
• In the distal convoluted tubule the tight junctions are much tighter compared to those of the PCT

Reabsorption of Urea and Chloride

• Half of urea is reabsorbed passively with urea transporters.
• Chloride is transported due to the lumen negative potential.

Maximal Reabsorptive Capacity (Tm)

• Tm refers to the maximum reabsorption rate of a substance before it appears in urine; expressed as the amount of solute reabsorbed per minute.
• Assessing how quickly nephrons process solutes.
• Glucose at 350 mg/min means, beyond this quantity, that there will be glucose in the urine.

Renal Threshold

• Renal threshold is the plasma concentration at which reabsorption stops, which refers to the blood level of the substance.
• Glucose's example is at 160-180 mg/dL.
• Transport maximum occurs and reabsorption stops when the plasma glucose concentration exceeds the usual levels; as a result, the substance is excreted in the urine.
• There must be a correlation between the blood glucose of the patient and the glucose in the urine.

Proximal Convoluted Tubule

• The proximal convoluted tubule is a major site of tubular reabsorption due to high metabolism and brush borders.
• Substances such as urea, salt, water, amino acids, and glucose are reabsorbed by it.

Early PCT

• Sodium reabsorption happens through the reabsorption of all other solutes such as amino acids, glucose etc

Late PCT

• Sodium and chloride reabsorption

Loop of Henle

• Thin descending: Thin membranes with no brush borders and few mitochondria with low reabsorption simple diffusion of water and some solutes
• Thick ascending: high metabolic activity that performs active reabsorption of sodium, potassium, and chloride using the NKCC2 cotransporter

Early Distal Tubule

• There is a first and second portion
• First: Forms Macula Densa in the JGA

Distal Convoluted Tubule and the Collecting Duct

• The Collecting Duct is divided into the cortical, and Medullary collecting ducts

Late Distal Tubule and Cortical Collecting Tubules

• Contain principal cells that reabsorb sodium via Epithelial Sodium Channels (ENaC) and NKA pump while secreting potassium via the Renal Outer Medullary Potassium channel (ROMK).
• Aldosterone controls sodium reabsorption.
• Potassium-Sparing Diuretics inhibit the activity of the principal cells.

Medullary Collecting Duct

• The final site for urine processing and whose permeability to water and urea is controlled by ADH.

Countercurrent Mechanism

• The osmolarity of the filtrate changes as it is absorbed along the different parts of the nephrons
• The water can be easily reabsorbed because of the hypertonic state of the medulla

Tubular Reabsorption: Control of Water Loss

• How concentrated or diluted your urine is, depends upon the body's state of hydration
• Tubular secretion: passages of substances from the blood in the peritubular capillaries into the tubules

Acid-Base Balance

• Regulation occurs via eliminating either hydrogen or bicarbonate ions

Bicarbonate Ions

• If the body is in alkalosis the bicarbonate is removed.
• If the body is in acidosis the hydrogen is removed.

Cells that perform acid-base balance

• Includes the intercalated cells, alpha and beta cells for secreting hydrogen ions, and either reabsorbing or secreting the bicarbonate

Glomerular Filtration Tests

• Includes measuring the rate by which the kidneys remove a filterable substance from the blood; sample used is 24-hour urine

Characteristics of an ideal clearance test substance

• The susbtance cannot be reabsorbed or secreted by the tubules

Clearance Tests

• Includes urea, inulin radioculides beta 2 microglobulin etc.