Renal System Part 1: Filtration and Reabsorption

Overview of the Renal System

• Primary Function: Blood Filtration: The kidneys are primarily responsible for filtering the blood to remove metabolic waste and foreign chemicals.

  • Removal of Foreign Chemicals: This includes the elimination of pesticides, drugs, food additives, and other substances foreign to the body.

• Regulation of Extracellular Fluid (ECF) and Blood Pressure: The kidneys manage the volume of extracellular fluid, which directly impacts systemic blood pressure.

• Osmolarity and Ion Regulation: The kidneys maintain the balance of key ions, most notably sodium ($Na^+$) and potassium ($K^+$).

• Gluconeogenesis: While not part of normal daily physiology, the kidneys can produce glucose from non-carbohydrate sources during periods of prolonged fasting.

• Hormone and Enzyme Production:

  • Renin: An enzyme produced by the kidney (hence the name "renal in origin"). It is a critical component of the pathway that generates Angiotensin II, a hormone that regulates sodium, potassium, and blood pressure.

  • The kidney produces other hormones, though they are not the primary focus of this specific discussion.

Gross Anatomy and Blood Supply

• Renal Arteries: These arteries branch directly from the Aorta. There is a left and a right renal artery supplying the two kidneys.

• Cardiac Output: The kidneys receive approximately $20\%$ to $25\%$ of the total cardiac output, representing a significant volume of blood flow relative to their size.

• Renal Circulation Path: Blood flows from the renal artery into shorter branches, including the arcuate artery (named for its arc-like appearance), eventually reaching the afferent arterioles.

• Glomerulus: Derived from the term for a "small ball of yarn," the glomerulus is a spherical ball of capillaries where blood filtration occurs.

• Ureter, Bladder, and Pelvis:

  • Ultrafiltrate (pre-urine) passes from the nephrons into the renal pelvis.

  • The ureter carries urine from the renal pelvis to the bladder for storage and eventual excretion.

The Nephron: The Functional Unit of the Kidney

• Quantity: There are approximately $1,000,000$ nephrons per kidney, totaling about $2,000,000$ in a healthy adult.

• General Structure: A nephron consists of the various tubules and the associated vascular supply (glomerular capillaries).

• Types of Nephrons:

  • Cortical Nephrons: These comprise the majority of nephrons in the human kidney. They have relatively short loops of Henle that dip only a short distance into the renal medulla. They are surrounded by peritubular capillaries.

  • Juxtamedullary Nephrons: These have very long loops of Henle that extend deep into the renal medulla. These are associated with specialized blood vessels called vasa recta (meaning "long straight vessels").

• Countercurrent Exchange: Because the flow of fluid in the nephron lumen is opposite to the flow of blood in the vasa recta, extensive countercurrent exchange is possible.

• Comparative Anatomy (Kangaroo Rats): Desert-dwelling mammals like kangaroo rats have a much higher ratio of juxtamedullary nephrons. This allows them to produce highly concentrated urine to conserve water, though this process is "expensive" in terms of $ATP$ consumption.

Mechanics of Glomerular Filtration

• Ultrafiltration: The process where fluid and small dissolved solutes pass across the capillary wall into Bowman’s capsule.

• Glomerular Filtration Rate (GFR):

  • For a healthy adult with two kidneys, the GFR is approximately $180\,L/day$ ($90\,L/day$ per kidney).

  • Total average blood volume is only $5\,L$, highlighting the massive volume of plasma processed daily.

• Urine Volume and Reabsorption: Average urine volume is only $1.7$ to $1.8\,L/day$. This indicates that nearly all ($99\%$) of the filtered volume is reabsorbed back into the blood.

• The Filtration Barrier:

  • Fenestrations: Large pores in the endothelial cells of the glomerular capillaries that allow water and solutes to pass.

  • Glomerular Basement Membrane: Composed of extracellular matrix molecules like collagen and glycoproteins. It acts as a physical barrier based on size.

  • Podocytes: Specialized cells that wrap around the capillaries with "foot processes." The gaps between these processes are called filtration slits.

  • Slit Diaphragm: A complex of proteins providing a molecular sieve.

• Size Selection: Small solutes (glucose, sodium), small peptides, and small proteins (like hCG - human chorionic gonadotropin) are filtered. Large proteins like serum albumin and blood cells (RBCs, WBCs) are excluded. Presence of large proteins in urine typically indicates pathology.

• Filtration Pressures:

  • Hydrostatic Pressure ($P_H$): The blood pressure in the glomerular capillaries, approximately $55\,mmHg$. This is the primary driving force for filtration.

  • Colloid Osmotic Pressure ($\pi$): Caused primarily by serum albumin in the plasma, which tends to keep fluid in the vessels. It is approximately $30\,mmHg$.

  • Fluid Pressure ($P_{fluid}$): The pressure of the non-compressible fluid already present in Bowman’s capsule and the tubules, approximately $15\,mmHg$.

  • Net Filtration Pressure: $55\,mmHg - (30\,mmHg + 15\,mmHg) = 10\,mmHg$.

Regulation of Glomerular Filtration Rate (GFR)

• Site of Regulation: The afferent arteriole is the key site. Constriction increases resistance and decreases GFR; dilation decreases resistance and increases GFR.

• Autoregulatory (Intrinsic) Control: Mechanisms originating within the kidney that maintain a constant GFR ($180\,L/day$) even when mean arterial pressure fluctuates between $80\,mmHg$ and $180\,mmHg$.

  • Myogenic Response: Smooth muscle cells in the afferent arteriole respond to stretch. High blood flow stretches the wall, opening stretch-sensitive sodium channels. This depolarization opens voltage-gated calcium channels, causing contraction and vasoconstriction to reduce GFR.

  • Tubuloglomerular Feedback: The ascending limb of the loop of Henle passes next to the glomerular capillaries. Specialized macula densa cells sense increased fluid flow. They respond by releasing the paracrine factor adenosine, which binds to receptors on the afferent arteriole smooth muscle, causing vasoconstriction and reducing GFR.

• Extrinsic Control:

  • Neural (Sympathetic): Sympathetic neurons release norepinephrine which binds to alpha-adrenergic receptors on the afferent arteriole, causing vasoconstriction. This occurs during "fight or flight" to divert blood to skeletal muscles.

  • Hormonal (Atrial Natriuretic Peptides - ANP): Released by cardiac muscle in the atria in response to stretch (increased blood volume). ANP causes vasodilation of the afferent arteriole, increasing GFR.

Reabsorption and Secretion Processes

• Filtration Fraction: About $20\%$ of the blood entering the afferent arteriole is filtered. The remaining $80\%$ passes through the efferent arteriole to peritubular capillaries. Of that filtered $20\%$, over $19\%$ is reabsorbed.

• Sodium ($Na^+$) Reabsorption:

  • Primary Active Transport: The sodium-potassium ATPase ($Na^+/K^+\text{-ATPase}$) pump on the basolateral membrane pumps $3\,Na^+$ out and $2\,K^+$ into the cell using $ATP$. This creates a low intracellular sodium concentration.

  • Apical Entry: Sodium enters the cell from the lumen via the epithelial sodium channel (ENaC), which is always open, moving down its concentration gradient.

• Glucose Reabsorption:

  • Secondary Active Transport: On the apical membrane, the Sodium-Glucose Cotransporter (SGLT) uses the potential energy of the sodium gradient to pull glucose into the cell against its gradient (symport).

  • Facilitated Diffusion: Glucose exits the basolateral side via GLUT carrier proteins and enters peritubular capillaries.

• Water Reabsorption: Water follows solute (primarily sodium) through aquaporins via osmosis.

• Urea: Approximately $40\%$ to $50\%$ of urea is reabsorbed. While a waste product, it can be recycled for biochemical reactions.

• Secretion: Movement of substances from peritubular capillaries into the tubule lumen.

  • Penicillin: A foreign organic molecule that is not efficiently filtered but is actively secreted by the kidney to be eliminated.

Glucose Handling and Transport Maximums

• Saturation: Unlike filtration, which does not saturate (it increases linearly with plasma concentration), reabsorption is mediated by transporters (SGLT) and is therefore saturable.

• Transport Maximum ($T_m$): For glucose, this is approximately $375\,mg/min$.

• Renal Threshold: The plasma concentration at which the transport maximum is reached and glucose begins appearing in the urine. This is approximately $300\,mg/dL$ (or $300\,mg$ per $100\,mL$).

• Normal Levels: Normal plasma glucose is around $100\,mg/dL$. Reabsorption is usually $100\%$.

Clinical Implications: Diabetes and SGLT2 Inhibitors

• Diabetes Mellitus Type 1: Autoimmune destruction of pancreatic beta cells leads to high blood glucose (hyperglycemia). When plasma glucose exceeds the renal threshold, glucose appears in urine.

• Diabetes Mellitus Type 2: Resistance of peripheral tissues to insulin. Diagnosed through a glucose tolerance test (plasma glucose $\ge 126\,mg/dL$ at baseline).

• Osmotic Diuresis and Polyuria: High glucose in the tubule lumen creates an osmotic effect that prevents water reabsorption, leading to frequent urination (polyuria).

• SGLT2 Inhibitors (Canagliflozin):

  • Mechanism: Specifically inhibits the SGLT2 transporter (responsible for $90\%$ of glucose reabsorption) in the proximal tubule.

  • Effect: Glucose stays in the pre-urine and is eliminated, lowering blood glucose levels.

  • Side Effects: Increased risk of urinary tract infections (UTIs) due to high sugar in the urinary tract, increased urination, and potential hypoglycemia.

Questions & Discussion

• Question: What is the fluid moving in the lumen of the nephron called?

• Answer: It is called ultrafiltrate. It can also be thought of as "pre-urine" because it undergoes significant modification (reabsorption and secretion) before it becomes final urine at the end of the collecting duct.