8.1 Kidney Notes

Structure and Function of the Kidney

• The kidneys filter about ¼ of your heart's output.

• Kidney functions:

  • Handle essential ions.

  • Reabsorb substances to keep body fluids balanced.

  • Remove waste through urine.

  • Control blood pressure and volume.

  • Help make red blood cells.

• Kidneys are key for keeping your body stable.

• An adult kidney:

  • Is shaped like a bean.

  • Is about the size of your fist.

  • Weighs about 5 ounces.

• The right kidney sits a bit lower than the left because of the liver above it.

• Ribs mostly protect both kidneys, which are between the T-12 and L-3 vertebrae.

• The hilus is the kidney's medial surface.

• Hilus: A concave area where ureters, blood vessels, and nerves enter the kidney.

• The kidney has an outer fibrous capsule and is surrounded by fatty tissue.

• Fatty tissue functions:

  • Protects from injury.

  • Helps hold the kidney in place.

• Kidneys are retroperitoneal organs, meaning they're behind the peritoneal cavity.

Nephron

• Nephron: The kidney's basic working unit.

• Each kidney has around 1 million nephrons.

• Nephrons manage water and material concentration by:

  • Filtering blood.

  • Reabsorbing what's needed.

  • Removing waste as urine.

• Nephron functions:

  • Gets rid of body wastes.

  • Controls blood volume, pH, and pressure.

  • Balances electrolyte levels.

• Each nephron has two parts:

  • The glomerular capsule (renal corpuscle).

  • The renal tubule.

• These connect through the tubule to collecting ducts.

• Functions of nephron parts:

  • Glomerular capsule (renal corpuscle): Filters blood.

  • Renal tubule: Reabsorbs necessary materials.

  • Collecting ducts: Carry waste as urine for excretion.

• A kidney section shows:

  • Outer cortex.

  • Inner medulla.

• The outer cortex contains:

  • Glomeruli.

  • Convoluted tubules (proximal and distal) of the nephron.

  • Blood vessels.

• The inner medulla has:

  • Loop of Henle.

  • Cone-shaped renal pyramids.

• Cortical columns: Cortex parts that go through the medulla to the renal pyramids.

• Each pyramid makes a kidney lobe.

• Renal pelvis: The kidney's center, a funnel-shaped tube connecting to the ureter as it exits the hilus.

• Calyces: Pelvis extensions that collect urine, draining into the renal pelvis and then the ureter.

• The ureter carries urine to the bladder for storage.

Nephron Structure and Function

• Nephron: The kidney's functional unit for blood filtration and reabsorption.

• Nephrons are two types:

  • Cortical nephrons.

  • Juxtamedullary nephrons.

• Cortical nephrons:

  • Make up 85% of nephrons.

  • Start in the cortex's outer part.

  • Have shorter loops of Henle that go a short way into the medulla.

• Juxtamedullary nephrons:

  • The other 15% of nephrons.

  • Start deeper in the cortex.

  • Have longer, thinner loops of Henle that go all the way into the medulla.

Renal Corpuscle and Blood Supply

• Nephrons get blood from two systems:

  • Glomerulus.

  • Peritubular capillary network.

• Glomerulus: Between two arterioles (afferent and efferent).

  • Arterioles are high resistance, creating a high-pressure system that forces fluid and solutes out of blood into the glomerular capillary.

• Peritubular capillaries: Low-pressure vessels for reabsorption, not filtration.

  • They surround the tubules, allowing quick solute and water movement.

• Efferent arterioles:

  • Are deep in the renal cortex.

  • Become long, thin-walled vasa recta vessels.

• Vasa recta:

  • Run alongside the loops of Henle in the medulla.

  • Help exchange solutes and water in the kidney.

• Glomerulus: A mass of capillaries in a thin, double-walled Bowman capsule.

  • Blood flows via the afferent arteriole into glomerular capillaries and out through the efferent arteriole to peritubular capillaries.

  • Solutes and fluids filter from the blood through the capillary membrane into Bowman's space.

  • The blood filtered into Bowman's space is called filtrate.

• Renal corpuscle: The glomerulus and its Bowman capsule.

• The glomerular capillary membrane has three layers:

  1. Capillary endothelial layer.

  2. Basement membrane.

  3. Single-celled capsular epithelial layer.

• Endothelial cells have fenestrations (small pores) for blood filtration.

• The epithelial layer around the glomerulus connects to the Bowman capsule.

• Podocytes (foot processes) are epithelium extensions in the basement membrane.

  • Podocytes form slit pores for filtrate passage.

• Basement membrane: Between epithelial and endothelial layers.

  • Its spaces determine the glomerulus's size-dependent permeability.

  • Normally, it stops red blood cells and plasma proteins from passing into the filtrate.

  • Damage causes red blood cells and proteins to leak into the filtrate, indicating glomerular disease.

Nephron Tubule

• The nephron tubule has four segments:

  1. Proximal convoluted tubule (coiled) draining Bowman's capsule

  2. Loop of Henle

  3. Distal convoluted tubule

  4. Collecting tubule joining other nephron tubules to collect filtrate

• Filtrate passes through these segments to the renal pelvis.

  • It starts at the proximal convoluted tubule to the descending limb of Henle, then the ascending loop back to the cortex.

  • The ascending loop becomes the distal convoluted tubule, which drains into the collecting tubule.

• The entire tubule is lined with a single layer of epithelial cells on a basement membrane.

  • Cell structure varies for different functions.

• Proximal tubule epithelial cells are fine with villi (fingerlike projections) to increase reabsorption area.

  • They have many mitochondria to help active transport.

• Loop of Henle epithelial cells have fewer mitochondria, meaning less reabsorption and metabolic processes.

Urine Formation

• Kidneys filter the entire plasma volume about 60 times daily.

  • They use nearly 25% of resting body energy to remove waste as urine.

• Around 47 gallons of glomerular filtrate with water, nutrients, and essential ions are removed from blood plasma daily.

• By the time filtrate enters collecting ducts, about 0.5 gallons of urine has formed, with 99% of water and nutrients reabsorbed into the blood.

• Three processes are needed:

  1. Filtration

  2. Reabsorption

  3. Secretion

• Filtration happens in the renal corpuscle, while reabsorption and secretion occur in renal tubules.

Filtration

• Filtration occurs in the glomerulus across the porous membrane between capillaries and Bowman’s capsule.

  • It's mechanical because it doesn't need energy.

• Fluids and solutes (water, glucose, amino acids, nitrogenous wastes) are forced through the membrane by high hydrostatic blood pressure in the capillary bed.

• Capillary pores stop blood cells and most blood proteins from passing through.

  • Plasma proteins stay in capillaries to keep the osmotic pressure of glomerular blood and prevent water loss.

• Normal glomerular filtration rate (GFR) is 120-125 ml/min or 180 L/day.

  • This is possible because of the large surface area of glomerular capillaries, high membrane permeability, and moderate net filtration pressure.

• GFR increases with higher arterial (and glomerular) blood pressure and decreases with higher glomerular osmotic pressure (often from dehydration).

• Maintaining a constant GFR is key for proper water and nutrient reabsorption from the filtrate.

  • Too rapid flow means needed substances aren't fully reabsorbed.

  • Too slow flow means nearly everything is reabsorbed, even wastes.

Regulation of Renal Blood Flow and GFR

• Three mechanisms regulate renal blood flow and GFR despite arterial blood flow changes:

  1. Renal autoregulation

  2. Nervous system control

  3. Hormonal control

• Renal autoregulation: GFR is controlled by adjusting afferent and efferent arteriole diameter, letting the kidney set its blood flow rate.

  • Efferent arteriole constriction increases resistance to outflow from glomeruli, raising glomerular pressure and GFR.

  • Afferent arteriole constriction lowers renal blood flow, glomerular pressure, and GFR.

  • The kidney can maintain a constant GFR despite blood pressure changes through this system.

• Nervous system: Kidneys are heavily innervated by the sympathetic nervous system.

  • Sympathetic activity (fight or flight) sends blood to the heart, brain, and muscles.

  • Nervous system control can override renal autoregulation.

  • Sympathetic nerve fibers narrow the afferent arteriole, followed by epinephrine release from the adrenal medulla, which decreases renal flow and GFR.

• Hormonal control: Renal blood flow and GFR are controlled by the renin-angiotensin-aldosterone system (RAA).

  • The RAA system responds when blood pressure drops too low.

  • Angiotensinogen is a pre-enzyme made by the liver.

  • Renin is released by juxtaglomerular (JG) cells of the nephron when blood pressure drops.

  • Renin turns angiotensinogen into angiotensin I.

  • Angiotensin I turns into angiotensin II in the lungs.

  • Angiotensin II increases vasoconstriction, raising peripheral blood pressure.

  • Angiotensin II releases aldosterone from the adrenal cortex.

  • Aldosterone increases sodium and water reabsorption from the filtrate.

Tubular Reabsorption and Secretion

• Glomerular filtrate is transported from Bowman’s capsule to nephron tubules.

• As filtrate moves through the tubule:

  • Needed solutes and fluids are reabsorbed into peritubular capillaries from tubular fluid.

  • Waste products are secreted from peritubular capillaries into tubular fluid.

• Tubular reabsorption happens as needed substances move through tubule segment membranes to reach peritubular capillaries.

• Water and ion reabsorption are hormonally regulated and can be passive or active.

• Most tubular reabsorption is in the proximal convoluted tubule (PCT).

  • Glucose and amino acids are almost fully reabsorbed in the PCT, along with water and other ions.

  • Sodium $(Na^+)$, chloride $(Cl^-)$, potassium $(K^+)$, and bicarbonate $(HCO_3^-)$ are 65%-80% reabsorbed from the filtrate.

• Filtrate moves from the PCT into the loop of Henle.

• The loop of Henle reabsorbs more sodium $(Na^+)$ and chloride $(Cl^-)$ than water.

• The thinner descending limb of the loop of Henle is very permeable to water and somewhat permeable to urea, sodium $(Na^+)$, and other ions.

  • Water is reabsorbed from the filtrate as it moves down the descending limb.

• As it enters the thin ascending limb, solutes including sodium $(Na^+)$, chloride $(Cl^-)$, potassium $(K^+)$, bicarbonate $(HCO_3^-)$, calcium $(Ca^{+2})$, and magnesium $(Mg^{+2})$ are reabsorbed while water stays in the filtrate.

  • The filtrate becomes more dilute.

• The filtrate then enters the thick ascending limb, which is also impermeable to water.

  • About 20%-25% of the filtered sodium $(Na^+)$, chloride $(Cl^-)$, and potassium $(K^+)$ are reabsorbed here, plus a little calcium $(Ca^{+2})$ and magnesium $(Mg^{+2})$.

• Once the filtrate reaches the distal convoluted tubule (DCT), about 10% of sodium $(Na^+)$ and chloride $(Cl^-)$ and 20% of water are left.

  • This tubule segment has two cell types: intercalated cells and principal cells.

  • Intercalated cells reabsorb potassium $(K^+)$ and excrete hydrogen $(H^+)$.

  • Hydrogen $(H^+)$ secretion is paired with bicarbonate $(HCO_3^-)$ reabsorption.

  • Principal cells are stimulated by aldosterone.

  • Aldosterone moves sodium $(Na^+)$ from the urine filtrate into principal cells, then into the interstitial fluid and peritubular capillaries.

  • Potassium $(K^+)$ moves from peritubular capillaries into principal cells and then into the urine filtrate.

• Tubular secretion removes unwanted substances from the blood, like urea.

  • The body also concentrates the filtrate and eliminates excess potassium ions and drugs like penicillin.

  • Bicarbonate $(HCO_3^-)$ and hydrogen $(H^+)$ secretion helps control blood pH.

• The final urine composition is determined by glomerular filtration, tubular reabsorption, and tubular secretion.

Endocrine Function of the Kidney

• Besides filtering blood, kidneys act as an endocrine organ, making chemical mediators that travel through the blood to affect other body sites.

• Kidneys affect blood pressure through:

  1. The renin-angiotensin-aldosterone system (RAA).

  2. Red blood cell production regulation via erythropoietin formation.

  3. Calcium metabolism through vitamin D activation.

• Erythropoietin: A kidney-made hormone that regulates red blood cell differentiation in bone marrow.

  • Low tissue oxygen levels, caused by anemia, cardiac or pulmonary disease, or high altitude, trigger erythropoietin formation.

• Vitamin D activation occurs in the kidneys.

  • Active Vitamin D boosts calcium absorption from the GI tract and regulates calcium deposition in bone.

  • Vitamin D has two forms: cholecalciferol (from skin via sun's ultraviolet rays) and ergocalciferol (synthetic).

  • Both forms must be chemically activated.

  • Inactive Vitamin D forms are converted to 25-hydroxycholecalciferol in the liver and to 1,25-dihydroxycholecalciferol in the kidneys.