EO

Chapter 25 & 26 Review Notes

Nephron Structure and Function

  • The nephron is the functional unit of the kidney.
  • Urine, the final product of regulation, is eliminated at the papilla of the pyramids, flowing from the collecting ducts into the minor calyces.

Organization of the Nephron

  • The diagram provided is a simplified view; in reality, nephrons are interwoven.
  • The arcuate artery and vein mark the boundary between the cortex and medulla.
  • The cortical radiate artery supplies blood to the afferent arteriole, which leads to the renal corpuscle.
  • The glomerulus, a high-pressure capillary, is where filtration occurs due to hydrostatic pressure differences across the filtration membrane.
  • The efferent arteriole carries blood away from the glomerulus to the peritubular capillaries.
  • Peritubular capillaries surround the tubules and facilitate exchange with the filtrate.

Reabsorption and Secretion

  • Excess substances in the blood can be eliminated by secretion via the cells lining the tubular parts of the nephron.
  • Essential nutrients need to be reclaimed through reabsorption.
  • Less than 1% of the filtrate volume becomes urine, emphasizing the importance of reabsorption.

Filtration Membrane and Nutrient Reabsorption

  • Nutrients broken down during digestion are small enough to pass through the filtration membrane; if not reabsorbed in the proximal convoluted tubule, they will be lost in urine.
  • Understanding the structure of the nephron lining is crucial for understanding its function. Expect questions on matching nephron sections with their lining and function in the exam.

Renal Corpuscle and Filtration

  • Filtration occurs in the renal corpuscle.
  • The visceral layer of the inner capsule consists of podocytes (squamous cells with foot-like extensions).
  • The capillary lining is simple squamous epithelium (endothelium).
  • A fused basement membrane lies between these layers, facilitating filtrate formation.

Specialization for Absorption and Secretion

  • Beyond the renal corpuscle, cells are specialized for absorption and secretion.
  • Cuboidal cells with microvilli enhance surface area for reabsorption in the proximal convoluted tubule.
  • Reabsorption refers to reclaiming nutrients that were filtered out of the blood into the nephron as filtrate.

Proximal Convoluted Tubule

  • The proximal convoluted tubule is lined with microvilli cells to maximize reabsorption.
  • About 65% of sodium, glucose, and amino acids are reabsorbed in this segment, along with water and other electrolytes.
  • Active transport (red arrows) and passive processes (blue arrows) are involved in reabsorption and secretion.
  • pH regulation occurs via secretion of hydrogen ions, and some drugs are also secreted here.
  • Bicarbonate is reabsorbed into the blood to neutralize acidity. Hydrogen ions (acidic components), are secreted.

Loop of Henle

  • The filtrate moves down the descending limb of the loop of Henle, which is permeable to water only.
  • Water reabsorption is passive, driven by osmotic or hydrostatic pressure gradients.
  • The ascending limb is more permeable to solutes.
  • The distal convoluted tubule is under hormonal regulation.

Hormonal Regulation

  • Remember which hormones act on which part of the nephron and their functions.
  • Aldosterone stimulates sodium reabsorption, leading to water retention. Sodium and chloride act as osmotic forces.
  • Atrial Natriuretic Peptide (ANP) antagonizes aldosterone, stimulating sodium secretion, leading to water loss and lower blood volume and pressure.
  • ANP is secreted from the right atrium when blood pressure is elevated.
  • ANP is a vasodilator, while aldosterone is a vasoconstrictor.

Distal Convoluted Tubule and Hormones

  • The distal convoluted tubule is the main site of hormonal regulation of electrolytes and water.
  • Parathyroid hormone (PTH) stimulates calcium reabsorption and activates vitamin D in the kidneys, enhancing calcium absorption in the small intestine.
  • Chloride follows sodium due to electrical gradients. Sodium is the main cation, and chloride is the main anion in extracellular fluid.

Collecting Duct

  • The collecting duct is also influenced by aldosterone, leading to sodium reabsorption and potassium secretion.
  • Increased blood potassium levels stimulate aldosterone secretion, enhancing potassium secretion into the filtrate.
  • The collecting duct is a major site for pH regulation via intercalated cells, which have microvilli similar to those in the proximal convoluted tubule.
  • Principal cells in the cortex respond to hormones and regulate potassium secretion.
  • Antidiuretic hormone (ADH) reduces urine volume by stimulating water reabsorption in the cortical part of the collecting duct.

Renal Clearance

  • Renal clearance is a diagnostic tool for assessing glomerular filtration rate (GFR).
  • It measures the volume of plasma cleared of a substance (usually creatinine) per unit time.

Urine Properties

  • Urine is slightly acidic and clear.
  • Color varies based on ingested food, vitamins, and drugs. Cloudy urine indicates a urinary tract infection.

Kidney Stones

  • High calcium levels and alkaline pH in urine can lead to kidney stone formation in the renal pelvis.
  • Blockage by kidney stones increases pressure behind the blockage.

Urinary Tract Anatomy

  • The trigone region of the bladder is marked by the openings of the ureters and the bladder neck, which is continuous with the urethra.
  • The urethra has internal and external sphincters.
  • The male urethra has three sections: prostatic, membranous, and spongy.

Juxtaglomerular Apparatus

  • The juxtaglomerular apparatus (JGA) includes the afferent arteriole, distal part of the ascending limb of the loop of Henle, and macula densa cells.
  • Macula densa cells monitor salt concentration in the filtrate.
  • High salt concentration indicates rapid filtrate flow and inadequate reabsorption.
  • To reduce GFR: constrict the afferent arteriole and contract glomerular mesangial cells, which reduces the filtration membrane.
  • Low salt concentrations result in secretion of vasodilatory chemicals from macula densa cells to the afferent arteriole, increasing GFR.
  • Granular cells secrete renin when stimulated by low blood pressure or sympathetic activation.

Glomerular Capsule Structure

  • The parietal layer is the outer wall of the capsule, while the visceral layer is intimate with the glomerulus, forming the filtration membrane.
  • Podocytes have elongated feet, and the spaces between them are filtration slits.
  • The filtration membrane consists of blood capillary endothelial cells, the basement membrane, and podocytes.
  • The slit diaphragm repels macromolecules, and mesangial cells phagocytize any that get through, preventing clogging.

Nephron Complexity and Juxtamedullary Nephrons

  • The tubular parts of the nephron are highly interwoven.
  • The distal convoluted tubule is continuous with the collecting duct, and the proximal convoluted tubule is connected to the capsule.
  • The afferent arteriole originates from the cortical radiate artery, while the efferent arteriole is continuous with the peritubular capillaries.
  • Juxtamedullary nephrons have a long loop of Henle and vasa recta for water reabsorption, producing concentrated urine. They make up 15% of nephrons, while cortical nephrons make up 85%.

Chapter 26: Body Fluids, Acid-Base Balance, and Electrolytes

  • This chapter focuses on fluid compartments, regulation of water content, acid-base balance, and electrolyte regulation.

Fluid Content in the Body

  • Total body fluid is about 40 liters.
  • Two-thirds (25 liters) is intracellular fluid (inside cells).
  • The remaining 15 liters is extracellular fluid.
  • Extracellular fluid includes interstitial fluid (12 liters) and plasma (3 liters).
  • Water content varies: infants (73%), adults (males > females), and elderly (lower water content).

Solutes in Body Fluids

  • Body fluids consist of water (solvent) and solutes.
  • Solutes are categorized as electrolytes and non-electrolytes.
  • Electrolytes are ionic compounds that dissociate in water into charged particles.
  • Non-electrolytes are organic compounds that do not dissociate in water.
  • Electrolytes include salts (cations other than H^+ and anions other than OH^-, acids, and bases.

Electrolytes vs. Non-Electrolytes

  • Electrolytes are more abundant solutes and contribute to osmotic force.
  • Non-electrolytes (e.g., glucose) are not charged, while electrolytes (e.g., salts, acids, bases) are.
  • Electrolytes mobilize water between compartments via osmotic and hydrostatic forces.

Cations and Anions in Fluid Compartments

  • Cations are positively charged, and anions are negatively charged.
  • Intracellular fluid's main cation is potassium (K+), while extracellular fluid's main cation is sodium (Na+).
  • Interstitial fluid and plasma are similar in electrolyte composition.
  • Plasma is more similar to intracellular fluid in protein content than interstitial fluid.
  • Intracellular fluid has the highest protein content, followed by plasma, and then interstitial fluid.
  • Albumin in plasma contributes to osmotic force, drawing water back into capillaries.
  • Extracellular fluid's main anion is chloride (Cl-), following sodium.

Buffers and Chemical Buffer Systems

  • Buffers resist changes in pH caused by metabolic reactions.
  • Chemical buffers in body fluids are the first responders to pH fluctuations.
  • Chemical buffers include phosphate, carbonic acid-bicarbonate, and proteins.
  • The phosphate buffer system, using hydrogen phosphate, predominates in intracellular fluid.
  • The carbonic acid-bicarbonate buffer predominates in extracellular fluid (mainly plasma), transporting about 70% of carbon dioxide in the blood.
  • Proteins, with amphoteric amino acids, serve as buffers by neutralizing hydrogen ions (amino group) or releasing hydrogen ions (carboxylic group).

Physiological Buffer Systems

  • Physiological buffer systems have a greater capacity than chemical buffers.
  • The respiratory buffer system is stimulated by pH or carbon dioxide level fluctuations in the brainstem, acting within minutes.
  • Respiratory compensation corrects pH changes caused by metabolism (metabolic acidosis or alkalosis).
  • Renal compensation corrects problems arising from respiratory acidosis or alkalosis, taking longer to act.

Water Intake and Output Balance

  • Maintaining water balance involves balancing water intake and output.
  • The hypothalamus is central to visceral control, stimulated by osmoreceptors when osmolarity increases (2-3%).
  • Increased osmolarity activates the thirst center and ADH secretion to conserve water and dilute blood.

Homeostatic Imbalances of Water Content

  • Dehydration results from excessive fluid loss (diarrhea, vomiting, sweating, blood loss), reducing extracellular fluid and dehydrating cells.
  • Kidney deficiency or excessive water intake can cause hypertonic hydration, leading to cell swelling and bursting.

Edema

  • Edema is the atypical accumulation of fluid in interstitial spaces.
  • Factors include cardiovascular problems, leaky valves or permeable capillaries, low colloid osmotic force (hypoproteinemia), high blood pressure, or lymphatic system issues.

Electrolyte Imbalances

  • Potassium (K+): needed for resting membrane potential.
  • Excess extracellular potassium lowers resting membrane potential, reducing excitability.
  • Too low extracellular potassium (e.g., from aldosterone hypersecretion) causes hyperpolarization and non-responsiveness.
  • Calcium (Ca2+):
    • Hypercalcemia (too much calcium) inhibits muscles and nerves, causing arrhythmia.
    • Hypocalcemia (too little calcium) causes muscle tetany, increasing nerve and muscle excitability.