Renal System Overview and Anatomy

Renal System: Overview

• Kidneys have general functions including:

  • regulating plasma volume
  • removing wastes
  • regulating electrolyte balance
  • regulating acid-base balance
  • selective reabsorption and secretion of molecules

• The urinary system's structure includes both macroscopic and microscopic components

  • Components are glomerular filtration, rate regulation, and mechanisms of salt and water reabsorption.

• Regulation of water, Na/K balance, and acid-base balance is also key functions.

Mammalian Kidney Structure

• The mammalian kidney has distinct regions:
  • nephron
  • renal pelvis
  • renal pyramid
  • cortex
  • medulla
  • renal artery
  • renal vein
  • ureter

Types of Nephrons

• Two types of nephrons:
  • Cortical nephrons (most common) - short loops of Henle
  • Juxtamedullary nephrons - long loops of Henle; fraction of the loop length affects urine concentration ability

Kidney Tubules

• Kidney tubules are made of epithelial cells with features:
  • brush border
  • tight junctions
  • apical (luminal) side
  • basolateral (serosal) side
  • lumen
  • blood

Urine Formation

• Urine formation involves three fundamental processes:
  • Filtration: water and solutes are filtered into Bowman's capsule.
  • Reabsorption: Solutes and water are reabsorbed into the bloodstream.
  • Secretion: Certain substances are actively secreted from the blood into the renal tubule.

Glomerular Filtration Properties

• Glomerular filtration lets water, salts, glucose, urea, and amino acids into Bowman's capsule.
• Cells and most proteins are NOT filtered.
• The filtrate is the fluid in Bowman's capsule.

Glomerular Filtration: Driving Force

• Glomerular filtration is driven by pressure.
• Afferent arteriole is wide and has low resistance to inflow.
• Efferent arteriole has high resistance and outflow.
• The high resistance outflow pathway allows for pressure to build up in the glomerulus.

Glomerular Filtration Dependence

• Glomerular filtration depends on these factors:
  • Net filtration pressure (10 mm Hg in humans)
  • Porous nature of glomerular endothelium (fenestrated capillaries)
  • Porous nature of visceral layer of Bowman's capsule (filtration slits)
  • Electrical charge on the basement membrane of the glomerulus repels negatively charged proteins

Juxtaglomerular Apparatus (JGA)

• JGA involves:
  • Macula densa cells in the distal tubule
  • Granular cells in the afferent arteriole
  • Distal tubule cells
  • JGA senses low flow and responds by releasing prostaglandins to dilate afferent arterioles to increase blood flow and GFR (glomerular filtration rate)

Glomerular Filtration Rate (GFR)

• GFR is the volume of filtrate produced per minute by both kidneys.
• GFR is typically 120 ml/min or 180 L/day.
• Total blood volume filtered every 40 minutes.

GFR Regulation

• Renal blood flow (RBF) determines GFR
• Arterial pressure varies from 80-200 mm Hg.
• RBF is highly regulated to maintain a constant GFR. - >RBF = > GFR and <RBF = < GFR

Intrinsic Regulation

• Renal autoregulation compensates for changes in RBF and GFR:
  • Myogenic- Smooth muscle contraction in the afferent arteriole occurs in response to stretch.
  • Tubuloglomerular feedback- Macula densa releases prostaglandins to adjust afferent arteriole diameter.

Extrinsic Regulation

• Sympathetic nervous system innervates afferent arterioles.
  • Constriction leads to lower RBF and GFR.
• Hormonal control
  • Angiotensin II signals general vasoconstriction, increasing blood pressure and GFR.
  • Aldosterone increases blood volume and blood pressure, increasing GFR

Mechanisms of Salt and Water Reabsorption

• Different nephron segments have different ion transport and water permeability.
• This sets conditions for recovering valuable resources, removing wastes, and concentrating urine.

Proximal Convoluted Tubule (PCT)

• Filtrate starts at 300 mOsm.
• Sodium, potassium, and chloride are transported from the lumen to the blood.
• Water follows by osmosis.
• Glucose and amino acids, and phosphate are reabsorbed.
• ~65-70% of filtrate is reabsorbed.
• Filtrate remains at 300 mOsm.

Loop of Henle

• Juxtamedullary nephrons have a primary impact.
• 20% of salt and water is reabsorbed in the loop of Henle.
• The loop of Henle uses a countercurrent multiplier system.

Thin Descending Limb

• Relatively low ion transport.
• Water is permeable, and follows the osmotic gradient out of filtrate.
• Filtrate becomes progressively more concentrated as it moves down the limb

Thick Ascending Limb

• Sodium, potassium, and chloride are actively transported across.
• Low water permeability.
• Tissue surrounding the limb becomes hyperosmotic.
• Salts are trapped in the tissue by recirculation via vasa recta.
• Filtrate becomes dilute (~100 mOsm).

Distal Convoluted Tubule (DCT)

• Hypotonic filtrate enters the DCT.
• Regulation of Na/K balance and pH balance occurs.
• Hormonal control (aldosterone).

Countercurrent Multiplier

• Loops of Henle establish an osmotic gradient in the renal medulla.
• Longer loops create steeper gradients

Vasa Recta

• Maintains the gradient established by the countercurrent multiplier.
• Recirculation of fluids and salts in vasa recta traps salt in interstitial tissues.

Collecting Duct

• Last 10-15% of filtrate.
• Permeable to water, follows osmotic gradient.
• Water permeability is under hormonal control (ADH)

Renal Control of Electrolyte Balance

• 85-90% of sodium and potassium are reabsorbed in PCT and Loop of Henle.
• 10-15% is regulated by aldosterone, and occurs in the DCT.

Acid-Base Balance

• In mammals, plasma pH is regulated by A and B cells in the DCT and collecting ducts.
• H+ is secreted, and HCO3- is reabsorbed.

Aquaporins

• Water channels in cell membranes allowing for water transport.

Explore the essential functions of the renal system, focusing on kidney anatomy and nephron types. Understand the roles of different components like glomerular filtration and the significance of nephron structures in regulating bodily fluids and wastes. This quiz will enhance your knowledge of mammalian kidney structure and function.