The Renal System

The Renal System Overview

  • General functions of the kidney

  • Structure of the urinary system: macroscopic and microscopic

  • Glomerular filtration: rate and regulation

  • Mechanisms of salt and water reabsorption

  • Regulation of:

    • Water balance

    • Na/K balance

    • Acid-base balance

Functions of the Kidneys

  • Regulation of plasma volume

  • Waste removal

  • Regulation of electrolyte balance

  • Regulation of acid-base balance

  • All result from selective reabsorption and secretion of molecules by the kidney.

Kidney Structure

  • Mammalian kidney components:

    • Nephrons

    • Renal pelvis

    • Renal arteries and veins

    • Renal pyramids

    • Cortex

    • Medulla

    • Ureters

Types of Nephrons

  • Two types:

    • Cortical Nephrons:

      • Most common

      • Short loops of Henle

    • Juxtamedullary Nephrons:

      • Long loops of Henle

      • Important for urine concentration

Urine Formation Processes

  1. Filtration in the glomerulus

  2. Reabsorption of solutes and water in renal tubules

  3. Secretion of waste into renal tubules

Glomerular Filtration (GF)

  • What gets filtered into Bowman’s capsule:

    • Water, salts, glucose, urea, amino acids

  • What does not get filtered:

    • Cells, most proteins

  • Fluid in Bowman’s capsule is called "filtrate."

Factors Affecting Glomerular Filtration

  • Filtration driven by pressure differences

  • Net filtration pressure: 10 mm Hg

  • Porous structures that aid filtration:

    • Fenestrated capillaries: glomerular endothelium

    • Visceral layer of Bowman’s capsule: filtration slits

  • Charge on basement membrane repels negatively charged proteins

Regulation of Glomerular Filtration Rate (GFR)

  • GFR determined by renal blood flow (RBF)

  • RBF varies with arterial pressure

  • GFR = 120 mL/min or 180 L/day

  • Intrinsic regulation mechanisms:

    • Myogenic response - smooth muscle contraction in afferent arteriole occurs in response to stretch (will reduce RBF)

    • Tubuloglomerular feedback - a mechanism where the macula densa cells in the distal tubule sense changes in sodium chloride concentration and water flow, leading to adjustments in glomerular filtration rate (GFR) by altering the diameter of the afferent arteriole.

  • Extrinsic regulation mechanisms:

    • Sympathetic nervous system - leads to contracting of afferent arterioles and < RBF and GFR; leads to contraction of podocytes reducing the number of filtration slits (<GFR); can override autoregulation

    • Hormonal controls (e.g., Angiotensin II, Aldosterone)

      • Angiotensin II signals general vasoconstriction which increases blood pressure and increases GFR

      • Aldosterone increases blood volume, increases blood pressure, increases GFR, and promotes sodium reabsorption in the kidneys, which further enhances fluid retention and contributes to maintaining systemic circulation.

Mechanisms of Salt and Water Reabsorption

  • Different nephron segments have varying:

    • Ion transport mechanisms

    • Permeabilities to water

  • Leads to:

    • Recovery of valuable resources

    • Loss of toxic waste products

    • Concentration of urine.

Proximal Convoluted Tubule (PCT) Functions

  • ~65-70% of filtrate reabsorbed here

  • Sodium (Na), potassium (K), and chloride (Cl) transported into blood

  • Water follows by osmosis

  • Glucose, amino acids, and phosphates are reabsorbed

Loop of Henle

  • Juxtamedullary nephrons play a key role

  • 20% of salt and water reabsorbed (not regulated)

  • Establishes counter-current multiplier system

Distal Convoluted Tubule (DCT) Functions

  • hypotonic filtrate enters the DCT, where further reabsorption of sodium and chloride occurs, leading to the dilution of the tubular fluid.

  • Regulation of Na/K balance and pH

  • Responds to hormonal control (e.g., aldosterone)

Thin descending limb / thin ascending

  • relatively low level of ion transport.

  • water is permeable and follows osmoticgradient out of filtrate

  • filtrate becomes progressively moreconcentrated as it moves down limb

Thick ascending limb

  • Na/K/Cl transported across•

  • ascending limb very low permeability to water, which allows for the reabsorption of sodium and chloride ions while preventing water from following osmotically.

  • tissue surrounding the limb becomes hyperosmotic

  • salts trapped in tissue by recirculation via vasa recta (counter current exchange)

  • filtrate becomes dilute (100 mOSM)

Countercurrent multiplier: Significance

  • loops of Henle

  • • contributes to the establishment of an osmotic gradient in interstitial tissue of renal medulla

  • longer loop, steeper gradient

Vasa recta: significance

  • maintenance of gradient established by counter-current multiplier

  • •via recirculation of fluids and salts in vasa recta

  • serves to trap salt in the interstitial tissues

Collecting Duct Functions

  • Last 10-15% of filtrate processed

  • Permeable to water under hormonal control (ADH regulation)

  • water follows the osmotic gradient set up by the counter-current multiplier of the loop of Henle

ADH mechanism

  • ADH secretion stimulated by low BP, high osmolarity

  • ADH binds GPCR on collecting duct cells

  • cAMP is stimulated

  • stimulation exocytosis and insertion of aquaporins into collecting duct cell membranes

  • Increases the rate of diffusion and the reabsorption of water

Renal control of electrolyte balance

  • 85-90% of Na and K reabsorbed in proximal tubule and loop of Henle, independent of hormones

  • 10-15% regulated by aldosterone• regulation occurs in DCT

  • aldosterone promotes

    • Na reabsorption

    • K secretion

Acid-Base Balance in Kidneys

  • A and B cells in DCT and collecting duct regulate plasma pH

  • HCO3- reabsorbed and H+ secreted

  • A cells primarily manage acid-base regulation by adjusting proton pumps based on urine pH.