Urinary System
Chapter 25
Overview 1. Structures/Organs
2. Location (Kidneys) – T12 to L3
– 150 g
Kidney Structure1. Renal hilum 2. Protective Tissue
– Fibrous capsule; fat capsule
3. Internal Structure – Cortex– Medulla – Pelvis and Calyces
Blood Supply
Nephrons 1. Overview2. Renal Corpuscle
– Glomerulus – Bowman’s Capsule
3. Function of Renal Corpuscle 4. Tubules
– PCT– Loop of Henle – DCT – Collecting Ducts
Tubule - Histology
Nephron Capillary Beds
1. Peritubular
2. Vasa Recta
Juxtaglomerular Apparatus • 1. JGA
– Granular cells – Juxtaglomerular cells – Macula densa
• 2. Filtration – Membrane – Process
How Urine is Formed • Three Steps to forming Urine
– 1. Glomerular Filtration
• Blood moved into glomerulus
• Forced Out: “Filtrate”
– 2. Tubular reabsorption
• Glucose; Amino Acids; 99% Water; Salts, etc. reclaimed by kidneys
– 3. Tubular secretion
• What is not needed ------- Urine
Glomerular Filtration 1. Passive Process
2. Glomerular Blood Pressure
3. Passage of Molecules
4. Pressures – Net Filtration Pressure
– Glomerular Hydrostatic Pressure
•5. Regulation – Intrinsic (in Kidney)
– Extrinsic (Nervous and Endocrine)
Intrinsic and Extrinsic Controls
Tubular Reabsorption
1. Introduction– Filtrate enters PCT – “Transepithelial”
process • Transcellular or
paracellular routes
2. Reabsorption of Sodium – Active/Transcellular – Out of the Tubule: “Primary Active Transport” (Na-K pump)– Secondary Active Transport at Luminal face
• With glucose, amino acids
Tubular Reabsorption 3. Nutrients, Water and Ions
– Reabsorption via secondary active transport (glucose, amino acids, ions, lactate, vitamins)
• Cotransport with sodium ions
– Water follows Na+
– Passive Tubular Reabsorption
• Aquaporins
Reabsorption – PCT
Reabsorption – Other Tubules and Collecting Ducts
Tubular Secretion 1. Some substances not reabsorbed.
2. Tubular Secretion: Reverse of reabsorption
3. Filtrate from Peritubular capillaries ------ filtrate in tubules
– H+ ; K+ ; NH4+
4. Functions– Disposal of drugs
– Urea
– Excess K+
– Blood pH
Osmotic Gradient
(Regulating Urine Concentration and Volume)
• Osmolality: number of solute particles dissolved in one liter of water
• One function of the kidney’s is to keep the solute load of body fluids constant at 300mOsm (milliosmol = 1 osmol = 1 mole of nonionizing substance/1L) which is ‘isotonic’
• This is done by regulating urine concentration and volume and is
accomplished via countercurrent mechanism
Counter Current Mechanism
• Countercurrent means substances flowing in opposite directions
• Involves flow of filtrate through loops of Henle (Juxtamedullary nephrons) and blood flow in vasa recta
• PCT filtrate = 300mOsm (same as blood plasma). Increases to 1200 as goes to the deepest part of the medulla
Countercurrent Multiplier
• Establishes an osmotic gradient • Descending Limb (LOH)
– Impermeable to solutes and permeable to water
• Ascending Limb (LOH)– Permeable to solutes and
impermeable to water– By time reaches DCT becomes
very dilute (100) or hypotonic
• Collecting ducts become permeable to urea
Countercurrent Exchanger
• Maintains the osmotic gradient
• Blood flow is sluggish
• Permeable to water and NaCl– Allows blood to make passive
exchanges with surrounding interstitial fluid and achieve equilibrium
• As blood flows deep into the medulla, it loses water and gains salts (hypertonic)
• As flow moves up to cortex the reverse occurs
• It helps to maintain the medullary osmotic gradient by removing salts
Formation of Dilute and Concentrated
Urine
• If AHD hormone is not released, as filtrate flows through the ascending loop it is dilute; a hypo-osmotic filtrate will continue through the collecting duct
– CT’s are impermeable to water (low aquaporins); no reabsorption of water occurs – Na and other ions can be removed from the DCT making the filtrate more dilute
• Concentrated Urine– ADH is released will increase the number of aquaporins in the CT’s – ADH release depends on the level of body hydration
• 99% of water in filtrate is reabsorbed into the blood and less than 1L/day of concentrated urine is excreted (ability to produce concentrated urine allows us to survive in times of low water availability)
• Diuretics: enhance output of urine
Summary of Nephron Function
Ureters
Bladder and Urethra
Animation Link
Control of Continence and Micturition