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