Lecture 18The Urinary System

5 Functions of the Urinary System

1. Regulate blood volume and blood pressure: by adjusting volume of water lost in urine releasing erythropoietin and renin

2. Regulate plasma ion concentrations: sodium, potassium, and chloride ions (by controlling

quantities lost in urine) calcium ion levels (through synthesis of calcitriol)

3. Help stabilize blood pH: by controlling loss of hydrogen ions and bicarbonate ions

in urine

4. Conserve valuable nutrients: by preventing excretion while excreting organic waste

products

5. Assist liver to detoxify poisons

Urinary System Organs

Kidney – produces urine

Urinary bladder – provides a temporary storage reservoir for urine

Paired ureters – transport urine from the kidneys to the bladder

Urethra – transports urine from the bladder out of the body

Quick Facts on the Kidneys

The kidneys filter 200 liters of blood daily, allowing toxins, metabolic wastes, and excess ions to leave the body in urine

Approximately one-fourth (1200 ml) of systemic cardiac output flows through the kidneys each minute

All the blood in the body is filtered 60 times everyday (2½ times every hour)

Internal Anatomy of the Kidney

A frontal section shows three distinct regions Cortex – the light colored, granular outer region Medulla – the inner region that exhibits cone-shaped medullary (renal) pyramids

6-18 Pyramids are made up of parallel bundles of urine-collecting tubules Renal columns are inward extensions of cortical tissue that separate the

pyramids A Lobe is a medullary pyramid and its surrounding capsule Papillae – drain urine from a lobe into a minor calyx

Draining passages Minor Calyces – small branches between the lobes and the major calyces Major calyces – large branches of the renal pelvis Renal pelvis – flat, funnel-shaped tube lateral to the hilus within the renal sinus

Urine flows through the pelvis and ureters to the bladder

Renal Pyramids Contain Nephrons

1. A Filter (the Renal Corpuscle): Glomerular capsule encloses a Glomerulus a fine network of

capillaries

2. A Tubule (3 sections): Proximal Convoluted Tubule (PCT) Loop of Henle is bent back on itself in

the center Distal Convoluted Tubule (DCT) Site of reabsorption & secretion

3. A Collecting Duct Water conservation device

A nephron is composed of three regions

Filtration Membrane

Figure 25.7a

The glomerulus is a filter that lies between the blood and the interior of the glomerular capsule

It is composed of three layers Fenestrated endothelium of the glomerular capillaries Visceral membrane of the glomerular capsule (podocytes) Basement membrane composed of fused basal laminae of the other

layers

2 Types of Nephrons

1. Cortical nephrons – 85% of nephrons; located in the cortex

2. Juxtamedullary nephrons: Are located at the cortex-medulla junction Have loops of Henle that deeply invade the medulla Have extensive thin segments Are involved in the production of concentrated urine

Capillary Beds

Every nephron has two capillary beds Glomerular capillaries Peritubular capillaries

Each glomerulus is: Fed by an afferent arteriole Drained by an efferent arteriole

Blood pressure in the glomerulus is high because: Arterioles are high-resistance vessels Afferent arterioles have larger diameters than efferent

arterioles Fluids and solutes are forced out of the blood throughout the

entire length of the glomerulus Peritubular beds are low-pressure, porous capillaries adapted

for absorption that: Arise from efferent arterioles Cling to adjacent renal tubules Empty into the renal venous system

Vasa recta – long, straight efferent arterioles of juxtamedullary nephrons

Juxtaglomerular Apparatus (JGA)

The distal tubule lies against the afferent (sometimes efferent) arteriole

Arteriole walls have juxtaglomerular (JG) cells Enlarged, smooth muscle cells Have secretory granules

containing renin Act as mechanoreceptors

Macula densa cells Tall, closely packed distal tubule

cells Lie adjacent to JG cells Function as chemoreceptors or

osmoreceptors

Mechanisms of Urine Formation

The kidneys filter the body’s entire plasma volume 60 times each day

The filtrate: Contains all plasma components except protein Loses water, nutrients, and essential ions to become

urine

The urine contains metabolic wastes and unneeded substances

Mechanisms of Urine Formation

Urine formation and adjustment of blood composition involves three major processes Glomerular filtration Tubular reabsorption Tubular secretion

Glomerular Filtration Rate (GFR)

The total amount of filtrate formed per minute by the kidneys

Factors governing filtration rate at the capillary bed are: Total surface area available for

filtration Filtration membrane

permeability Net filtration pressure (NFP)

Changes in GFR normally result from changes in glomerular blood pressure

GFR is directly proportional to the NFP

Glomerular Filtration

The glomerulus is more efficient than other capillary beds because: Its filtration membrane is significantly more permeable Glomerular blood pressure is higher It has a higher net filtration pressure

Plasma proteins are not filtered and are used to maintain osmotic pressure of the blood

If the GFR is too high: Needed substances cannot be reabsorbed quickly enough and are lost in

the urine

If the GFR is too low: Everything is reabsorbed, including wastes that are normally disposed of

Three mechanisms control the GFR Renal autoregulation (intrinsic system) Neural controls Hormonal mechanism (the renin-angiotensin system)

Absorption in Renal Tubules and Collecting Ducts

PCT reabsorbs substances including: Sodium, all nutrients, cations,

anions, and water Urea and lipid-soluble solutes Small proteins

Loop of Henle reabsorbs: H2O, Na+, Cl, K+ in the

descending limb Ca2+, Mg2+, and Na+ in the

ascending limb

DCT absorbs: Ca2+, Na+, H+, K+, and

water HCO3 and Cl

Collecting duct absorbs: Water and urea

Proximal Convoluted Tubule

Loop of Henle

~ 1200 mOsm

~ 300 mOsm

and produce the initial dilute urine

200 mOsm

Additional 10% of original waterpassively reabsorbed

Permeable to WaterNot to Solutes

Active reabsorption Na+

Na+/K+-Cl- symporter

Na+-H- antiporter

Passive reabsorption Na+

Additional 40% of original NaCl reabsorbed

(Filtrate ~ 100 mOsm)

Permeable to SolutesNot to Water

How Kidney Tubules Create an Osmotic Gradient

The Countercurrent Multiplier

Vasa Recta: Countercurrent Exchange

The vasa recta is a countercurrent exchanger that: Delivers blood to the cells in the area While maintaining the osmotic gradient

Proximal Convoluted Tubule

Loop of Henle

~ 1200 mOsm

~ 300 mOsm (Filtrate ~ 100 mOsm)

Common ProcessProducing initial dilute urine

200 mOsm

Distal Convoluted Tubule

Med

ullary C

ollectin

g D

uct

Variable ProcessProducing concentrated urine

Additional 40% of original NaCl reabsorbed

Permeable to SolutesNot to Water

Additional 10% of original waterpassively reabsorbed

Permeable to WaterNot to Solutes

IF plasma > 300 mOsm

Pituitary releases ADH

Insertion of aquaporins into luminal membrane

Water (& urea) Reabsorbed

Water Reabsorption

How Kidney Tubules Produce Concentrated Urine

Summary of Urine Concentration

Two things are required to concentrate urine A high osmotic gradient in the kidney’s medulla The presence of ADH

The loop of Henle is the primary structure responsible for maintaining the hypertonic medullary interstitium

The presence of ADH is mediated by blood plasma osmolality as sensed by the hypothalamus

The final concentration of urine cannot exceed the mOsm level of the most concentrated part of the hypertonic medullary interstitium.

Proximal Convoluted Tubule

Loop of Henle

~ 1200 mOsm

~ 300 mOsm

(Filtrate ~ 100 mOsm)Distal Convoluted Tubule

Med

ullary C

ollectin

g D

uct

What flows out of the filtrate in the descending limb?

What flows out of the filtrate in the ascending limb?

What hormone mediates the process in the previous question?

1.

2.

3.

When the DCT in this region is permeable to water, will water flow into or out of the tubule?

6.

Does the filtrate have higher osmolality in the Descending or Ascending limb?

4.

This difference in osmolality is called the?

5.

What hormone mediates water permeability in the tubules?

7.

What condition stimulates its release?

8.

In addition to water, what solute flows out of the duct in this region?

9.

What does this solute help maintain?

10.

As water is removed from the filtrate what happens to the filtrate volume?

11.

Summary Quiz on Concentration of Urine