BIO – 255 Anatomy & Physiology

Chapter 25 – Urinary System

Urinary System Functions?

Excretion – blood filtering Gee Whiz! - each day the kidneys filter

about 180 liters of plasma About 25% of total cardiac output goes to

the kidneys!!! Renin and erythropoietin production Vitamin D activation

Urinary System Anatomy

Kidneys, Ureters, Urinary bladder, and Urethra – Figures 25.1, 25.2 and 25.3

Urinary System What is the nephron?

There are about 1 million per kidney! Figure 25.4 and 25.5

Urinary System Juxtaglomerular apparatus – this is the site

of renin production The cells determine blood pressure by

stretch of the afferent arteriole wall Figure 25.6

Urinary System Nephron Structure

Renal corpuscle = the Bowman’s capsule and the glomerulus

The inner layer of the Bowman’s capsule is composed of podocytes

Spaces between podocyte projections are called filtration slits

Figure 25.7

Urinary System The glomerular capillary walls are lined

with openings called fenestrae The filtration membrane = capillary wall,

basement membrane, and podocytes Figure 25.7

Urine Production Urine production occurs at the nephron It occurs in three stages:

Filtration – fluid is moved from the blood into the Bowman’s capsule This fluid is called filtrate

Reabsorption – movement of substances from the filtrate back into the blood

Secretion – active transport of wastes into the urine

Figure 25.8

Urine Production Filtration

Total plasma volume is filtered ~60 times per day The kidneys use about 20-25% of all oxygen

in the body (at rest) but they only make up about 1% of total body weight

About 180 L of filtrate per day, but only about 1.8 L of urine (reabsorption is major)

Urine Production Filtration Membrane

The filtration membrane is a barrier to large particles in the blood Cells, most proteins, etc.

Fluid is pushed out of the capillary by the filtration pressure Blood pressure in the glomerulus is

extremely high Figure 25.9

Urine Production Filtrate is similar to plasma, but urine is not

The primary changes occur during: Reabsorption – this occurs in the tube system

of the nephron Proximal tubule – most solutes and water are

reabsorbed here For most molecules this occurs by secondary active

transport (cotransport) with Na+ ions Na+ is actively transported out of the cell into the

interstitial fluid This generates the concentration gradient to power

the cotransport mechanism Figure 25.12

Urine Production By the end of the proximal tubule, 65% of

the filtrate volume has been reabsorbed

Urine Production Loop of Henle

Descending limb extends into the medulla the interstitial fluid has an extremely high

solute concentration (1200 mOsm/kg) This causes the movement of water out of

the nephron A few solutes enter the nephron

At the bottom of the loop, an additional 15% of filtrate has been removed

Figure 25.14

Urine Production The ascending limb is impermeable to

water, but additional solutes are removed This causes a dilution of the filtrate (100

mOsm/kg) Figure 25.14

Urine Production Distal tubule and collecting duct

More ions are absorbed Na+ and Aldosterone

Water movement through the distal tubule and collecting duct walls is under hormonal control

Antidiuretic Hormone (ADH) is the primary regulator – What does it do?

More later

Table 25.1

Urine Production Some materials are not actively removed

from the filtrate Urea, uric acid, phosphates and other toxic

substances become more concentrated

Urine Production Secretion

Some substances are actively transported into the urine in the proximal and distal tubules Drugs, urea, excess K+

Blood pH regulation and H+ secretion Figure 25.8

Urine Formation CD

Urine Production Regulation of Urine Concentration and

Volume Osmolality is a measure of how many

solutes are dissolved in a fluid Dilute urine (~65 mOsm/kg) Concentrated urine (~1200 mOsm/kg) Blood and most other body fluids (~300

mOsm/kg) The composition of urine is manipulated to

ensure this remains constant

Urine Production The concentration of urine depends

on maintaining a high solute concentration in the medulla water and solutes move from the

filtrate into the medullary interstitial fluid at the loops of Henle

The vasa recta are more permeable to water than solutes

Figures 25.13 and 25.14

Urine Production Concentration of urine depends on

ADH ADH activates a second messenger

system that leads to more active aquaporins in the distal tubule and collecting duct This allows water to be removed from

the filtrate and enter the blood “anti”-diuretic

Figure 25.15 and from other text

Urine Production Formation of dilute urine occurs in

the absence of ADH Aquaporin function is inhibited so

water is retained in the filtrate Figure 25.15

Reg. of Urine Concentration & Volume What if a person could not produce

enough ADH? Diabetes insipidus (10-20 liters of

urine per day) Figure 25.15

Reg. of Urine Concentration & Volume Renin-Angiotensin-Aldosterone

Mechanism When BP decreases, the kidneys secrete

renin Renin is secreted by the cells of the

juxtaglomerular apparatus Renin activates a plasma protein called

angiotensin Active angiotensin causes

vasoconstriction and the release of aldosterone from the adrenal cortex

Reg. of Urine Concentration & Volume Aldosterone increases Na+ and Cl-

reabsorption by the kidney What effect would this have on urine

concentration and volume? Angiotensin can also stimulate the

release of ADH Figure 25.15 and from other text

Reg. of Urine Concentration & Volume Atrial Natriuretic Hormone (ANH)

Is secreted by the cells of the right atrium when they are stretched

ANH blocks the secretion of ADH and the reabsorption of Na+

Effect?

Other Information Diuretics (such as Lasix and Diuril)

inhibit Na+ reabsorption. Effect? Kidney stones occur when minerals

crystallize in the renal pelvis Ultrasound treatment Risk factors are bacterial infections, urine

retention, high blood Ca2+ , and alkaline urine.

Micturition (Urination) There are smooth and skeletal muscle

sphincters that close off the urethra When about 200 ml of urine is in the

bladder, the urge to urinate occurs The skeletal muscle can over-ride the urge After another 2-300 ml, another urge is felt This can also be suppressed When the volume reaches about 600 ml the

urge is constant and eventually micturition occurs (whether you like it or not)