The Excretory System

The body has 2 kidneys

They hold ~ ¼ of our blood at any one time

They play a major role in homeostasis

Each has a mass of ~ 500 g

are made up of about 1 million nephrons

1. Remove Wastes

2. Balance Blood pH

3. Maintain Water Balance

4. Blood Pressure

Such as urea and uric acid from the blood

Uric acid is formed from the breakdown of DNA (nucleic acids)

Urea is formed in a process called deamination

It occurs in the liver

It is the conversion of amino acids in the blood to carbohydrates (urea is produced)

1.) NH3 is removed from amino acids (ammonia – highly toxic…0.005

mg can kill you)

2.) 2 molecules of NH3 combine with CO2 to form urea

(urea is 1000X less toxic)

Both urea and uric acid enter the blood and will be removed from the body by the kidneys

The build up of uric acid is called gout

Kidneys remove ions (E.g. H+) from the blood keeping the pH in normal range between 7.3 and 7.5

CO2 from the peritubular capillaries combines with H20 in the cells that line the nephon producing carbonic acid….

Carbonic acid breaks apart to form H+ and HCO3

- ions

CO2 + H2O H2 CO3 H2CO3 H + + HCO3

- 

If the blood is too acidic the H+ is excreted in the urine and HCO3

- is reabsorbed back to the blood …buffering the blood

If the blood is too basic The HCO3

- is excreted in the urine and H+ is reabsorbed back to the blood

This is aided by changes in breathing rate

180 L moves through kidneys in one day

~ 2L of water is lost per day from perspiration, exhaling, urine

178 L are reabsorbed!

we can only survive a few days without water 1% loss of H2O = thirst,

5% loss of H2O = pain, collapse,

10% loss of H2O = death…E.g. heat exhaustion

Two hormones regulate water concentration in the blood:

antidiuretic hormone (ADH) aldosterone

Hormones are chemicals that travel in the blood and target cells in the body…. eliciting a response

Targets the nephrons distal tubule and collecting duct

It increases H2O reabsorption into the

bloodstream

It is produced by nerve cells in the hypothalamus

it is stored in the posterior pituitary gland and secreted when it is needed

Hypothalamus

Pituitary

H2O lost in the body causes H2O to move from the tissues into the bloodstream (shrinking body cells)

osmoreceptors in the hypothalamus detect low osmotic (H2O) pressure

The hypothalamus responds by: triggering thirst sending a nerve message to the

pituitary gland ….stimulating the release of ADH

ADH travels in the blood and targets the nephron

No ADH

ADH Present

ADH causes the distal tubule and collecting duct to become permeable to H2O

The kidneys reabsorb more H2O a more concentrated urine (dark yellow) is

produced

Cold weather, caffeine and alcohol all inhibit the release of ADH

reabsorption of the H2O can’t take place and

urine volume is increased

The kidneys help regulate blood pressure

Aldosterone: is produced and secreted by the

adrenal glands

It targets the ascending Loop of Henle, distal tubule and collecting duct

It increases the amount of Na+ reabsorption.. therefore increasing the reabsorption of H2O

This increases blood pressure

It is detected by osmoreceptors in the juxtaglomerular apparatus ………cells located in the afferent arteriole near the glomerulus

cells in the juxtaglomerular apparatus release rennin

Rennin converts angiotensinogen (a plasma protein secreted by the liver) to angiotensin (active form)

Produces and releases aldosterone

Aldosterone Increases Sodium reabsorption

Na+

Na+

Na+Na+

Na+Na+ Na+

Na+

Na+

Na+

Na+

angiotensin: constricts blood vessels causes aldosterone to be released from

the adrenal glands (both of which increase BP)

Aldosterone causes an increase in Na+ reabsorption from the ascending loop, distal tubule and collecting duct (nephron)

H2O follows by osmosis and moves into the bloodstream therefore increasing blood volume and BP

Draw and label the structures of the Urinary System

Renal arteries: branch from the abdominal aorta they supply the kidneys with

oxygenated, unfiltered blood

Renal veins: remove deoxygenated, filtered blood

Filter wastes from the blood

Are made up of 3 layers:

a.) the cortex: outer layer of connective tissue

b.) the medulla: inner layer..contains the nephrons (beneath the cortex)

c.) the pelvis: hollow chamber joining kidney to ureter

Are 2 tubes that filtered wastes from the kidneys passes through to reach the bladder

storage for urine

holds about 200 mL of urine

at 400mL stretch receptors signal the brain that it is “time to go….”

after 600 mL is collected all voluntary control is lost!

urine leaves the bladder through the urethra

sphincter muscles relax and urine is

voided

It is shorter in women and as a result women are more susceptible to bladder infections…..closer contact to the outside

The Nephron Handout: fill in structures

Note: the cortex contains: the afferent, efferent

arterioles and the Bowman’s capsule

The medulla contains: the Loop of Henle

are the functional units of the kidney

Afferent arterioles supply the nephrons with blood branch into the capillary bed called the

glomerulus

Blood leaving the glomerulus: Passes through the efferent arterioles travels to the peritubular capillaries

(they wrap around the tubules) to the venule then to the renal vein

The glomerulus is surrounded by the Bowman’capsule (funnel-like part)

This tapers into the proximal tubule

The proximal tubule connects to the loop of Henle

The loop of Henle connects to the distal tubule

The distal tubule empties into the collecting duct …these ducts merge in the renal pelvis

Your Assignment: Page 380, 1-3

It depends on 3 steps: Filtration Reabsorption Secretion

Filtration: the movement of fluid from the blood

(glomerulus) into the Bowman’s capsule

Reabsorption: is the movement of essential solutes and water

from the nephron back to the blood

Secretion: involves the transport of materials from the

blood into the distal tubule of the nephron

Filtration

Occurs at the glomerulus

filtration rate: if the pressure then filtration rate

blood enters nephron through the afferent arteriole (pressure is 2 kPa)

Then blood enters the glomerulus (pressure is 8 kPa) which acts like a high pressure filter  

Proteins, blood cells and platelets are too large to filter through glomerulus and move into the efferent arteriole

Plasma passes through the glomerulus into the Bowman’s capsule (E.g. Na+, Ca2+, glucose, urea, uric acid etc.)

Reabsorption

Solutes move from nephron into bloodstream

Most reabsorption (85%) occurs in the proximal tubule

It also occurs in the Loop of Henle and distal tubule

it occurs till the threshold level is reached

600 mL of fluid moves through kidney per minute… about 120 mL of that is filtered into the nephron…and from that only 1 mL of urine is made

Both active and passive transport occur (large molecules require active transport)

glomerular filtrate (dissolved solutes) travels through the proximal tubule  

the proximal tubule contains a rich supply of mitochondria (ATP production) and microvilli

  as the filtrate moves through the proximal

tubule, Loop of Henle and distal tubule, H2O and certain ions are reabsorbed back into the bloodstream

This maintains proper water and ion balance

Na+, glucose and amino acids are actively transported out of the nephron into the peritubular capillaries (bloodstream)

in the descending loop the filtrate is hypertonic and water diffuses out of nephron into bloodstream (osmosis)

At the ascending loop, the filtrate is hypotonic Na+ is actively transported out …..to keep water from moving the wrong way

Cl- and HCO3- ions follow the Na+ ions…

due to charge attraction (passive transport)

There is limited energy for active transport so some solutes (E.g. NaCl) stay …..

once the ions move out, an osmotic gradient is set up so water diffuses from tubules and Loop of Henle into bloodstream

filtrate becomes more concentrated as it moves along towards the distal tubule

Secretion

movement of wastes from the bloodstream into nephron

Occurs at the distal tubule and involves active transport

urea, histamine, NH3, K+ ions, H+ ions, HCO3 ions, minerals, drugs such as penicillin etc. move from the blood into the distal tubule

After secretion the urine moves into the collecting ducts

The collecting ducts lead to the renal pelvis then on to the ureters and bladder

Filtration glomerulus and Bowman’s capsule passive transport

Reabsorption proximal tubule (mainly), loop of Henle, distal

tubule active and passive transport

Secretion distal tubule active transport

water (95%) urea uric acid trace amino acids electrolytes: Na+, K+, Cl-, SO4

2-, PO4 +3

excess vitamins, minerals, etc

The pH of urine ranges from 4 to 5 but can get as high as 8.5

Remains in Blood

Filters into Capsule

Reabsorbed

RBC Water Water (most of)

WBC Amino acids Amino acids (all)

Platelets Glucose Glucose (all)Fats Salts Salts (some)

Blood Protein Urea, uric acid

Urea, uric acid (none)

Component

Plasma Filtrate Urine

Urea 0.03 0.03 2.0

Uric Acid 0.004 0.004 0.05

Glucose 0.10 0.10 0 to trace

Amino Acid

0.05 0.05 -----

Salt 0.72 0.72 ~1.5

Protein 8.00 ----- ----

Comparing Solutes in plasma, Nephron, and Urine

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