Ladies & Gentlemen,
today's topic is .................................The Human Excretory System
 
 

What we're dealing with here are the organs in the body that have a role in removing metabolic wastes.  Let's clarify what we mean by "metabolic wastes".  Metabolic wastes are by-products of metabolism (dah, no kidding).  That's fine & dandy as long as we know what metabolism means.


me-tab-o-lism \ma'tab-e-liz-em\ n the sum total of the chemical reactions that keep an organism alive


Examples of these "chemical reactions" would be things like synthesis, respiration, hydrolysis, & neutralization reactions.  Each of them have a role in keeping a living thing ticking; and in so doing, each produces certain waste products.  These waste products are referred to as metabolic wates.  Do we have the jist of this yet?

Let me put it this way:  humans must get rid of two types of wastes.  Wastes from the digestive system (feces) & wastes from metabolic activities (sweat & urine).  Removing digestive wastes (pooping) is called egestion.  Removing metabolic wastes is called excretion.  OK? OK.  Let's move on .......

 



Major Metabolic Wastes:

This table summarizes the four types of metabolic wastes produced by humans
(& other animals) & the type of chemical reactions that produce them.

METABOLIC WASTE

A BY-PRODUCT OF ....

water

dehydration synthesis & respiration

carbon dioxide

cellular respiration

salts

neutralization

urea

protein metabolism, deamination

         dehydration synthesis = reactions in which small molecules are combined to form large molecules

         cellular respiration = chemical reaction that releases energy from organic molecules (usually glucose)

         neutralization = reactions between acids & bases

         deamination = removal of amino groups from protein molecules


HUMAN EXCRETORY ORGANS - there are 4
 

1. the lungs

2. the liver

3. the skin

4. the KIDNEYS
(Urinary System)


 
 
 


EXCRETORY ORGAN #1 = THE LUNGS

Cellular respiration occurs in every living cell in your body. It is THE reaction that provides energy (in the form of ATP molecules) for cellular activities.  If respiration stops, the cell no longer has energy for cellular activities & the cell dies.  As respiration occurs carbon dioxide is produced as a waste product.  As the carbon dioxide accumulates in body cells, it eventually diffuses out of the cells & into the bloodstream, which eventually circulates to the lungs.  And here, in the alveoli of the lungs, carbon dioxide diffuses from the blood, into the lung tissue, and  then leaves the body every time we exhale.  We should note that some water vapor also exits the body during exhalation.

For a more detailed description of gas exchange in the lungs
(with pictures & everything) peek at my page about
the Human Respiratory System.


 
 


EXCRETORY ORGAN #2 = THE LIVER

The liver is a large, important organ.  In fact it is the largest internal organ in our bodies.  Its numerous functions make it "part" of the circulatory, digestive, and excretory systems.  You know what? The space below would make a nice spot for a chart summarizing the jobs of the liver .............
 

 Excretory Function 


 Some proteins & other nitrogenous compounds are broken down in the liver by a process called deamination.

 As a result of these reactions, a nitrogenous waste called urea is formed.
 

Digestive Function 


 The liver produces bile, which is temporarily stored in the gall bladder before being released into the small intestine where it helps "emulsify" (break down) lipid molecules.

 Circulatory System 


 The liver removes & breaks down old red blood cells.

 It is also responsible for maintaining "normal" levels of glucose in the blood. When stimulated by insulin, the liver removes glucose form the blood & converts it to glycogen for storage.  When stimulated by the hormone glucagon, the liver does the opposite: it breaks down glycogen, producing glucose, which is released into the bloodstream.

 The liver is also responsible for removing potentially hazardous chemicals from the blood.  It "detoxifies" the blood.  For this reason, alcoholics and other types of addicts have a higher incidence of liver disease.
 


 
 


EXCRETORY ORGAN #3 = THE SKIN

As you already know, sweat comes out of pores in your skin.  As you may not know, sweat is a mixture of three metabolic wastes: water, salts, & urea.  So as you sweat, your body accomplishes two things:

1) sweating has a cooling effect on the body, and

2) metabolic wastes are excreted.

So, how does the sweat form?  Ah, good question. 
Let's look at a picture & picture the sweat-making process.

I've colored the sweat gland & the duct (tube) leading to the skin surface a nice bright green so that they stand out.

What you should notice about the skin is that there are two layers.  The thin epidermis at the top, and the thicker dermis below.  The inner layer of skin (dermis) is where we find oil glands, hair follicles, fatty layers, nerves, and sweat glands.

Notice that the sweat gland is a tubular structure tangled with capillaries (the smallest of blood vessels).  This close association of tubes allows wastes (namely water, salts & urea) to diffuse from the blood  & into the sweat gland.  And then, when body temperature rises, the fluid (sweat) is released from the gland, travels through the tube (duct), & reaches the skin surface through openings called pores.

WA-LA !  Wastes have left the body !
 


 


EXCRETORY ORGANS #4 = THE URINARY SYSTEM

Ah, the process of making pee-pee.  Not as simple as you might think:

The urinary system (pictured here) is composed of two kidneys, two tubes called ureters, one urinary bladder, and another tube called the urethra.

You probably know that the kidneys filter blood, so it's no surprise that there are a couple of blood vessels attached directly to each kidney.  The full names of these vessels are the renal artery and the renal vein.

Three of the four major metabolic wastes produced by the body are filtered from the blood by the kidneys.  Any idea which ones?  They are water, salts, & urea (the 4th, carbon dioxide, is excreted by the lungs, remember?).  These are the same three wastes that sweat glands filter & excrete.  So, yes, sweat & urine are made basically from the same ingredients (except in different concentrations).  Eeeeeeeeeeeew, yuck !

Each kidney (which can be thought of as BIG filters) are made up of thousands of tiny filtering sub-units called nephrons.  More on those in a minute .............

The ureters are simply tubes that carry urine from the kidneys to the urinary bladder.

The bladder temporarily stores urine.  And the urethra is the tube through which urine leaves the body when you go pee-pee.

 

LAYERS OF THE KIDNEY

#1 - the CORTEX.
This layer is jam-packed with lots & lots of those nephrons that I just mentioned.
We'll get to the specifics on those in just a second.
Think of the cortex as the filtering layer of the kidney.

#2 - the MEDULLA.
The middle layer.  Think of it as the collecting layer.
Tubes carrying filtered wastes travel from the cortex, through the medulla towards the pelvis.

#3 - the PELVIS.
This is the area where all of the collecting tubules come together
& connect with the ureter (which is structure #4).
The ureter transports the wastes (urine) to the urinary bladder.

 

THE NEPHRON - the structural units of the Kidney
 
The outermost layer of the kidneys, the cortex, is composed of approximately 1,250,000 structural units called nephrons.  Blood is carried to the kidneys by the renal arteries, which branch into smaller arteries inside the cortex and then lead to clusters of capillaries called  glomeruli.

Each glomerulus is surrounded by a "C"-shaped structure called the Bowman's Capsule.  It is here that materials such as urea, salts, water, glucose, & others pass from the blood into the nephron.

These materials (referred to as the "filtrate") pass through the tubule, also known as the loop of Henle.  As the filtrate travels through the tubule, useful substances are reabsorbed into the surrounding capillaries (which connect to veins that will transport the "clean" blood back to the heart via the renal vein).

About 180 liters of filtrate is produced each day, but only 1.5 liters of urine.  So as you can see, most materials that initially enter the nephron are reabsorbed, leaving only the urea, salts, & some water in the tubule.  These metabolic wastes form urine, which is transported to the urinary bladder by the collecting tubule.


 

There you have it, the Human Excretory System.
Very important in helping to maintain (my favorite word) ............... homeostasis.
If we didn't excrete our metabolic wastes they would reach toxic levels and it would be "curtains".

So next time you sweat, or pee, or exhale, be sure to say "thanks kidneys (or lungs or sweat glands) for saving my life".
Just make sure nobody hears you.


Excretion Review Questions !!!!!!!
Until my web-skills get a little fancier, you'll have to write down your answers some place.

For the first bunch of questions (#1-8), your answer is liver, lungs, skin, or kidneys.

  1. excretes salts from its surface
  2. responsible for excreting carbon dioxide
  3. has functional units called nephrons
  4. produces both urea & bile
  5. organ in which blood loses urea & reabsorbs glucose and amino acids
  6. removes old red blood cells from the circulatory system
  7. maintains homeostasis by removing body heat, water, & other wastes
  8. adds or removes glucose from the blood as necessary

 

9.     Which is the correct pathway for the elimination of urine?

a) urethra, ureter, bladder, kidney
b) kidneys, urethra, bladder, ureter
c) bladder, ureters, kidney, urethra
d) kidneys, ureters, bladder, urethra

10.    In humans, for carbon dioxide to be excreted, it must pass from the blood into:

a) nephrons
b) alveoli
c) sweat glands
d) the liver

11.     A blockage in a ureter would interfere with:

a) urine entering the kidney
b) urine entering the bladder
c) urine leaving the body
d) urea entering the kidney

12.    Which is NOT a metabolic waste in humans?

a) carbon dioxide
b) oxygen
c) salt
d) urea
e) water

13.    In which form are nitrogenous wastes excreted in humans?

a) ammonia
b) uric acid
c) urea
d) amino acids

14.    Nitrogenous wastes are the result of metabolism involving:

a) lipids
b) carboohydrates
c) inorganic compounds
d) proteins

15.    In addition to water, the principal components of urine are:

a) amino acids & fatty acids
b) urea & salts
c) ammonia & bile
d) hydrochloric acid & urea


Use the diagram to answer questions #16-24.

16. The structural unit shown in the diagram is called:

a) an alveolus
b) a nephron
c) a sweat gland
d) a ureter

17. Into which structure does the filtrate first pass?

a) 1     b) 2     c) 3     d) 5

18. In which area are materials needed by the body reabsorbed?

a) 2     b) 3     c) 4     d) 5

19.  In which area is urine collected?

a) 2     b) 3     c) 4     d) 5

20.  Which of the following does not normally enter structure 3?

a) salts
b) red blood cells
c) urea
d) water
e) glucose

21.  Which structure is the glomerulus?
22. Which structure is the loop of Henle?
23. Which structure is the collecting tubule?
24. Which structure is the Bowman's Capsule?

CHECK YOUR RESPONSES <HERE>


 


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ANSWERS TO EXCRETORY SYSTEM REVIEW QUESTIONS

For the first bunch of questions (#1-8), your answer is liver, lungs, skin, or kidneys.

1. excretes salts from its surface - SKIN
2. responsible for excreting carbon dioxide - LUNGS
3. has functional units called nephrons - KIDNEYS
4. produces both urea & bile - LIVER
5. organ in which blood loses urea & reabsorbs glucose and amino acids - KIDNEYS
6. removes old red blood cells from the circulatory system - LIVER
7. maintains homeostasis by removing body heat, water, & other wastes - SKIN
8. adds or removes glucose from the blood as necessary - LIVER

9.     Which is the correct pathway for the elimination of urine?

a) urethra, ureter, bladder, kidney
b) kidneys, urethra, bladder, ureter
c) bladder, ureters, kidney, urethra
d) kidneys, ureters, bladder, urethra

10.    In humans, for carbon dioxide to be excreted, it must pass from the blood into:

a) nephrons
b) alveoli
c) sweat glands
d) the liver

11.     A blockage in a ureter would interfere with:

a) urine entering the kidney
b) urine entering the bladder
c) urine leaving the body
d) urea entering the kidney

12.    Which is NOT a metabolic waste in humans?

a) carbon dioxide
b) oxygen
c) salt
d) urea
e) water

13.    In which form are nitrogenous wastes excreted in humans?

a) ammonia
b) uric acid
c) urea
d) amino acids

14.    Nitrogenous wastes are the result of metabolism involving:

a) lipids
b) carboohydrates
c) inorganic compounds
d) proteins

15.    In addition to water, the principal components of urine are:

a) amino acids & fatty acids
b) urea & salts
c) ammonia & bile
d) hydrochloric acid & urea

Use the diagram to answer questions #16-20.

16. The structural unit shown in the diagram is called:

a) an alveolus
b) a nephron
c) a sweat gland
d) a ureter


17. Into which structure does the filtrate first pass?

a) 1     b) 2    c) 3     d) 5

18. In which area are materials needed by the body reabsorbed?

a) 2     b) 3     c) 4     d) 5

19.  In which area is urine collected?

a) 2     b) 3    c) 4     d) 5

20.  Which of the following does not normally enter structure 3?

a) salts
b) red blood cells
c) urea
d) water
e) glucose

21. Which structure is the glomerulus? #2
22. Which structure is the loop of Henle? #5
23. Which structure is the collecting tubule? #4
24. Which structure is the Bowman's Capsule? #3

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