Download EXCRETION Excretion- the process of getting rid

EXCRETION
Excretion- the process of getting rid of wastes after metabolism and of
removing excess heat to keep body temperature constant.
Sometimes the organs that get rid of wastes have a double function
(i.e. vacuoles get rid of wastes and also act as osmotic regulators in
balancing the cell’s contents to restore homeostasis).
Metabolism- all processes of the body working together.
Homeostasis- the ability of an organism to keep its processes the same
despite environmental changes.
The watery environment that a cell finds itself in will also
determine its method of getting rid of metabolic wastes.
Some waste products of metabolism that are removed by excretion
are:
- nitrogen compounds (i.e. ammonia, urea and uric acid) from the break
down of proteins and amino acids
- excess water & carbon dioxide from cellular respiration
- mineral salts built up during metabolism
All of these wastes are poisonous in high concentrations.
Land Environments
Since animals live on dry land, they cannot diffuse dissolved
ammonia into the air; it has to be changed into a chemical that can build
up in cells and then be transported safely through the blood. Since
these organisms live in a dry land environment, they cannot afford to
lose a great deal of water. The excretion of urine helps maintain
balance.
Filtration and Excretion in Humans
Like most complex organisms, humans have a filtering system to
filter out nitrogenous wastes and maintain osmotic regulation.
Excretory organs in humans are:
- lungs
- liver
- kidneys
- skin
Lungs
- get rid of CO2 and H20 (in form of H20 vapor) which are end-products
of aerobic cellular respiration
The Skin
The skin is also an excretory organ. It secretes water, salts and
some urea (through perspiration). This is important in regulating body
temperature.
The skin has two layers:
- an epidermis for protection
- the dermis which is the thick, active layer. The dermis contains
tough, fibrous connective tissue, blood and lymph vessels, nerves, sweat
and oil glands and follicles.
Sebaceous glands produce oily secretions which provide protective
coating to skin and hair and keep them soft and pliable.
Sweat glands absorb water, urea and minerals from the blood and
release them through pores in the skin.
Liver
-detoxifies blood by removing harmful substances such as bacteria,
drugs and hormones; these less poisonous substances formed are
returned to bloodstream and excreted by kidneys. Excessive harmful
materials can lead to cirrhosis which eventually causes liver to stop
functioning and death will occur.
- excretes bile which has some ingredients which are metabolic wastes;
failure to excrete bile properly leads to jaundice (yellowed skin because
its metabolic wastes are reabsorbed into blood).
- breaks down excess amino acids which cannot be stored in the body;
breaks them into ammonia which is very poisonous so it is changed into
urea which diffuses from liver into bloodstream and then onto kidneys
where it is filtered and then released in urine.
Kidneys
The two kidneys are the main organs of excretion and they also
maintain osmotic regulation by regulating the concentrations of
substances found in body fluids. If they did not perform these two
functions, a person would die.
Kidneys are as big as your fist; bean-shaped; and they lie on either
side of spine in lower back. Each kidney has an outer part known as
cortex, an inner part known as the pelvis (hollow; urine accumulates
here before going down ureters) and a central cavity called the medulla.
Nephron Filtration Centers
Inside each kidney (cortex & medulla) are ~1 million filtering units
called nephrons. Each nephron is made up (See Fig. 11.19 p. 375):
- a cup-shaped Bowman’s Capsule which narrows into a long
coiled tube
Nephric filtrate- fluid in Bowman’s capsule
In the center of each capsule is a mass of capillaries called the
glomerulus. The glomerulus forms from a small artery running
from one of two renal arteries. The renal artery takes
oxygenated blood to the kidneys. Another branch of renal
artery forms capillaries that branch out around the tubule.
Small networks of veins lead from the capillaries and end up in
one of two renal veins which take purified blood from the
kidneys back into general circulation & eventually back to heart.
- Proximal tubule- tube b/w Bowman’s capsule & loop of
Henle
- Loop of Henle- in medulla; removes water from filtrate by
osmosis; descending & ascending parts
- Distal tubule- tube that connects the loop of Henle to the
collecting duct; responsible for tubular secretion
- Collecting duct- carries urine into renal pelvis; 99% of H2O
that entered proximal tubule as nephric filtrate has been
returned to body as has glucose & amino acids; 1% lost is
returned by diet
The Filtering Process…Main function of kidneys
Blood enters the kidneys containing urea and excess salt. At any
time, 20% of blood is in kidneys. When proteins are broken down,
ammonia is released and converted into urea by the liver. See figure
11.18 p. 375. This is essential because urea is less toxic and requires
less water for excretion.
From the renal artery, blood passes into smaller arteries that lead to
the glomerulus in the Bowman’s capsule of the nephron unit. Because
of blood pressure, H2O and its dissolved materials are forced into
Bowman’s capsule while proteins, red and white blood cells, and
platelets stay in the blood in the capillaries. The continuous flow of
blood forces the H2O and dissolved particles from the Bowman’s
capsule into the proximal tubule.
Much of the re-absorption of needed materials takes place along
the proximal tubule which uses active transport to take needed
materials back to the capillaries that surround the tubules.
From here, filtrate moves to loop of Henle whose main function is
to remove H2O from filtrate. In the descending loop, H2O travels
through these special cells back to the capillaries via osmosis b/c of
increased Na+ conc. in medulla cells. At bottom of descending loop,
loss of H2O results in increased Na+ in filtrate and as filtrate moves up
ascending loop, Na+ is pulled into surrounding cells and Cl- follows.
H2O can’t reenter here b/c ascending loop is impermeable to it.
Basically in the tubules & loop of Henle, the blood is cleansed of salt and
urea and it has reabsorbed any needed materials and H2O. The capillaries turn
into veins and take the materials and H2O back into the renal vein to join the
general circulation and eventually get back to the heart. The materials left in the
tubules are excess H2O, salts and urea.
In the distal tubule, tubular secretion involves active transport to
pull H+ ions, creatinine & drugs out of blood and into filtrate. Fluid
from many distal tubules then enters common collecting duct which
carries urine into renal pelvis.
The water that remains with the urea and excess salt is called
urine. The collecting ducts lead from the outer layers of the kidneys to
the central cavity called the renal pelvis. A tube called the ureter
drains the central cavity of each kidney and leads to the urinary
bladder. When the bladder fills (max ~600 ml), muscular rings open (1
involuntary, one voluntary) and urine enters the urethra for excretion
from the body. Figure 11.17 p. 374
♂ urethra 20 cm long & merges with vas deferens to form single
urogenital opening
♀ urethra 4 cm long; separate reproductive and urinary openings.
When kidneys do not function, very serious, life-threatening
situations can occur. The kidney can be damaged by bacteria or
chemicals like mercury or lead. Waste accumulation in tissues (uremia)
can lead to poisoning, starvation and suffocation. Tissue filled with
waste cannot absorb food or oxygen. Symptoms of lack of waste
removal are fever, convulsions, coma and death. When the kidneys do
not filter the blood (renal failure), the person must go on hemodialysis
(kidney dialysis) where the blood from an artery is transferred to a
machine to be cleaned of waste products by osmosis & diffusion and
then transferred back to the person’s vein. Dialysis has limitations.
Renal failure can be caused by a number of conditions and it can
lead to many serious problems:
- abnormal conc. of salt & H20
- altered pH
- general deterioration of homeostasis.
Ideally, dialysis is a temporary measure used to replace normal
functioning kidney(s) until kidney(s) can function again on own or, in
more serious cases, a transplant becomes available.
Other species as potential donors? Read p. 380 (pigs)…implications?
Urine Output
Osmotic balance and the balance of salt are very important in
addition to getting rid of urea. The amount of water lost is important
and has to be controlled. For example, if you were ill or sweating from
heavy exercise, you might have to retain more water than usual. How is
this controlled?
The distal tubule and collecting duct permeability is controlled by
the hormone ADH (anti-diuretic hormone) which is secreted by the
pituitary gland. It allows more H20 to be removed from nephric filtrate
when body must conserve H20. When body has excess H20, ADH is
inhibited and more H20 is excreted in urine. Alcohol & caffeine block
ADH release and lead to higher volumes of urine being excreted.
Diabetes also results in high volumes of sugary urine (high blood sugar
overloads proximal tubule so glucose remains in filtrate and results in
H20 retention).
Other hormones control salt content and pH in the blood; red blood
cell production and vitamin D production in the skin.
Disorders of Excretory System
Kidney stones- form when chemicals precipitate out of urine & form
crystals; most formed from calcium oxalate crystals, others uric acid &
cystine crystals; more common in men; usually reoccur.
Causes:
- recurrent urinary tract infections (UTIs)
- not drinking enough H20
- inactive lifestyle
- too much vitamin C & D
Treatments:
- depends on size of stone: many pass on own; medications to
break down crystals; lithotripsy (ultrasound shock waves
break up stone); surgery for larger ones
- drink more H20
- change diet
- follow medical advice
Symptoms:
- severe pain in small of back or abdomen
- blood in urine
- nausea, vomiting
Diagnosis:
- complete exam & X-rays; blood & urine tests
Urinary tract infections (UTIs)
-more common in women (anus & urethral opening so close) so wipe
front to back to prevent infection); potentially serious; can permanently
damage kidneys or cause renal failure
- if bladder infected, cystitis
- if urethra infected, urethritis
-if kidney infected, pyelonephritis
Symptoms:
-painful urination (burning)
-frequent need to urinate, but no urine present
-brown or bloody urine
Sometimes:
-tender upper abdomen/lower back
-nausea, chills, fever, vomiting
Diagnosis:
-urine test
Treatment:
-antibiotics
-surgery if serious
Prevention:
-drink lots of H20
-wipe front to back
-maintain good personal hygiene