Download Excretory System. The kidneys regulate the amount of water, salts

Excretory System.
The kidneys regulate the amount of water, salts and other substances in the blood. The kidneys
are fist-sized, bean shaped structures that remove nitrogenous wastes (urine) and excess salts
from the blood. The ureters are tubes that carry urine from the pelvis of the kidneys to the
urinary bladder. The urinary bladder temporarily stores urine until it is released from the body.
The urethra is the tube that carries urine from the urinary bladder to the outside of the body. The
outer end of the urethra is controlled by a circular muscle called a sphincter.
Each kidney is composed of three sections: the outer (renal) cortex, the (renal) medulla (middle
part) and the hollow inner (renal) pelvis. The cortex is where the blood is filtered. The medulla
contains thecollecting ducts which carry filtrate (filtered substances) to the pelvis. The pelvis is a
hollow cavity where urine accumulates and drains into the ureter.
How blood is Filtered: The filtering units of the kidneys are the nephrons. There are
approximately one million nephrons in each kidney. The nephrons are located within the cortex
and medulla of each kidney. The tubes of the nephron are surrounded by cells and a network of
blood vessels spreads throughout the tissue. Therefore, material that leaves the nephron enters
the surrounding cells and returns to the bloodstream by a network of vessels. Each nephron
consists of the following parts: 1) glomerulus ; 2) Bowman’s capsule ; 3) proximal tubule ; 4)
loop of Henle ; 5) distal tubule ; 6) collecting duct.
NOTE : The glomerulus is a mass of thin-walled capillaries. The Bowman’s capsule is a
doublewalled, cup-shaped structure. The proximal tubule leads from the Bowman’s capsule to
the Loop of Henle. The loop of Henle is a long loop which extends into the medulla. The distal
tubule connects the loop of Henle to the collecting duct.
~ See Figure 11.19; p. 375.
A nephron is composed of the Bowman’s capsule, the proximal tubule, the loop of Henle, the
distal tubule and the collecting duct. Kidney Action :
Blood enters Bowman’s capsule through a tiny artery — (the renal artery). The artery branches
to form a glomerulus. Blood pressure forces some blood plasma and small particles into the
surrounding capsule — (this is called the nephric filtrate). Large particles such as blood cells
and proteins remain in capillaries. The nephric filtrate is pushed out of the capsule and into the
proximal tubule . This is where reabsorption begins. Only materials needed by the body are
returned to bloodstream — for example, 99 % of water, all glucose and amino acids and many
salts are reabsorbed. Osmosis , diffusion , and active transport draw water , glucose , amino acids
and ions from filtrate into surrounding cells. Small villi like projections help in the active
transport of glucose out of the filtrate and speed up the reabsorption process. From here, these
components return to the bloodstream.
The filtrate reaches the end of the proximal tubule and the fluid is isotonic with the surrounding
cells. Glucose and amino acids have been removed from the filtrate. The filtrate then moves to
the loop of Henle whose primary function is to remove water from the filtrate by osmosis. There
is in increase in sodium concentration in the cells of the loop of Henle as we move from the area
of the cortex to the inner pelvis of the kidney. This causes water to be drawn from the filtrate.
These high levels of sodium are a result of active transport and results in increased concentration
of filtrate (due to water loss).
Why doesn’t water on outside go back in? The water that left the descending loop cannot enter
the ascending loop because it is impermeable to water. As the water concentration in the filtrate
decreases, the chlorine concentration increases and causes it to diffuse out of the tubule. This
process is helped by the electrical attraction of chlorine to sodium. The filtrate moves to the
distal tubule where tubular secretion occurs. Active transport is used to pull hydrogen ions,
creatinine, drugs such as penicillin out of the blood and into the filtrate. Fluid from a
number of nephrons moves from the distal tubules to a common collecting duct which carries
what can now be called urine to the renal pelvis.
NOTE : Since the kidneys control what leaves and what remains in the nephrons, they
maintain the levels of water, ions and other materials nearly constant and within the limits to
maintain homeostasis.
Function of the Kidney :
The principal function of the kidney is to filter blood in order to remove cellular waste (urea,
uric acid and creatine) products from the body. At any given time, 20 % of blood is in the
kidneys. Humans can function with one kidney. If one ceases to work, the other increases in size
to handle the workload. The kidney has other functions but it is usually associated with the
excretion of cellular waste such as : 1) urea (a nitrogenous waste produced in the liver from the
breakdown of protein. It is the main component of urine) ; 2) uric acid (usually produced from
breakdown of DNA or RNA) and 3) creatinine (waste product of muscle action). All of these
compounds have nitrogen as a major component. The kidneys are more than excretory organs.
They are one of the major homeostatic organs of the body. They control water pH,
secrete erythropoietin (a hormone that stimulates red blood cell production) and activate vitamin
D production in the skin. That is why a doctor can tell so much from a urine sample.
Disorders of the Excretory System :
Our excretory system is very important in maintaining homeostasis. There are certain conditions
which can affect the excretory system. We will discuss some of these common disorders.
1. UTI (Urinary Tract Infection) is a very common disorder. If the bladder has become
infected, it is known as cystitis. If the urethra is infected., it is called urethritis. UTI is more
common in women than men because of differences in anatomy. Symptoms include painful
urination burning sensation), frequent urination (even if no urine present) and bloody or brown
urine. This can lead to chills, fever, nausea, vomiting and upper abdomen tenderness.
2. Kidney Infections result when an infection reaches the kidneys and becomes known as
pyelonephritis. Common causes can be infection from elsewhere in the body or obstruction of
the prostate gland (usually in older men). For children, infection can be caused by the tube that
drains urine from the kidneys and the bladder. NOTE : If left untreated, all UTI’s can lead to
permanent kidney damage and possible kidney failure. The general treatment is by antibiotics. A
person needs to maintain good personal hygiene when eliminating wastes (liquid and solid
forms) from the body. Also, a person should drink lots of water.
3. Kidney Stones are also a common kidney disorder. They form when chemicals in the urine
precipitate out and form crystals. The most common crystals are from calcium oxalate, while
others could be from uric acid and cystine. Kidney stones are more common in men than women
and can reoccur at any time. Factors such as recurrent urinary bladder infections, insufficient
water intake and consumption, low levels of physical activity, and too much Vitamin C and D
intake can lead to kidney stones. One of the best ways to decrease the occurrence of stones is to
drink lots of water and to change your dietary habits. Symptoms include severe back or abdomen
pain, blood in the urine, nausea and vomiting. Diagnosis involves a complete medical
examination, including X-rays. Treatment may vary from letting the stones pass through the
urinary tract to ultrasound shock (or lithotripsy) to disintegrate the stones to a small size that can
be passed through the urinary tract. Real large stones require surgery for removal.
Artificial Kidney Dialysis :
(Willem J. Kolff was the inventor of the artificial kidney dialysis machine. Dr. Gordon
Murray was the first North American scientist to develop and use kidney dialysis).
When the kidneys do not function properly, a person is said to be in renal failure and dialysis
must be performed artificially. Without this artificial kidney dialysis, toxic wastes build up in the
blood and tissues, and cannot be filtered out by the ailing kidneys. This condition is known as
uremia, which means literally "urine in the blood." Eventually this waste build-up leads to death.
Dialysis is a procedure that cleans and filters the blood when the kidneys are not functioning
properly. It is a treatment for people in the later stage of chronic renal insufficiency (kidney
failure). Sometimes dialysis is a temporary treatment. However, when the loss of kidney function
is permanent (as in endstage kidney failure), the patient must continue to have dialysis on a
regular basis. This treatment cleans the blood and removes wastes and excess water from the
body. Dialysis keeps a person with kidney failure alive, without it people with this condition
would die. The only other treatment for kidney failure is a kidney transplant.
There are two types of dialysis: hemodialysis and peritoneal dialysis. In hemodialysis, the
blood is removed from an artery and passed through an artificial kidney machine where it is
purified and vital substances added before it enters back into a vein in the patient’s body .
Peritoneal dialysis uses a filtration process similar to hemodialysis, but the blood is filtered
inside the body rather than in a machine. Both types of dialysis require surgery to prepare the
person’s body. Hemodialysis or kidney dialysis means "cleaning the blood", and works on the
principles of diffusion. Blood is circulated through a machine which contains a dialyzer (also
called an artificial kidney) The dialyzer has two spaces separated by a thin membrane. Blood
passes on one side of the membrane and dialysis fluid (dialysate) passes on the other side. The
wastes and excess water pass from the blood pass through the membrane into the dialysis fluid
which is then discarded. The cleaned blood is returned to the patient’s bloodstream.
Peritoneal Dialysis occurs inside of the abdomen in the peritoneal cavity that is lined with a thin
membrane called the peritoneum. This membrane surrounds the intestines and other internal
organs. Peritoneal dialysis is also called continuous peritoneal dialysis (CPD). In CPD, the
patient always has dialysis fluid in the peritoneal cavity, so the blood is constantly being cleaned.
The fluid is changed at regular intervals throughout the day using a catheter (usually made of
soft, nonirritating plastic inserted in the patient’s abdomen below the navel). Excess water and
wastes pass from the blood through the peritoneum into the dialysis fluid. This fluid is then
drained from the body and discarded. In most cases this treatment can be performed, at home or
at work without assistance. Dialysis is only a temporary solution. The best solution is for a
kidney transplant. Once a person is deemed suitable for a transplant, a donor must be found. The
best situation is for the donor kidney to come from a living family member as the likelihood of a
genetic match is much greater and therefore the likelihood of rejection is considerably less.
There are two types of kidney transplants: a living donor transplant and a cadaveric
transplant. In a living donor transplant, a kidney from a donor, usually a blood relative, is
transplanted into the patients body. The donor's blood group and tissue type must be compatible
with the patients, and extensive medical tests are done to determine the health of the donor. In a
cadaveric transplant, a healthy kidney from someone who has died suddenly is transplanted into
the patients body. Before a cadaveric donor's organs can be transplanted, a series of medical tests
have to be done to determine if the organs are healthy. Also, the family of the donor must give its
consent for the organ donation. The recipient of the new organ needs to take drugs to suppress
their immune system and prevent their body from rejecting the new kidney. Kidney
transplantation usually has very positive results for its recipients. They live longer, healthier lives
than is possible with dialysis treatments. Taking all of the necessary precautions and having a
healthy donor kidney may lead to a successful transplant, but complications are possible. There
is a greater short-term risk of death associated with the surgery, however, this risk is outweighed
by the long-term effects of continuous dialysis.