UNIT 4: Homeostasis
Chapter 9: Homeostasis: A Fine Balance
pg. 426 -
9.5: The Excretory System
pg. 446 - 454
Excretion in Invertebrates and Non-mammalian Vertebrates
Contractile Vacuole – is a structure in a single-celled organism that
maintains osmotic equilibrium by pumping excess fluid out of the cell.
Metanephridium – is an excretory organ in some invertebrates that is used
to reabsorb and eliminate wastes.
Malpighian tubules – is the main organ of excretion in insects, which is
used to carry wastes to the intestines.
Figure 2: A
Metanephridium of an
earthworm
Figure 3: Excretion through
Malpighian tubules in a
grasshopper
Figure 4: Saltwater birds get rid of
excess salt through salt glands in the
head.
The Human Excretory System
Nephron – is the tiny functional unit of the kidney that filters wastes from
the blood.
The functional unit of the kidney is the nephron. It regulates water balance
in the body and conducts excretion of waste metabolic waste products. The
organs of the excretory system are the kidneys (containing nephrons), ureter,
urinary bladder, and the urethra.
Figure 5: The human excretory system.
Kidneys
The role of the Kidneys: removal of wastes, balancing blood pH, and
maintaining the body’s water balance. There are two kidneys found in
mammals, one on the left and one right side of the abdomen. The mass of
one kidney is 150 g. The kidney acts as the filter for the circulatory system
therefore it receives 25% of the Cardiac output from the heart or
approximately 1.25 L/min. The renal artery delivers the blood to the kidneys,
where the blood is filtered, removing metabolic wastes, cleaning the blood,
and returning blood into the circulatory system through the renal vein.
There are three parts to the structure of the kidney, the outer level known as
the Cortex, the middle layer known as the Medulla, and the final layer, the
renal pelvis, which is hollow cavity space. The ureter is attached to the renal
pelvis and transport s blood to the bladder for storage. The urine stored in
the bladder is transported out of the body via the urethra. This occurs when
the bladder is full, approximately 300 to 400 mL.
Figure 6: Anatomy of the Kidney. Each kidney contains about 1 million nephrons.
Nephrons
Bowman’s Capsule – is a small folded structure in the human kidney that
encircles the glomerulus.
Glomerulus – is a network of capillaries within the Bowman’s capsule that
performs the first step in the filtration of blood.
Afferent Arteriole – is a vessel that supplies blood to the nephrons in the
human kidney.
Efferent Arteriole – is a vessel that carries away filtered blood from the
nephrons in the human kidney.
Peritubular Capillaries – is a net of capillaries in the nephrons that reabsorb
essential ions and minerals from the filtrate.
Proximal Convoluted Tubule – is the duct portion of a nephron that
connects the Bowman’s capsule to the loop of Henle.
Loop of Henle – is the U-shaped part of the duct that connects the proximal
convoluted tubules to the distal convoluted tubule.
Distal Convoluted Tubule – is the duct portion of a nephron that connects
the loop of Henle to the ducts that lead to the renal pelvis.
The Nephron is the functional unit of the kidney. There are approximately 1
million nephrons per kidney. The Nephrons will transverse across the Cortex
and Medulla of the kidney during the events of Excretion. When discussing
the role of the Nephron in the production of urine, the events are placed into
the regions of the Nephron.
The event of Excretion starts with Bowmen’s capsule and its interaction
with the Glomerulus. The Glomerulus is a series of capillaries surrounded
by Bowmen’s capsule. Blood enters the Glomerulus from the afferent
arteriole which originated from the renal artery. The blood contains
metabolic wastes, glucose, proteins, electrolytes, and water in its plasma.
The blood enters under high blood pressure which supports the process of
Filtration.
The blood leaves the Glomerulus through the Efferent arteriole under low
blood pressure, and enters the Peritubular capillaries where Reabsorption
will occur. The blood is then returned to the renal vein.
The waste products and water from the unfiltered blood will move from the
Glomerulus into Bowmen’s capsule, and from here move into the Proximal
convoluted tubule found in the Renal Cortex. The proximal tubule descends
into the Renal Medulla, as the blood travels to the Loop of Henle. The tube
makes a U-turn and now heads back up the Renal Cortex and is now called
the distal convoluted tubule.
The waste product that is created is now called Urine and enters the
Collecting Duct which transports urine to the region called the Renal Pelvis.
The urine is transported to the Bladder for storage through the Ureter.
Figure 7: A nephron and its blood circulation.
The Formation of Urine
The nephron is responsible for the formation of urine and the conservation
of water. The process of filtration creates a filtrate and a residue. The filtrate
is what passes through the filtering apparatus and the residue is the material
that cannot. The water, glucose, minerals, vitamins, amino acids, electrolytes,
and metabolic wastes (urea and uric acid) pass through the filter (Bowmen’s
capsule and Glomerulus apparatus), becoming the filtrate. Urine is
Hypoosmotic to the surrounding body tissues, therefore water will move
from the nephron into the body tissues to conserve water. The nephron is
responsible for conserving water and nutrients, balancing salts, and
concentrate wastes for excretion.
The formation of urine occurs in three events, Filtration, Reabsorption, and
Secretion.
Filtration occurs in the in the Bowmen’s capsule, Glomerulus apparatus.
Waste material enters the nephron for excretion.
Reabsorption occurs in the; Proximal tubule, Loop of Henle, and Distal
tubule. This is where water and nutrients are reabsorbed back into the blood.
Secretion is where material in the blood transferred back into the nephron.
Filtration
Filtration – is the process in which blood and fluid pass through a
selectively permeable membrane.
Urine formation begins in Bowmen’s capsule where the glomerulus brings a
rich blood supply for filtration under high blood pressure. Bowmen’s
capsule has a selectively permeable membrane (filter) and will only allow
water, ions, small nutrients (glucose, amino acids), and nitrogenous wastes
(urea and uric acid) to enter. The blood cells (red and white), platelets, and
plasma proteins are too large to pass through and stay in the capillaries.
The fluids and material that passes into Bowmen’s capsule is the ultrafiltrate of the blood. (passes through the filter) This process is called
Filtration.
The volume of blood that passes through a kidney per day is 1400 L.
Bowmen’s capsule removes about 180 L of fluid from the blood. The body
contains about 5 L of blood, with the plasma making up 2.75 L. (55%) This
means the kidneys filter the entire contents of blood plasma approximately
65 times a day. Only 1.5 L of filtrate is excreted as urine daily, the rest is
reabsorbed.
Figure 8: The movement of water, ions, and other molecules through the collecting
tubules and nephrons in the kidney.
Figure 9: Filtration occurs in the
Bowman’s capsule.
Reabsorption
Reabsorption – is the transfer of water, ions, and nutrients back to the
interstitial fluid via passive and active transport.
Aquaporin – is a membrane protein that passively transports water
molecules.
The filtrate that entered Bowmen’s capsule consist of urea, uric acid, water,
ions, and other molecules, and now moves into the proximal convoluted
tubule. Water, ions, and nutrients (glucose, amino acids) are returned to the
interstitial fluid by passive and active transport. Specialized protein pumps
move ions (Na+, K+, and Cl-) from the filtrate into the surround interstitial
fluid. Active transport (transport proteins) reabsorb amino acids, glucose,
and other nutrients out of the filtrate and back into the tissue fluids. All
materials will eventually enter the Peritubular capillaries.
The filtrate becomes Hypoosmotic to the interstitial fluid, since all these
materials have been removed. Therefore water follows by osmosis. More
water is removed by specialized proteins called aquaporins (water channels).
This ensures that the maximum amount of water is removed during
reabsorption.
The proximal tubule is very efficient, it allows for the reabsorption of 67%
Na+, K+, and Cl-, 65% of water, 50% of urea, and nearly all of the amino
acids, glucose and other nutrients.
Some Urea and other metabolic waste products are left behind in the
proximal convoluted tubule.
The highly concentrate waste materials and fluid will now move into the
Loop of Henle. Here additional water will be reabsorbed, which will
increase the concentration of waste material. In the upper potion of the Loop
of Henle more Na+ and Cl- are removed into the interstitial fluid, passively
and eventually actively. This process continuously reduces the volume of the
filtrate in the nephron.
The filtrate moves into the distal convoluted tubule where more water and
ions are removed. The urea and other nitrogenous wastes are unchanged.
The concentrated filtrate now moves into the collecting duct. As the fluid
and waste travels from the cortex to the medulla, even more water is
removed.
Figure 10: Reabsorption
occurs in the proximal
convoluted tubule, the loop of
Henle, and the distal
convoluted tubule.
Secretion
Secretion – is the removal of waste materials from the blood and
intercellular fluid.
During the process of urine formation, some wastes are secreted back from
the interstitial fluid back into the proximal tubule of the nephron. H+
(maintains pH); detoxified nitrogenous wastes from the liver (ammonia),
water soluble drugs (penicillin), and metabolites are secreted back into the
nephron.
By the time the urine reaches the end of the collecting duct and the renal
pelvis it is 4 X the concentration in comparison to the extracellular fluid.
The waste is transported to the bladder through the ureters.
See Table 1: Function and Roles of the Different Parts of the Nephron
and Collecting Ducts. Pg. 452
Figure 11: Secretion occurs
in the proximal and distal
convoluted tubules, and the
collecting duct.
Kidney Disease
Kidneys are responsible for maintaining water balance and
homeostasis, but they can be damage or contract diseases, and this
can impact other organs.
Urinalysis, the analysis of urine content, can be used to diagnose
various kidney ailments.
Diabetes Mellitus is caused by the insufficient secretion of insulin.
This causes blood sugar levels to rise. Excess sugar remains in the
nephron, causing a reverse of the osmotic pressure. Water is
retained in the nephron, and urination is increased.
Kidney Stones are caused by the buildup of mineral solutes,
oxalates, phosphates, and carbonates. When these solutes combine
with calcium they form crystals and eventually form stones. The
stones are lodged in the renal pelvis or ureters and can be very
painful. Ultra sound (high energy sound waves) is usually used to
break up these stones so they can be passed. Surgery can also be
used if the extracorporeal shock wave lithotripsy (ESWL) does not
work.
If a person is near total loss of kidney function they will be place
onto a dialysis machine (artificial kidney) which mechanically
filters their blood. Total loss of kidney function, the individual will
require a kidney transplant to survive.
Chapter 9: Summary
pg. 458
Chapter 9: Self-Quiz
pg. 459
Chapter 9: Review
pg. 460 - 65