Among the vertebrates
the main organ that performs the function of excretion is the kidney. Kidney
can function with modification in fresh water, in the sea and oh land. The
ancestors of vertebrates, the invertebrate chordate have segmental-shaped
arranged excretory structures throughout the body like the metanephridia in
earthworm. The primitive vertebrate hag fishes have kidney with segmental-form
arranged tubules. Kidneys contain numerous tubules not arranged segmental.
Excretion
in man
Excretion in Man
Metabolic
Wastes
Metabolic wastes are
the substances produced within the body by various chemical processes. The
wastes of the digestive process are removed i.e. eliminated. The only true
excretory products that are removed from the body during elimination are bile
pigment and salts of certain minerals. Bile pigments are called bilirubin
produced by breakdown of old worn out red blood cells and metabolites of
various hormones. Nitrogen containing molecules or nitrogenous wastes includes
urea, produced from deamination of amino acids. Uric acid is formed from the
breakdown of nucleic acids. Creatinine is derived from a nitrogen containing
molecule called creatine in the muscle cells.
Toxin substances such
as pesticides, drugs, food additives ingested into the body, and carbon dioxide
and water vapor produced during respiration are also metabolic wastes. The
waste present in the body cause serious hazards, thus are eliminated by
excretory system.
Excretory Organs
Kidneys, lungs, skin
and liver are the structures for the elimination of metabolic waste products.
Urea
Urea is the primary
nitrogenous waste product of humans and other living mammals. The advantages of
using urea as a nitrogenous waste product are:
(i)
Non-toxic: It can therefore be carried round the body in the
blood from the liver until it is removed by the kidneys.
(ii)
Very soluble: It does not require a great deal of
water to get rid of it and it is easily transported.
(iii)
A small molecule: It is easily filtered in the kidneys.
Ammonia is converted into urea in the liver by a cyclic reaction known as
ornithine cycle.
In ornithine cycle, two
molecules of ammonia and one molecule of carbon dioxide are used. One ammonia
molecule combines with CO2 and already available precursor from previous cycle
ornithine to form citrulline, subsequently ammonia combines to form arginine.
One molecule of water is made (two made, one used). The arginine is split by
arginase to form one molecule of urea. Ornithine is regenerated ready for the
next cycle.
Urea is transported in
the blood plasma from the liver to the kidneys. The metabolic pathways involved
in the production of urea are called urea cycle.
Liver as a Homeostatic
Organ
The major homeostatic
roles of liver are:
(a)
Carbohydrate metabolism: Inside the liver cells the glucose
is either built up into glycogen for storage or broken down into CO2 and water
with the release of energy. Considerable excess of glucose is converted into
lipids.
(b)
Fat metabolism:
(1)
Liver converts excess of carbohydrates to fats.
(2)
Liver removes excess of cholesterol from the blood and breaking it down or when
necessary synthesizing it. If glucose is in short supply, the liver cells
breakdown fats into fatty acids and glycerol for respiration.
(c)
Detoxification: Detoxification means the removal of
toxins or poisons. Detoxification is part of homeostasis and helps to maintain
the composition of blood in a steady state. The liver cells detoxify drugs,
poisons, chemicals, additives, pesticides, alcohol, nicotine etc. by absorbing
them and then changing them chemically. The major toxic substance is ammonia
which is converted into urea.
(d)
Deamination and urea formation: Excess of amino acids,
brought to the liver through the hepatic portal vein, is deaminated by the
liver cells. The amino group is converted to ammonia by the specific enzymes in
the liver cells. Ammonia enters ornithine cycle in which it reacts with CO2 to
form urea. The urea is then shed from the liver cells into the blood stream,
and taken to the kidney which eliminates it from the body through urine.
(e)
Storage of vitamins: The main vitamins stored in the liver
are fat soluble vitamins A, D, E, and K. The liver also stores water soluble
vitamin B and C especially those of the B group such as nicotinic acid, vitamin
B12 and folic acid.
(f)
Breakdown of red blood cells: Red blood cells have a
life span of 120 days. They are then broken-down by phagocytic macrophage
(special WBCs) cells in the liver, spleen and bone marrow. Inside the
macrophages hemoglobin is broken down into heme and globin. Globin is broken
down into amino acids. The iron is removed from heme. The remaining part of the
molecule is converted to bilirubin, which is yellow and a component of bile.
The accumulation of bilirubin in the blood causes jaundice. The iron may be
then reused by the cells in the bone marrow to make more hemoglobin, or iron
may be stored in the liver. The yellow pigment found in urine called urochrome,
is also derived from the breakdown of heme, but this pigment is deposited in
the blood and subsequently excreted by the kidneys.
Urinary System in Man
There is a pair of
kidneys just above the hipbone, at the back of the abdominal cavity, one on
each side of the spine against the abdominal wall. The left kidney lies
slightly above the right. The kidneys are red brown in color and bean shaped.
The kidneys receive blood from the aorta via renal arteries and the renal veins
return blood to the inferior vena cava. Urine formed in the kidneys passes by a
pair of ureter to the bladder where it is stored until it is released via the
urethra. Two sphincter muscles surround the urethra where it leaves the
bladder, one of which is under voluntary controls. The process of control
release of urine is known as urination.
Excretory system in man |
Internal Structure of
Kidney
A sagittal section of
the kidneys shows two distinct regions, an outer darker the cortex and an inner
lighter region the medulla. The cortex is covered by tough transparent membrane
the capsule. The cortex and medulla of each kidney has nephrons. The medulla is
composed of tubular part of the nephron and blood vessels, which together form
the renal pyramids. All the pyramids project into the pelvis which leads into
the ureter.
Gross
anatomy of kidney
Nephron:
The nephron is the structural and functional unit of kidney. There are about
one million nephrons in each kidney. There are two type of nephrons (a)
cortical nephron (b) juxtamedullary nephron.
Cortical
nephron: The nephrons arranged along the cortex are known as
cortical nephron, have relatively short Loop Of Henle. Juxtamedullary nephron
(juxta means, close): These nephrons have their renal (glomerular) capsule
close to the junction of the cortex and medulla.
They have long loop of
Henle which extend deep into the medulla. The two types of nephrons have
different uses. Under normal conditions of water availability the cortical
nephrons deal with the control of blood volume, whereas, when water is in short
supply, increased water retention occurs, through the juxtamedullary nephrons.
Structure
of Nephron
The nephron consists of
a long tubule. It is coiled. It is closed at one end and open at the other. At
the close end of the tubule in the cortex, the wall of the nephron is expanded
and folded into a double walled cup-shaped chamber, the Bowman’s capsule,
within the enfolded portion of the Bowman’s capsule is a network of capillaries
the glomerulus (pl: glomeruli). The tubule consists of:
(a) Proximal tubule
(b) Loop of Henle
(c) Distal tubule
(d) Collecting duct.
Structure Of Nehphron |
Proximal
Tubule: It begins from the Bowman’s capsule. It is highly
coiled. This part of the nephron is called proximal tubule. It is present in
the cortex.
Loop
of Henle: The tubule then plunge down into the medulla, forms
a sharp loop, then again straighten up and return to the region of Bowman’s
capsule. It is thin walled region of the nephron.
Distal
tubule: At the end of the loop, the nephron becomes highly
coiled. This part of the nephron is the distal tubule.
Collecting
duct: The nephron finally straightens out and merges into
a collecting duct. Collecting ducts join with even larger ducts, which
eventually empty into the renal pelvis.
Peritubular
Capillaries: Blood travels from the renal artery,
into afferent arterioles, then into the glomerular capillaries. These
capillaries do not merge into veins. Rather, they merge into efferent
arterioles (so-called because it conducts blood away from the glomerulus) which
divide into a second set of blood capillaries, known as Peritubular
capillaries. In nephrons that are positioned almost entirely in the cortex,
peritubular capillaries thread profusely around proximal tubule, the loop of
Henle, and the distal tubule. From there, the peritubular capillaries merge
into renal veins that lead away from the kidney. In juxtamedullary nephrons
additional capillaries extend down to form a loop of vessels called vasa recta.
Functioning
Of Kidney
The function of the
kidney is to form urine. It involves three processes in the nephrons:
glomerulus filtration, tubular re-absorption and tubular secretion.
Glomerular
filtration: In filtration all compounds of the
blood except blood cells and most proteins can move into the glomerulus. From
there the filtrate passes into the nephron tubule. Active transportation is not
involved. Hydrostatic pressure in the glomerulus created by the heart
contractions is greater in the capillaries than the fluid tissues pressure in
the tubule. The pressure drives out a certain amount of fluid and solutes from
the blood. Thus the relatively high pressure difference across the glomerulus
is the reason of filter of fluid into the tubule of the nephron. The glomerular
filtrate contains sugar, amino acids, salts, nitrogenous wastes specially urea
and other dissolved substances.
Tubular
re-absorption: During re-absorption, about 90% of the
water and most of the solutes that enter the nephron are returned to the
bloodstream. As the filtrate flows along the nephron, selective re-absorption
rapidly alters its composition. For instance, sodium ions are actively
transported across the proximal tubule walls and into interstitial fluid.
Chloride ions being negatively charged are attracted by the positively charged
sodium ions and passively follow them in the same direction. On the average 75%
of the Na and Cl ions present in the filtrate leaves the proximal tubule. The
outward solute movement sets up an osmotic gradient and water passively move
out. The solutes and water flow into capillaries surrounding the tubule. Thus,
although re-absorption into capillaries is bulk flow process, water is
reabsorbed only because solutes are reabsorbed first. In loop of Henle H2O and
Na is reabsorbed.
Tubular
secretion: Some substances appear in the urine in greater
amounts than were filtered into the nephron. They are actively transported from
the peritubular capillaries into the nephron tubule, an event known as tubular
secretion. These substances include K and H ions, drugs and some toxic
substances. These include many metabolic wastes, such as uric acid, creatinine
and assorted products of hemoglobin breakdown.
Filtration, re-absorption and secretion in a nephron |
Hormonal
Control of Kidney Function
Antidiuretic
hormone (ADH): (Diueresis, increased excretion of
urine). It is also called vasopressin. When ADH is released from the posterior
pituitary, it is picked up by the blood stream and transported to the kidneys.
ADH acts on cells of the distal tubules and collecting ducts making them more
permeable to water. With increased ADH secretion, more water is reabsorbed. As
a result, urine volume decreases, and its salt concentration increases and
fluid volume leaving the body is more diluted.
Aldosterone:
It
is a hormone produced and secreted by the adrenal glands and has major
influence over sodium re-absorption. The kidneys themselves help to control
aldosterone secretion.
Counter Current Mechanism
It is in the interest
of terrestrial animals to conserve water. In mammals, this function depends on
loop of Henle. Mammals are the only vertebrates which can produce markedly
hypertonic urine. The loop of Henle concentrates sodium chloride in the medulla
of the kidney. This then causes osmotic flow of water from the collecting ducts
there by concentrating the urine and making it hypertonic to blood. In
achieving this loop of Henle follows the principle of countercurrent
multiplier.
Counter Current Mechanism |
As the renal fluid
flows along the ascending limb, salt is actively removed from it and deposited
in the surrounding tissue fluid. From there the salt diffuses into, and
equilibrates with, the fluid in the descending limb. This active transfer of
salt takes place at all levels of the loop of Henle. At any given level the
effect of this is to raise the concentration in the descending limb above that
in the ascending limb. The effect at any one level is slight, but the overall
result is multiplied by the length of the loop. As the renal fluid flows down
the descending limb towards the apex of the loop it becomes more and more
diluted. As the renal fluid passes down the collecting duct water passes out of
it by osmosis. This raises its solute concentration. Meanwhile, the water is
taken away in the blood stream. The characteristics U- shape of loop of Henle
provides a counter current system which establishes and maintains this high
concentration.
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