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Excretory System
Urinary System
 Excretion is the removal or elimination or metabolic wastes from the body.
 The important metabolic wastes are the following:
1. Respiratory wastes: Co2 and H2o are by products by exhalation
(lungs).
Excess H2o is eliminated inform of urine (Urinary System) & sweat
(skin).
2. Nitrogenous wastes: are formed by deamination of amino acid,
breakdown of nucleic acid etc… it includes ammonia (NH3), uric acid
etc…
 The body's internal environment consists of all fluids that are not inside
the culls – that is blood and the interstitial fluid. Together, is the body's
extra – cellular fluid (ECF).
 The Urinary System is an organ system that helps keep the volume and
composition o (ECF) within to tolerable range. It does this by balancing the
daily losses of water and solutes with daily intare.
 The Urinary System works wit the other system of the body to help
maintain hemostasis.
Function of Excretion:
1- Elimination of excess water and excess harmful solutes from internal
environment (waste product).
2- Removal of excess it ion (acid – base balance )
3- Maintaining body temperature.
Water gains & Losses
The body gain water through:-Absorption of water from liquid and solid food in the gut
-Metabolism breakdown of organic molecules giving water.
The body lasses water through:-Evaporation form lungs and skin.
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-Sweating, by mammals that sweat.
-Elimination, in faces.
Solute gains & Losses
Mammals gain solutes mainly by:-Absorption from the gut.
-Secretion from cells.
-Metabolism.
-Respiration.
Mammals typically loss solutes by:-Urinary System.
-Respiration.
-Sweating, by some species.
URINARY SYSTEM OF MAMALS
ex: human.
It consists of:
 A pair of kidneys that filter water, mineral ions, organic wastes &
other substances from the blood, forming a fluid called urine.
 Tow ureters (tubular channels to transport urine).
 One urinary bladder (a muscular sac which stores the urine).
 One urethra (a muscular tube which passes urine to outside).
Kidney structure:
Each kidney composed of:
 An outer capsule (renal capsule).
 Cortex (outer functional layer).
 Medulla (inner functional layer).
 Internally, each kidney divided into several lobes.
 Each lobe contain blood vessels and numerous slender tubes known as
nephrons (the functional unit of the kidney), which continues with
the tube like collecting duct then open in kidney's central cavity
(renal pelvis).
 Each human kidney has more than a million nephrons extend from the
kidney cortex down through the medulla.
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Component of Nephron
1. Glomerulus (malpigian corpuscles):
-Is the first portion of the nephron where water and solutes are
filtered from the blood (blood filtering unit). It composed of: A cup-shaped portion (Bowman's capsule) that receives water and
solutes being filtered this blood.
 Glomerulus capillaries: Set of blood capillaries inside the Bowman's
capsule (highly permeable to water and solutes.
2. Proximal convoluted tubule:
-Is the tubular portion (closest to the capsule) into which water and
solute enters after being filtered from of the blood.
3. Loop of Henle:
-The hairpin-shaped, that functions in the re-absorption of water and
solutes.
-It has two limbs discending and ascending.
-It located in medulla layer.
4. Distal convoluted tubule:
-Is the tubular portion of a nephron most distant from the capsule, it
is a region of water and sodium re absorption.
-It ends as a collecting duct, which is a part of a duct system leading
into kidney's central cavity and then into ureter.
Each nephron interacts closely with two set of blood capillaries:
 Glomerulus capillaries.
 Peritubular capillaries: is a set of blood capillaries around the tubular
part of the nephron. It functions in re-absorption of water and solutes
back into body and in secretion of some substances.
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URINE FORMATION
 Urine is a fluid that ribs the body of water and solutes that are in excess
of the amounts required to maintain extra cellular fluid.
 Flow of urine (urine action) from the bladder is a reflex action.
*When the bladder is full, the sphincter around its neck opens,
smooth muscle in its balloon-like wall contracts, and also urine is
forced out through the urethra. Skeletal muscle surrounds the
urethra. It is contraction, which is under voluntary control,
prevents urination.*
Urine formed in kidneys (nephron) by three processes:
1. Filtration.
2. Re-absorption.
3. Secretion.
Filtration:
 Blood pressure generated by heart's contractions derives filtration, which takes
place at the Glomerulus.
 Pressure filters blood by forcing water and all solutes except proteins
out from the glomerular capillaries. The protein-free filtrate moves out
from the cupped part of the nephron, into the proximal tubule.
Tubular reabsorption:
 Proceeds along the nephron's tubular regions. Most of filtrate's water
and solutes move out from the space enclosed by the nephron tubular
capillaries. This by diffusion or active transport.
 Maximum amount of re-absorption take place in proximal tubule.
Tubular secretion:
 Proceeds at the tubular wall but in opposite direction of reabsorption.
 Cell making up the wall accept solutes from the peritubular capillaries,
then secrete them into nephron's lumen.
+
+
 The main solutes are &potassium ions (H& K).
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 The process also prevents certain metabolites (such as uric acid) & foreign
substances (such as drugs) from accumulating in blood.
Factors Influencing Filtration
1-Blood pressure is high in glomerular capillaries. The arterioles delivering blood to
glomerulus have a wider diameter and less resistance to flow than those carrying
blood away.
2-Glomerular capillaries are highly permeable (10 to 100 times more permeable to
water & small solutes) than others capillaries, so the kidney can filter about 180
liters per day.
3-The neural, endocrine& local control maintain the flow even when blood pressure
changes. It directed to the kidneys at a given time.
Reabsorption of water and sodium
- Kidneys adjust how the body excretes or conserves the water and sodium as in
following steps:
1. From the Bowman's capsule the filtrate moves to the proximal tubule.
2. Some sodium ions pump out of the filtrate (by active transport mechanism) to
the interstitial fluid of adjacent capillaries.
-
3. Other ions ex: cl follows sodium into interstitial fluid.
4. Water also leaves the filtrate (by osmosis) to interstitial fluid of adjacent
capillaries.
5. NOTE: The nephron wall is highly permeable in this region (about 2/3 of the
filtrate's water is reabsorbed here).
6. The filtrate then enters the loop of Henle.
7. Interstitial fluid is saltiest around the hairpin turn of the loop of Henle.
8. Water moves out of the filtrate, by osmosis, before the turn.
9. The fluid left behind becomes saltier until it matches the interstitial fluid.
10. The loop wall after the turn is impermeable to water, but sodium is pumped out
by active transport mechanism.
11. The interstitial fluid become saltier & attracts more water out of the filtrate
just entering the loop.
12. Fluid arriving at the distal tubule is dilute.
13. The stage is set for adjustments, which can lead to urine that is highly dilute,
concentrated or anywhere in between.
HORMONE- INDUCED ADJUSTMENT
The cells of distal tubules & collecting duct have receptors for ADH & Aldosterone.
ADH (antidiuretic hormone):
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-If the hypothalamus detects a drop in extracellular fluid volume, it calls for
secretion of ADH.
-ADH makes the tubule walls more permeable to water, so more water is reabsorbed
and urine gets more concentrated.
-When body holds too much water, the secretion of ADH decreases, the kidney
tubules become less permeable, less water is reabsorbed, and urine remain dilute.
Aldosterone:
-Secreted by adrenal cortex.
-Enhance sodium reabsorption.
-Stimulates cells of distal tubules and collecting ducts to reabsorb sodium faster and
excrete less sodium. Conversely, when there is too much sodium, Aldosterone
secretion slow. Less sodium is reabsorbed, so more is excreted.
THE ACID- BASE BALANCE
 Kidneys help to keep the extracellular from becoming too acidic or too basic.
 This by controls of concentration of H+ & other dissolved ions.
 Buffer system (neutralization only), respiration, & urinary excretion all work in concert
to provide control over acid-base balance of ECF.
 Normally, the extracellular pH of human body should be maintained between 7.37 &
7.43.
 Acid lower the pH and bases raise it.
+
The control or removal of H is:
1/ Temporary removal by buffering system, which neutralizes excess H+ in blood &
interstitial fluid, the carbon dioxide forms during the reactions is exhaled from the
lungs:
+
-
H
+ HCO3
H2CO3
Bicarbonate
CO2
+
H2O.
carbonic acid
+
H
+ any ions
neutral compound.
2/ Permanent elimination, in the kidney:
+
H
-
+ HCO3
CO2
+ H2O.
*Carbon dioxide eliminates by respiratory system & H2O go to nephron.
Or:
+
H +
phosphate ions or ammonia
excreted in urine.
Kidney function:
1. Control of blood pressure.
2. Acid-base balance.
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3. Fluid & electrolyte balance.
4. Osmoregulation & ions control (is water & salt balance in the body fluid).
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