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1. Removal of harmful and unwanted nitrogenous chemical wastes from the body is known as ‘excretion’. Organs
concerned with the formation,storage and elimination of urineconstitute the excretory system’. Important rolein
maintainingthe homeostatic (steady sate) condition the body.
Waste products – (1) Carbon dioxide(2) Nitrogenous metabolic wastes (3) Excess salts and vitamins,Water,(5) Bile
pigments.
In humans, the term urinary system is more appropriate than the excretory system for the elimination of nitrogenous waste products)
1) Carbon dioxide and water (respiration)
2) Nitrogenous wastes are urea, uric acid and ammonia.1) Produced mainly in the liver from the dead
protein-remains of tissues thatare brought to it.2) Extra amino acids broken down in lever to produce
usableGlucoseand the urea. Urea is highly poisonous;and is excreted out through the kidneys
3) Excess salts such as common salt(NaCl) and even some excess water-solublevitamins (Band C) need to
be eliminated.Salts aremainly given out through the kidneys.
4) Water is taken in with food and beverages, in quantities.Excess quantity of water is removed which also
serves a useful purposeof dissolvingthe harmful materials to carry them out.
THE EXCRETORY ORGANS
3. Lungs: Excretion by lungs in theform of CO2 in the expired air.
2.Sweat glands:Excretion by sweat glands is incidental.Theseglands are primarily concerned with cooling (thermoregulation). The sweat secreted also carries with it small amount of nitrogenous
wastes. Sweat glands pass out sweat only when required for cooling.So truly they are not excretory.
~External Structure of the kidney
two bean-shapeorgans
10 cm long and 6 cm wide, Located on either side of the backbone and protected by the last two ribs.
Right side kidneyis at a slightly lowerA tube, the Ureter, arises the notch (hilum) in the median surfaceof the kidney
Ureter connects behind withthe urinary bladder in the lower part of the abdomen. Front(Top) end of the Ureter (connected into kidney) is somewhat
expandedand is called pelvisurine produced constantly flows through the ureters and is collects in the urinary bladder End of Ureter is somewhatprojecting
(into the bladder) and act like valves to prevent backflowof urine when the bladder contracts to pass out the urine. Sphincter
(circular muscle) guardsthe openingof the bladder intothe Urethra, and relaxes only at the timeofurination
(micturition)
~Internal Structure of the kidney
2main regions- an outer dark cortex and an inner lighter medulla. Medullais composed of finely striped
substancearrangedin several conicalpyramids.Apexof each pyramid (Papilla) projects into the pelvis of
the kidney.The kidney is composed of enormous numberof minute tubules called uriniferoustubulesor nephrons or renal
tubules or justkidney tubules. These are the structuralas well as functional units
~Structureof a Kidney tubuleEach kidney tubulehas thefollowing parts Bowman's capsuleis a
thin-walled (single-cell thick epithelium) cup, something likea hollow ball pressed deep on
one side. Its hollow internal space continues into the tubule. The outer concavityof thecup lodgesaknot-like mass of blood
capillaries, called Glomerulus.TheBowman's capsuleand theglomerulustogetherarecalled Malpighian
capsule or renal capsule.
~Proximal convolutedtubule (PCT)is thestartingconvoluted regionof thetubule("proximal" means nearer, i.e.
nearer to the Bowman's capsule). Both theBowman's capsuleand the proximal
convoluted part lie in the cortexgivingcortex a dotted appearancein sectional view
~MiddleU-shaped part (Loop of Henle) is shaped like a hair-pin;it is not convoluted.It runs in medulla to turn backand to re-enter
the cortexto continueinto the next convoluted regionof thetubule.
~Distal convolutedtubule (DCT)(again lying in the cortex) is theend part of the kidneytubule
("distal" means farther, i.e. farther or away from theBowman's capsule). It opens into a collecting duct. Thecollecting duct receives the contents of many kidney tubules

2. and pours it as urinein thepelvis of the kidney. Henle's loops and collecting ducts lie in the medulla giving it a striped
appearance
URINIFEROUS TUBULES number in both kidneys: Approximately 2 million Each singletubule : 4—5 cm long Total length of all tubules together : more than 60 kmThis great length
provides a huge surface for reabsorption of usablesubstances specially water, as the contents move through them. Blood flowingthrough kidneys per minute 1-liter Glomerular filtrate
produced in 24 hours 160 liters Urineproduced from glomerular filtrate after reabsorption per day 1.2 liter
-BLOOD SUPPLY TO THE KIDNEY TUBULES
A pair of renal arteries branch offfrom thedorsal aortato enter therespective kidneys Each renal
artery branches and re-branches severaltimes to give rise to arterioles;each such arteriole enters a
Bowman's capsuleunder thename of Afferent arteriole (afferent : to bring to).This afferent arteriole breaksinto anumber of
capillarieswhich form aknot-like mass (Glomerulus) closelyfittinginsidethe Bowman's capsule The reuniting capillariesof theglomerulus
formthe Efferent arteriole (efferent : to carry away).Theefferent arterioleafter emerging from the Bowman's capsule runs ashort distance and breaks up into
a secondary capillary network (vasa recta)which surrounds therenal tubule, and rejoins to forma vein(Fig. 9.3). By
uniting again and againwith other veins of thekidney, it ultimately forms the renal veinwhich leaves he
kidney at the median surfaceto pour theblood intothe posterior venacava.
Function of the Kidney—
Production ofurine the production ofurine occurs in three major steps; (I)ultrafiltration, (Il)reabsorption and (Ill)tubular secretion.
(l) ULTRAFILTRATION The blood flowsthrough theglomerulusunder greatpressurewhich is much greater than in the capillaries elsewhere. The reason for this greater pressure is that the
efferent(outgoing) arteriole is narrower thanthe afferent(incoming) arteriole. This high pressure (hydrostatic pressure) causes theliquid part of the
blood to filter outfrom theglomerulus intothe renal tubule. This filtration under extraordinary forceis
called Ultrafiltration. Duringultrafiltration almost all the liquid partof theblood (plasma alongwith most of its organic and inorganic substances
including urea, glucose, amino acids, etc.) comes out of theglomerulus and passes into the funnel-shaped cavity
of the Bowman's capsule(Fig. 9.4). Thefluid entering therenal tubuleis called the Glomerular filtrate. Theglomerular filtrate consists
of water, urea, salts,glucoseand other plasma solutes. The thicker partof theblood left behind in theglomerulus after ultrafiltration,namely, thetwo kinds of
corpuscles, proteins, and other largemolecules carriedforward through theefferentArteriole the blood proceeding
away from theglomerulus is relatively thick.
(11) REABSORPTION The glomerular filtrateentering the renal tubuleis not urine. It is an extremely dilutesolution containinga lotof
useful materials includingglucoseand some salts such as thoseof sodium. As the filtrate passes down the tubule, much of the water is reabsorbed
together with the usefulsubstances. But their reabsorptionis only tothe extent
that the normal concentrationof the bloodis not disturbed. This is called
selectiveabsorption.
(Ill) TUBULAR SECRETION Certain substances like potassium(K+)in thenormal course, and alarge number of foreign chemicals including drugs like
penicillin are passed intotheformingurine in the distal convoluted tubule. This passageinvolves
the activity of the cells of the tubular wall, and hence it is called tubular secretion.
The filtrateleft after reabsorption and tubular secretion is called urine.

3. All these steps involved in urine production requireenergy, hence theoxygen demand of the kidneys is to 7 times higher than what is required
by muscles.
Urine excretion — Final urine passesin collectingducts tothe pelvisand through theureter intothe urinary
bladderby ureteral peristalsis(waves of constriction in theureters) and dueto gravity. Urine is expelled from the urinary bladderthrough the
urethra(in thepenis in males, and directly females) by relaxation of the sphincter musclelocated at theopening of theurinary bladder in the urethraunder
impulsefrom thenervous system Such a process is called micturition.
Physical properties of urine
Colour : Clear yellow (due to pigment urochrome). Colour varies with the diet.
Volume : I to 1.5 litres per day but varies.
pH : 5 to 8 i.e., usually it is slightly acidic(pH = 6). Protein diet makes it more acidic while vegetablediet makes it alkaline.
Odour : if kept standing,the smell of urine become strong, ammonia-likedue to bacterial activity otherwisefaintsmell.
Specific gravity :1.003 to 1.035
Taste: Normally, it is saltish,When sweetish due to diabetes, When tasteless,it is due to much water urine,a condition called diabetes
insipidus (due to insufficientADH hormone) means tasteless.
CONSTITUENTS OF URINE The normal human urine consists of about 95% of water and 5% of solid wastes dissolved in it. The percentage of
the solid wastes may slightly vary according to the food taken and according to the time after taking food but usually these are approximately asfollows :
Organic in (g/L) grams per litre of urine Inorganic in (g/L) grams per litre of urine
Urea 2-3 Sodium chloride 9-0
Creatinine 1-5 Potassiumchloride 2-5
Uric acid 0-7 Ammonia 0-6
Others 2-6 Others 2-5
Besides the normal constituents, the urine may pass out certain hormones and also certain medicines like the antibiotics and the excess vitamins.

4. Abnormal constituents in urine
(i) Blood cells — Known as haematuria, in this condition blood passes with urinedue to some infection in urinary tract,kidney
stones or tumour. — In a condition called glycosuria,
(ii) Glucose excess glucose passes with urine due to diabetes mellitus (sugar diabetes)
(iii) Albumin —Due to high blood pressure, to increased permeabilityof the capsulemembraneon account of bacterial
infection, albumin proteins may be secreted out via the urine.
(iv) Bile pigments —- Due to anaemia, hepatitis (jaundice) or liver cirrhosis, urine may contain: bilepigments.
REGULATION OF URINE OUTPUT
The pituitary gland and Diuresis (increase: production of urine) Concentration of the water reabsorption is controlled by antidiuretic
hormone(ADH) secreted by the posterior lobethe Pituitary gland. If ADH secretion is reduced here is an increased production
of urine, this is called alled 'diuresis'. Substances that increase the formation of urine are called 'diuretics e.g.,liquid, tea, coffee, alcohol, etc.
OSMOREGULATION
The kidney while removing wastes like urea from blood also regulates its composition, i.e., thepercentageof water and salts. This function is called
osmoregulation — it implies theregulation osmotic pressureof the blood.
ARTIFICIAL KIDNEY If one kidney is damaged or removed for some reason, the other kidney aloneis sufficientfor excretory needs and theperson can lead a normal life.But failure
of both the kidneys would lead to death. Artificial kidney is a dialysis machine. The patient's blood is led from the radial artery in
his armthrough the machinewhere the urea and excess salts are removed and the purified blood is returned to a vein in the same arm. In cases of permanent damage to thekidneys,
dialysisisto be repeated for about twelve hours twice a week.