Excretory system:the kidney

Excretory System of human being
The removal of waste products or metabolic waste like uric acid,ammonia and urea from the body by cell metabolism is known as excretion.The metabolic waste are removed by a excretory organs.Excretion helps to maintain homeostasis.The excretory organ of human beings is a pair of kidney.
KIDNEY:
Kidney is a bean shaped organ measuring about 10cm in length and 5cm in width.It weighs about 159 gm.In human being the left kidney lies slightly above the right kidney.The kidney is covered by tough connective tissue which in turn is covered by fatty layer.It keeps the kidney warm and protect from injury.Each kidney has outer convex and inner concave surface.The concave surface has helius from where enter renal artery,nerve and comes out renal vein and pelvis.
Each kidney is internally differentiated into outer cortex and inner medulla.Outer cortex is light brown in colour and is mesodermal in origin whereas innner medulla is dark brown in colour and is ectodermal in origin.Medulla makes no of folds known as pyramid.The cortex that extend between pyramid is renal column of Bertini.
The functional and structural unit of kidney is Nephron.Each kidney consists of millions of nephrons.Nephron is 2 cm in length and 25 nanometer in diameter.Each nephron has malphigian capsule and tubule.The proximal end of malphigian capsule has Bowman's capsule with glomerulus.Bowman's capsule is double wall cup like structure.Concavity of bowmans capsule has glomerulus which is network of blood capillaries.Glomerulus get blood supply from afferent
artery which contain nitrogenous waste and the rest of blood is carried by efferent artery.The tubule is very long and differentiated into into many region.
The tubule starts from bowmans capsule as a narrow neck.Behind the neck tubule makes a few coil known as proximal convoluted tubule(PCT) which runs posteriorly and opens into loop of Henle.Loop of Henle has descending tubule loop and ascending tubule loop.The ascending loop runs continually into distal convoluted tubule.In the cortex lies bowmans capsule with glomerulus,PCT and DcT whereas in the medulla lies Loop of Henle and collecting duct.The effert artery break down and cover the tubule.
From the helix of each kidney arises pelvis which is triangular in shape and runs posteriorly as ureter.Ureter is muscular tube and 25 cm long.It runs down in posterior abdominal cavity each from either side of vertebral column.Ureter opens into urinary bladder which is a muscular organ with centre cavity.In Urinary bladder opens two ureter and the opening of urethra.SO,it is triangular in shape.In urinary bladder urine is temporarily stored.The posterior region of urinary bladder elongates as neck which is provided with internal valve.The neck is continuous with the tubular urethra which is also provided with external valve.The valve opens only in the time of micturation.In female urethra is 4 cm long and open outside urethral orifice.In the male,urethra is 20cm long and carry both urine and semen ,so called urino genital duct;and opens as penis.

PHYSIOLOGY OF URINE FORMATION:
Human is ureotelic as urea is excretory waste.Urea is produced in liver and is released in blood stream by the process known as Kreb cycle.Excess amino acid is converted into ammonia which is toxic and is converted into urea..Blood carry urea into kidney and release outside as urine.Therefore,urine formation involves two steps:Urea formation and Urine formation.
Urine formation occurs in nephron of kidney and involves following process:
1)Ultrafiltration:
The filtration though semi permeable membrane under high blood pressure is known as ultra filtration and the filtrate is ultra filtrate.It takes place in malphigian body.The wall of bowmans capsule and glomerular cappilaries is very thin and semi permeable.The efferent arterial is narrower than afferent arterial.So,the blood pressure in bowmans capsule is high.Therefore,nitrogenous waste together with water and semi permeable molecules are filtered into lumen of bowman capsules known as ultra filterate.The rest of the blood from glomerulus is carried by effert artery which breakdown into peritubular cappilary network
and envelop the tubule except collecting duct.The glomerular filtrate is isotonic and it content mainly water,uric acid ,amino acid,glucose,ketone bodies, creatinine,sodium except WBC and RBC,
the glomerular hydrostatic pressure(GHP)=75mmHg
Osmotic pressure (OP) =20 to 25 mmHg
Filtrate hydrostatic pressure(FHP) =20 mm hg
Net hydrostatic pressure =GHP-(OP+FHP)
=25 mm hg
glomerular filtrate per min =125 mm from both kidney

2)Selective Reabsorption:
As the filtrate passes down the tubule many useful substances are taken back into the blood of the cappilary network that surrounds the tubule(peritubular cappilaries).So,the composition of O.P and pH changes.This is called selective reabsorption.It occurs in two ways:Passive absorption by diffusion and Active absorptiontaking place against the concentration gradient using ATP.These are amino acid,glucose,uric acid,keratinin vitamin,hormones,etc.About 65 to 80%of glomerular filtrate are absorbed in Proximal convoluted tubule.so,the filtrate becomes isotonic which passes through loop of henle.
The wider region of descending loop is impermeable to the water,urea and inorganic ion but the narrower region is permeable to water .So,more water is drawn back
to peritubular cappilaries.So,the filtrate become hypertonic to the blood plasma.Now,the hypertonic fluid reaches the ascending limb.The ascending limb is impermeable to water along the entire length but the wider region is permeable to inorganic ion.So,the filterate becomes hypotonic which passes to Distal convoluted tubule.
In the distal convoluted tubule as the plasma water falls the posterior pituitary tube releases Anti diuretic hormone(ADH) which increases the permeablity of water to DCT,collecting tubule and duct so that more water is reabsorbed in this region and forms concentrated urine.When the level of plasma become normal ADH secretion is stop so that permeablity also decreases.So,less water is reabsorbed forming diluted urie.
In the DCT active absorption of Na+ also take place under the influence of Aldesterone hormone secreted by Adrenal gland.So,the filtrate become hypotonic to blood plasma which passes into collecting duct as urine.
3)Tubular secretion:
The peritubular cappilaries has some excretory waste which is released into the filtrate of tubule called tubular secretion.These solid include uric acids,pigments,drugs,ammonia etc especially in PCT and DCT.The removal of H+ and ammonia from blood helps to maintain the pH of blood.
MICTURATION OR URINATION:
The urinary bladder serve to store the urine temporarily and also pass it out at regular interval.This removal of urine from the bladder is micturation.It is reflex process in grown up child and adult.The urinary bladder is supplied by parasympathetic and sympathetic nerve.Stimulation of Sympathetic nerve causes the relaxation of muscle of Urinary Bladder and constriction of internal valve.So,urine is blocked in the bladder.Accumulation of urine in bladder
(400ml to 500ml)create pressure in the muscle of bladder so that parasympathetic nerve is stimulated.It causes the contraction of bladder and relaxation of internal valve leading into the outflow of urine through urethre.The urethra also posssess external valve which opens only at time of micturation.Urine is released outside through urethral orifice.

URINE:
Urine is transparent,light yellow,acidic fluid.The yellow colour is due to presence of urocrome which is produces by breaking down of haemoglobin from worn out RBC.The colour may be affected by the type of food,vitamin suppliment and othe medicine.The pH ranges from 7-8depending upon acidic or basic food ondiet.A normal adult produces 1-1.5litre uring per day.