Lymphatic System

· Lymph is formed by extravasation or filtration of blood from the arteriolar end of capillaries into the tissue spaces.
· Similar in composition to plasma except for the absence of large molecular weight proteins.
· Lympth or tissue fluid → lymph capillaries → lymph sinuses → lymph hearts → veins
· Lacks RBCs and thrombocytes.
· Lymph helps in absorption of fat in the small intestine.

· The lymphatic system consists of lymphatics or lymph vessels- thin wall delicate vessels of different sizes.
· Larger vessels open into the venous system
· Lymph heart: small rhythmically contractile in nature.

The frog has 2 pairs of lymph hearts:
· One pair is located anteriorly below scapulae and opens into the subscapular vein.
· Other pair located on either side of urostyle and open into femoral veins.
· Lymph spaces: about 20 subcutaneous lymph sinuses under the skin (example: dorsal, lateral and abdominal etc.)

Excretory System

· Excretory system is concerned with the removal of metabolic wastes from the body.
· Kidney is the main organ of excretion.
· Others are the liver, large intestine and lungs.
· Development of Kidney System:
· IN TADPOLE: Kidney is pronephric. The mode of excretion is ammonotelic.
· IN ADULT FROG: The kidney is mesonephric and the mode of excretion is ureotelic. 
· Note: Frog is Uricotelic during hibernation and aestivation.

· Dark red in colour, roughly oval in shape with the internal margins irregularly lobulated.
· Located on either side of the abdominal aorta just below vertebral column in the sub-vertebral lymph space, retroperitoneal or covered ventrally by peritoneum.
· Ureter emerges from the posterior fourth of its outer margin and passes backwards to open into the dorsal side of the cloaca.

· Supplied by:
· 4-5 pairs of renal arteries.
· Renal portal vein
· Urine is collected in the thin-walled, bilobed, elastic urinary bladder at the ventral side of cloaca by the sphincter.


· Units of excretion: Nephronsor Uriniferous tubules.
· The total number of nephrons in each kidney ranges from 2000 to 2500.
· Each nephron consists of 2 parts.
i) Malpighian corpuscle formed by Bowman capsule and glomerulus.
ii) Tubule

· Bowman's capsule:
· Cup-shaped blind end of nephron.
· Lined with flattened squamous epithelium folded to form visceral and parietal epithelium.

· Diameter of afferent arteriole in the glomerulus is greater than that of efferent arteriole → increases hydrostatic pressure in glomeruli for ultrafiltration.
· Tubule is not divisible into PCT, DCT and Henle’s loop. The tubule is lined with columnar cells which are ciliated at the neck of a nephron.
· Each tubule runs dorsally, makes coils, runs ventrally, makes second coils, runs dorsally and opens into Transverse collecting tubule.
· The transverse collecting tubules open on the inner side of the kidney into Bidder’s canal and on the outer side of the kidney into the ureter.
· Few ciliated funnels are present on the ventral surface of the kidney known as nephrostomes which pour excretory matter from the coelom to the renal vein.
· Ureter – It is also called urinary or Wolffian duct.
· In male frogs, the ureter is also called the urinogenital duct because it conducts both sperms and urine.

· Physiology of Urine Formation

· Filtration of the metabolic waste (particularly urea) from blood and its subsequent excretion as urine occurs by the following processes:

· Due to the greater diameter of afferent arteriole than efferent arteriole, hydrostatic pressure in glomerular capillaries is increased → plasma including urea, electrolytes, glucose, amino acids filter out through the glomerulus into Bowman’s capsule and then pass into tubules.

· Useful substances like glucose, amino acids, electrolytes and much water is reabsorbed by the tubules.
· Process of reabsorption is selective.


· Certain harmful/toxic substances are actively secreted from blood into the tubules.
· The filtrate now is called urine.

· Functions of Kidney
· Excretion of metabolic waste from the body.
· Regulation of water content of the body.
· Acid-base balance.
· Production of blood cells in the embryonic period.

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