Excretory

• The organisms that excrete ammonia are called ammonotelic and the process is called ammonotelism
• the process of excreting urea is called ureotelism and the animals which excrete urea is called ureotelic
• the process of excreting uric acid is called uricotelism and the animals which excrete Uric acid are called uricotelic
• human excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder and urethra
• these are reddish brown, bean-shaped structures situated between the levels of thoracic and third lumbar vertebra
• kidneys are mesodermal in origin
• they develop from mesodermal nephrostomes and mesomeres
• the right kidney is lower and smaller than the left kidney because the liver takes up much space of the right side
• the outer surface of each kidney is convex and the inner concave surface has a notch called hilum
• layers of kidney

  • Outermost- renal fascia
  • adipose layer
  • innermost- renal capsule
• LS of kidney shows outer cortex and inner medulla
• ureter is expanded as a funnel shape cavity called renal pelvis
• the free end of pelvis has number of cup like cavities, these are called major calyces and minor calyces
• The medulla projects into the calyces as conical processes called renal pyramids or medullary pyramids
• the tip of the pyramids is called renal papillae
• the cortex spreads in between the medullary pyramids as renal columns called columns of Bertini
• Glomerulus along with Bowman’s capsule is called the Malpighian body or renal corpuscle with filters out large solutes from the blood and delivers small solutes to the renal tubule for modification
• the afferent arteriole is short and wide that supplies blood to glomerulus and the efferent arteriole is narrow and long carrying blood away from the glomerulus
• Bowman’s capsule is a double walled, cup-like structure that surrounds the glomerulus
• the outer parietal wall of glomerulus is composed of flattened calls and the inner visceral wall is composed of a special type of less flattened cells called podocytes
• The loop of Henle has two parts; descending limb and ascending limb
• The proximal convoluted tubules (PCT) are highly folded and coiled structures where most of the absorption occurs
• PCT - Proximal Convoluted Tubule
• PCT is a highly coiled network and is restricted to the cortical region of the kidney
• Henle’s loop is quite narrower and U-shaped having a descending limb that ends into the medulla and an ascending limb that extends back from the medulla into the cortex
• DCT - Distal Convoluted Tubule
• DCT is thicker and highly coI lied structure situated in the cortex like the proximal tubule. The coils of both the convoluted tubules are intermingled
• last part of nephron is called collecting or straight tubule, which is lined by cuboidal cells
• PCTN - Peritubular Capillary Network
• vasa recta is absent or highly reduced in cortical nephron
• in some nephrons the loop of Henle is very long and runs deep into the medulla is called juxtamedullary nephrons
• Functions of Kidney

  • Regulation of water and electrolyte balance
  • Regulation of arterial pressure
  • Excretion of metabolic waste and foreign chemicals
  • Secretion of hormones like renin
• Ureters


  The pelvis of each kidney is continued as a ureter and angles out at the hilum. It is a long and muscular tube that extends posteriorly and opens into the urinary bladder.
• Urinary Bladder


  A thin walled, pear-shaped, white transparent organ present in the pelvic cavity that temporarily stores the urine.
• Urethra


  A membranous tube that conducts urine to the exterior. The urethral sphincters keep the urethra closed except during voiding of urine.
• Urine Formation


  1. Glomerular filtration

  2. Reabsorption

  3. Secretion
• Glomerular Filtration


  The first step of urine formation where blood is filtered by the glomerulus. Kidneys filter about 1100-1200 ml of blood per minute, which is roughly 1/5th of the blood pumped out by each ventricle of the heart in a minute. The filtration occurs through the endothelium of glomerular blood vessels, the epithelium of Bowman's capsule, and the basement membrane between them. The podocytes (epithelial cells of Bowman's capsule) are arranged to form filtration slits or slit pores.
• Passive transport


  The movement of water by osmosis and urea by simple diffusion.
• Active transport


  The reabsorption of glucose and amino acids.
• Tubular Secretion


  An important step in urine formation where certain chemicals in the blood that are not removed by filtration from the glomerular capillaries are removed by this process, helping to maintain ionic and acid-base balance of the body.