Anatomy

• Excretory System


  Structures that mainly produce and transport the nitrogenous waste products for the excretion
• Kidneys


  Remove waste products of metabolism, excess water and salts from blood and maintain pH
• Ureters


  Convey urine from the kidneys to the bladder
• Urinary bladder


  Muscular reservoir of urine
• Urethra


  Channel to the exterior
• Kidney


  • Complex bean shaped, reddish brown color organs
  • Convex lateral margins and a concavity on the medial side is known as hilum
  • Upper and lower ends and anterior and posterior surfaces
  • Human body consists of two kidneys
  • Size - Length: 11cm; Width: 6 cm and Thick: 3 cm
  • Weighs approximately 150g
  • Right kidney is slightly lower than the left kidney

  
• Anatomical Location of Kidneys

  
• Coverings of Kidney


  • Renal Capsule / False Capsule
  • Renal Fascia / Middle Fatty Capsule / Peripheric Fat
  • Innermost Covering: Fibrous capsule or true capsule

  
• Organs Associated With Right Kidney


  • Superiorly: Right Adrenal gland
  • Anteriorly: Right lobe of the liver, Duodenum and the Hepatic flexure of the colon
  • Posteriorly: Diaphragm, Muscles of the posterior abdominal wall

  
• Organs Associated With Left Kidney


  • Superiorly: Left Adrenal gland
  • Anteriorly: Spleen, Stomach, Pancreas, Jejunum and Splenic flexure of the colon
  • Posteriorly: Diaphragm and muscle of the posterior abdominal wall

  
• Hilum


  • Concave medial border of the kidney
  • Attachment to the upper expanded end of the ureter – Renal Pelvis
  • Lies more or less in the transpyloric plane, a little medial to the tip of the ninth costal cartilage
  • Right Kidney: Below transpyloric plane
  • Left Kidney: Above transpyloric plane
  • Renal artery, Renal vein, Lymphatic vessels, Nerves and Ureters enters and leaves the kidney at the level of hilum

  
• Regions of the Kidney


  • Renal Cortex
  • Renal Medulla

  
• Renal Cortex


  • Tissue lying between the bases of the pyramids and the surface of the kidney, forming the cortical arches or cortical lobules
  • Light and dark striations, the light lines are called medullary rays
  • Tissue lying between adjacent pyramids is also a part of the cortex – Renal columns
  • Each pyramid comes to be surrounded by a 'shell' of cortex, the pyramid and the cortex around it constitute a lobe of the kidney

  
• Renal Medulla


  • Triangular areas of the renal tissue
  • Each pyramid has a base directed towards the cortex, and an apex that is directed towards the renal pelvis, and fits into the minor calyx
  • Pyramids shows striations that pass radially towards the apex

  
• Gross Internal Structure


  • Hilum leads into a space called the renal sinus
  • Renal sinus is occupied by the upper expanded part of the ureter
  • Renal pelvis divided into two or three parts called major calyx
  • Each major calyx divides into a number of minor calices
  • The end of each minor calyx is shaped like a cup
  • A projection of kidney tissue called a papillae fits into a cup

  
• Uriniferous Tubules


  • The functional parts of the kidney
  • Each uriniferous tubules consists of and excretory part called nephron and of a collecting duct / tubules
  • Collecting tubules draining different nephrons join to form larger tubules – Papillary Duct
  • Each papillary duct opens into a minor calyx at the apex of the renal papilla
  • Each kidney contains one or two million nephrons

  
• Parts of a Nephron


  • Renal Corpuscle / Malpighian Corpuscle
  • Renal Tubules

  
• Renal Corpuscle


  • The glomerulus is formed by the invagination of a tuft capillaries into the dilated, blind end of the nephron – Bowman's capsule
  • Capillaries are supplied by an afferent arterioles and are drained by an efferent arterioles
  • Diameter of afferent arteriole is greater than the efferent arteriole
  • Two cellular layers separate the blood from the glomerular filtrate in Bowman's capsule: Capillary Endothelium and Specialized Epithelium of Capsule
  • Endothelium of glomerulus is fenestrated with pores that are 70 – 90 nm in diameter
  • Endothelium of glomerular capillaries is completely surrounded by the glomerular basement membrane along with specialized cells – Podocytes
  • Podocytes have numerous pseudopodia that interdigitate to form filtration slits along the capillary wall
  • Glomerular basement membrane doesn't contain visible gaps or pores
  • Stellate cells called mesangial cells are located between the basal lamina and the endothelium

  
• Proximal Convoluted Tubule (PCT)


  • About 15mm long and 35 micrometer in diameter
  • Wall is made up of a single layer of cells that interdigitate with one another and are united by apical tight junction
  • Between the cells are extension of the extracellular space called the lateral intercellular spaces
  • Luminal edges of the cells have a striated brush borders due to the presence of many microvilli
• Loop of Henle


  • Descending portion and the proximal portion of the ascending limb are made up of thin, permeable cells
  • Thick portion of the ascending limb is made up of thick cells containing many mitochondria
  • Specialized cells at the end form the macula densa, which is close to the efferent and particularly the afferent arteriole
  • Macula, the neighboring lacis cells, and the renin secreting granular cells in the afferent arteriole form the juxtaglomerular apparatus
• Distal Convoluted Tubule (DCT)


  • About 5mm long
  • Epithelium is lower than PCT, few microvilli are present and no distinct brush border
• Collecting Duct (CD)


  • About 20 mm long and pass through the renal cortex and medulla to empty into the pelvis
  • Epithelium is made up of principal cells (P cells) and intercalated cells (I cells)

  
• Renal Medullary Interstitial Cells (RMICs)


  Contain lipid droplets and are a major site of cyclooxygenase 2 (COX – 2) and prostaglandin synthase (PGES) expression

  
• Epithelium Lining the Renal Tubules


  • Neck is lined by simple squamous epithelium continuous with that of the glomerular capsule
  • PCT are lines by cuboidal or columnar cells having a prominent brush border
  • Thin segment of LH is lined by low cuboidal or squamous cells and the tick segment is by cuboidal cells
  • DCTs lined by cuboidal cells without a brush border
  • Collecting tubules lined by simple cuboidal or columnar epithelium

  
• Renal Blood Vessels


  • Renal artery divides into a number of lobar arteries
  • Lobar arteries divide into two or more interlobar arteries that enters the tissue of the renal columns and run towards the surface of the kidney
  • At the base of pyramids, the interlobar arteries divide into arcuate arteries. They lie parallel to the renal surface at the junction of the cortex and medulla

  
• Prostaglandin synthase (PGES)


  Enzyme that catalyses the production of prostaglandins
• The Nephron


  • Functional unit of the kidney

  
• Epithelium Lining the Renal Tubules


  • Single layer of epithelial cells supported on a basal lamina
  • Neck lined by simple squamous epithelium
  • PCT lined by cuboidal or columnar cells with a prominent brush border
  • Thin segment of LH lined by low cuboidal or squamous cells, thick segment by cuboidal cells
  • DCT lined by cuboidal cells without a brush border
  • Collecting tubules lined by simple cuboidal or columnar epithelium

  
• Renal Blood Vessels


  1. Renal artery divides into lobar arteries

  2. Lobar arteries divide into interlobar arteries

  3. Interlobar arteries divide into arcuate arteries

  4. Arcuate arteries divide into interlobular arteries

  5. Interlobular arteries form afferent arterioles

  6. Blood circulates through glomerular capillaries

  7. Efferent arterioles emerge from glomeruli

  8. Efferent arterioles divide into capillaries

  9. Capillaries drain into interlobular veins, arcuate veins, interlobar veins

  10. Efferent arterioles near medulla form descending vasa recta

  11. Descending vasa recta form capillary plexus around Loop of Henle

  12. Ascending vasa recta drain the capillary plexus

  
• Types of Nephrons


  • Cortical Nephron
  • Juxtamedullary Nephron
• Cortical Nephron


  • Located entirely in the cortex
  • Include most nephrons
  • Length of the tubes are short
• Juxtamedullary Nephron


  • Present at the border of the cortex and medulla
  • Length of the tubules are long
• The Ureters


  • Long, smooth muscle walled tubes connecting the renal pelvis to the urinary bladder
  • Human has two ureters, each 25cm long and 3mm wide
  • Upper half lies on the posterior abdominal wall, lower half in the true pelvis
  • Cross the upper end of the external iliac artery, then turn towards the bladder
  • Run obliquely in the bladder wall for about 2cm
  • No valve or sphincter at the lower end

  
• Layers of Ureters


  • Outer layer: Fibrous tissue
  • Middle layer: Inner longitudinal and outer circular smooth muscle layers
  • Inner layer: Transitional epithelium lining with longitudinal folds
• Abdominal parts of Right and Left ureters


  1. Run downwards with a slight medial inclination

  2. Cross the upper end of the external iliac artery

  3. Come to lie on the lateral wall of the pelvis

  4. Finally turn towards the bladder

  
• Blood Supply to The Ureters


  • Supplied by branches of the renal, internal iliac, and inferior vesical arteries

  
• The Urinary Bladder


  • Pear or tetrahedron shaped, muscular reservoir for urine
  • Opens into the urethra at its lowest point, the neck
  • Apex gives attachment to the median umbilical ligament
  • Peritoneum covers only the superior surface
  • Maximum capacity of 500ml
  • Empty bladder has four triangular surfaces

  
• Surfaces of Urinary Bladder


  • Posterior (base/fundus)
  • Superior
  • Anterior (apex)
  • Inferolateral

  
• Layers of the Bladder Wall


  • Outer layer: Serous layer
  • Middle layer: Three layers of smooth muscle
  • Inner layer: Transitional epithelium
• Inner Layer of Bladder Wall


  • Transitional epithelium lining can change shape and move over one another when bladder is distended
  • Epithelium is capable of withstanding osmotic changes
  • Mucous membrane has numerous folds (rugae) in the empty bladder