intro the the kidney

week 4 ctb

functions of the kidney

• endocrine and homeostatic organs

1. homeostasis

   • filters plasma

   • regulates excretion of metabolic waste and toxins in urine

   • rebasorption of of essential nutrients

2. endocrine:

• production and release of: 

  • erythropoietin

  • active form of vit D

  • renin- regulation of BP

homeostatic kidney

• contributes to homeostasis by removing substances from plasma

• selectively retains certain substances and excreting others

• reabsorption of essential nutrients/ions

• excretion of metabolic waste/toxins

• regulation of body fluid composition

• regulation of total body fluid volume (regulation of BP)

• key processes: filtration, reabsorption, secretion to eventually form urine 

• macro and micro structures reflect their functions

uniferous tubules

• made uo of numerous uniferous tubulues and associated blood vessels in cortex and medulla

• uniferous tubule = nephron + collecting duct

• plasma is filtered in nephron and solutes are reabsorbed/secreted

• filtrate to be excreted is collected in collecting duct to be excreted as urine

uniferous tubule diagram

nephron

• functional unit of kidney

• 1 million per kidney

• tubular structure made of epithelial cells

parts of a nephron

• renal corpuscle (glomerulus and Bowman’s capsule)

• proximal covoluted tubule (PCT) 

• loop of Henle

• distal convoluted tubule (DCT)

• collecting duct

microvasculature

• nephrons have 2 arterioles and 2 capillary networks

• blood enters glomerulus via afferent arteriole and leaves via efferent arteriole

• high pressure glomerular capillaries: 

  • filtration

• low pressure peritubular capillaries:

  • reabsorption/secretion

renal corpuscle

• made up of glomerulus (glomerular capillaries) and Bowman’s capsule

• filters blood to form an initial filtrate

• Bowman’s capsule appearance is like a double walled cup that surrounds glomerular capillaries

• outer layer = parietal layer

  • simple squamous (structural)

• inner layer = visceral layer

  • modified simple squamous (podocytes → filtration)

• mesangiel cells provide support between glomerular capillary loops

• arteriole at either end of glomerular capillary bed facilitates high pressure in glomerulus for filtration

filtration barrier

• podocytes wrap around glomerular capillaries and their interlinked foot processes form filtration slits

• filtration barrier formed by:

  1. glomerular capillary endothelium: fenestrated

  2. basement membrane: negatively charged

  3. epithelial cells (podocytes): interdigitating foot processes and filtration slits

• limits passage of substances from blood based on size, charge and shape

epithelia of nephron

• after glomerular filtration, filtrate passes into Bowman’s capsule

• flows along nephron tubule segments

• tubule made up up of epithelial cells

• epithelial layers adapted for functional role in different parts of nephrons

proximal convoluted tubule (PCT)

• tubule segment

• unmodified filtrate leaves Bowman’s space and enters PCT

• simple cuboidal cells with microvilli forming brush border to increase SA

• approx 70% of water, sodium, chloride, AAs, glucose reabsorption

• secretion of some drugs/waste molecules

loop of Henle: thin limbs

• 3 main parts with different characteristics:

  1. thin descencing limbs

  2. thin ascending limbs

  3. thick ascending limbs

loop of Henle: thin descending limb 

• low permeability to ions

• no active reabsorptiom or secretion of solutes

• high permeable to water

• simple squamous epithelium

loop of Henle: thin ascending limb

• permeable to ions but essentially no active reabsorption or secretion of solutes (passive only)

• completely impermeable to water

• simple squamous epithelium with tight junctions

loop of Henle: thick ascending limb

• highly permeable to ions: active reabsorption of sodium/other solutes

• impermeable to water

• simple cuboidal cells

• the 3 sections of LOH function to generate hyperosmolar interstitial fluid in medulla (relative to plasma)

• important in urine conc

distal convoluted tubule (DCT)

• divided into early and late parts

• simple cuboidal cells

• highly permeable to ions/solutes: active reabsorption of sodium and other solutes

• secretion of K+ and H+

• variable water permeability depending on precise part of distal tubule and presence of ADH

• early distal tubule passes back to vascular pole of its own renal corpuscle to form: juxtaglomerular apparatus

juxtaglomerular apparatus: JGA

3 components to JGA:

1. macula densa: specialised cells within part of early distal tubule that passes next to vascular pole of its own renal corpuscle

2. granular or juxtaglomerular cells in afferent arteriole

3. extraglomerular mesanglial cells (Lasic cells)

involved in tubuloglomerular feedback and control of BP

collecting duct

• final site for urine processing

• simple columnar cells

• water permeability of medullary collecting duct under hormonal control by ADH

• surrounded by a medullary interstitial fluid with a high conc of solutes

• key role in regulating degree of urine concentration by controlling water reabsorption

body fluid homeostasis

• maintaining a constant volume and stable composition of body fluids essential for homeostasis

  • including electrolute balance and BP regulation

• requires precise matching of intake and output

• some poorly regulated, insensible water losses

• most water output is via kidneys (urine)

• kidneys play major role in body fluid volume and composition regulation

ICF

• male total body water: 42 litres (60% body weight)

• varies with age, sex, body fat

• ICF= 28 litres

• distributed in different compartments separated by semi permeable cell membranes

• composition of fluids varies between compartments (cells) as cell membrane is permeable to water but not to most electrolytes

ECF

• 14 litres

• further divided into 2 main compartments

• interstitial fluid (1.1 litres): surrounds cells

• plasma (3 litres): non cellular component of blood

• composition of plasma and ECF is similar due to equilibration across capillary membrane

• capillary membrane highly permeable to water and electrolytes but not to most plasma proteins

body fluid composition

ECF:

• sodium is main cation (+ve)

• chloride is main anion (-ve)

ICF: 

• potassium is main cation

• phosphate and organic anions are main anion

• potassium is main osmotically active electrolyte

• different compartments have different compositions but are in osmotic equilibrium

• sodium is main osmotically active electrolyte

• by regulating Na+, kidneys can directly regulate body fluid volume