Lecture Seven

-Excretion o Blood is filtered through the kidneys complex system whereby urine is formed by modification of the filtrate via the reabsorption of required molecules and elimination of waste products -Regulation of blood volume and pressure o Control the extracellular fluid of the body by producing either a small volume of concentrated urine or large volume of dilute urine -Regulation of the solute concentration in the blood o Major ion concentrations are regulated -E.g. Na+ , K+ , Cl- , Ca2+

-Regulation of pH levels of the extracellular fluid o Secretion of H+ to assist with maintaining correct acidity -Regulation of red blood cell synthesis o Kidneys secrete erythropoietin, which stimulates the production of red blood cells -Regulation of the synthesis of Vitamin D o Vitamin D is required for calcium absorption from the intestines o Kidneys play a major role in regulating the blood Ca2+ levels not just through its reabsorption but by activating vitamin D

• Lie either side of the vertebral column on the posterior abdominal cavity wall • T12-L3 • The liver is superior to the right kidney thus it lies slightly lower than the left • Kidneys are surrounded by several layers

• Renal capsule - layer of fibrous connective tissue • Layer of adipose tissue • Renal fascia – layer of connective tissue • Final layer of adipose tissue

-Hilum – opening for the o Renal artery and nerves to enter o Renal vein and ureter to exit -Renal sinus is the cavity which opens after the hilum o Filled with connective and adipose tissue -Kidney is made up of an outer cortex and inner medulla

-Renal pyramids o Base form the boundary between cortex and medulla Renal papillae o Point of the pyramids Minor calyces o Renal papillae extend into this funnel o N = 8-20 Major calyces o Several pyramids minor calyces merge into major calyces o N = 2-3 Renal Pelvis o Major calyces merge to form Ureter o Renal pelvis forms a small diameter tube the ureter which extends to the bladder Renal columns o In between renal pyramids

• Functional units of the kidneys - 1.3 million in each kidney

• Renal corpuscle – filters blood • Proximal convoluted tubule (PCT) – returns filtered substances back to blood • Loop of Henle (nephron loop) – conserve water and solutes • Distal convoluted tubule (DCT) – additional wastes added to filtrate

Collecting duct -Connects to several DCT and carries fluid from cortex to medulla and empties into the papillary duct Papillary duct -Empties into the minor calyx

• Renal corpuscle • Proximal Convoluted Tubule (PCT) • Loop of Henle (descending limb then ascending limb) • Distal convoluted Tubule (DCT) • Collecting Duct • Papillary Duct • Minor Calyx • Major Calyx • Renal Pelvis • Ureter • Urinary Bladder • Urethra

-Comprised of; o Glomerulus – network of capillaries which receives blood from the afferent arteriole and exits at the efferent arteriole o Bowman’s capsule – capsule that surrounds the glomerulus and where fluid is filtered into from the capillaries and then flows to the PCT. -Filtration membrane allows fluid to flow into the capsule (first major step of urine formation).

• Juxtaglomerular cells – smooth muscle cells forming an arrangement around the afferent arteriole at the entry point to the glomerulus • Macula Densa – between the afferent and efferent arteriole lies a section of the DCT which in this section contain specialised cells • Juxtaglomerular apparatus – the contact of the juxtaglomerular cells and Macula Densa.

-Function = secretes renin (enzyme) which assists with the regulation of blood pressure and formation of filtrate

• Longer than the DCT • Outer basement membrane with simple cuboidal epithelial cells • Inner surface has many microvilli projections

• Thick portions are simple cuboidal epithelium cells - Thin portions are simple squamous epithelium cells

• Simple cuboidal epithelium cells • Smaller cells than the PCT • Less microvilli

• Simple cuboidal epithelium cells - Larger in diameter compared to the rest of the renal tubule

• Filtration: The process of non-selectively forcing small molecules and water out of the blood into the Bowman’s capsule where it is called filtrate • Tubular Reabsorption: The process of returning water and solutes back into the blood as the filtrate flows through the renal tubule • Tubular Secretion: The movement of drugs and toxic by-products from the blood into the filtrate - Movement of substances are either active or passive, similar to that of reabsorption

Capillary endothelial layer: o Contains many pores (fenestrations) Basement Membrane o Has spaces between fibbers Epithelial layer- Podocyte o Has foot processes creating filtration slits

-Renal fraction = % of total cardiac output that enters the kidneys o Resting, healthy adults = 21% -Glomerular Filtration Rate (GFR) = the amount of filtrate (plasma) that enters the Bowman’s capsule

o Renal blood flow rate (per min) = cardiac output mL/min x renal fraction n = 5600 ml x 21% = 1176 mL/min o Renal plasma flow rate -Renal blood flow rate x amount of plasma in blood -1176 x 55% = 650 mL/min o GFR = Renal plasma flow rate x 19% (filtration fraction) = 650 x 19% = 125 mL/min -In a day = 180 L

-Filtration Pressure = pressure gradient in the renal corpuscle

• Outward pressure of blood in the capillaries • Forces solutes and fluid out of capillaries and into Bowman’s capsule • Greater pressure in glomerulus compared to other capillaries • Efferent arteriole is smaller in diameter compared to afferent arteriole and capillaries

-Inward pressure of the filtrate in the Bowman’s capsule pressing back on the capillaries