BISC 101 Lecture Notes - Lecture 15: Distal Convoluted Tubule, Proximal Tubule, Efferent Arteriole

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BISC 101 – Lecture 15 – Osmoregulation and Excretion: Part II
Excretory System
Excretory systems regulate solute movement between internal fluids and the
external environment
Excretory systems produce urine by refining a filtrate derived from body fluids
Key functions of most excretory systems:
oFiltration: Pressure 0 filtering of body fluids
oReabsorption: Reclaiming valuable solutes
oSecretion: Toxins and other solutes from the body fluids moved to filtrate
oExcretion: Removing the filtrate from the system
Kidneys
Kidneys: Excretory organs of vertebrates, function in both excretion and
osmoregulation
oAccount for less than 1% of body weight
oReceives about 20% of resting cardiac output
Mammalian Excretory Systems: Found centered on the paired kidneys and is the
principal site of water balance and salt regulation
Each kidney is supplied with blood by a renal artery and drained by a renal vein
oRenal Artery: Supply blood to the kidneys
oRenal Vein: Blood drained from the kidneys
Ureters: Muscular duct where urine exits from the kidneys
Urinary Bladder: Storage where urine is drained from the ureters
Urethra: Tube where urine is expelled through
The mammalian kidney has to distinct regions
oOuter Renal Cortex
oInner Renal Medulla
Minor Calyx: Collects urine from one renal medulla
Major Calyx: Collects urine from minor calyces
Renal Pelvis: Collects urine from all major calyces
Nephron: The functional unit of the kidney
oGlomerulus: Nephron consists of a single long tubule and a ball of
capillaries called the glomerulus
oBowman’s Capsule: Surrounds and receives filtrate from the glomerulus
~ 1 million nephrons, total tubule length of 80 km, in one adult kidney
Filtration
Filtration: Pressure filtering of body fluids
Filtration occurs as blood pressure forces fluid from the blood in the glomerulus
to lumen of Bowman’s capsule
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Fenestrated Endothelium: Pore like openings of capillaries that are permeable to
small molecules but not to blood cells or plasma proteins
Podocytes: Specialized capsule cells that re permeable to small molecules but
not to blood cells or plasma proteins
Small molecules include salts, glucose, amino acids, vitamins, nitrogenous
wastes, and other small molecules
Pathway of the Filtrate
1. Bowman’s capsule
2. Into the nephron
i. Proximal convoluted tubule
ii. Loop of Henle
iii. Distal convoluted tubule
iv. Collecting duct
Cortical nephrons: Confined to the renal cortex
o80% of nephrons in the human kidney are cortical
Juxtamedullary Nephrons: Have loops of Henle that descend into the renal
medulla
o20% of nephrons in the human kidney are juxtamedullary
oOnly mammals and birds have juxtamedullary nephrons
oThese nephrons enable mammals to produce urine that is hyperosmotic
to body fluids, thereby conserving water
Blood Vessels and Nephrons
Afferent Arteriole: A branch of the renal artery (supplies blood to kidneys) that
divides into the capillaries of the glomerulus
oSupplies blood to the nephrons
Efferent Arteriole: Formed when capillaries converge as they leave the
glomerulus
oThe efferent arteriole subdivides again to form the peritubular capillaries
oPeritubular Capillaries: Surrounds the proximal and distal convoluted
tubules
Additional capillaries extend downwards to form the vasa recta
oVasa Recta: Serves the loop of Henle
The Nephron Organization of Blood Filtration
1. Proximal Convoluted Tubule
Reabsorption of ions such as K+, Na+, Cl-, HCO3- back into blood
Water moves in by osmosis to the blood
Nutrients such as glucose, and amino acids are reabsorbed into the blood
o90% of HCO3- is reabsorbed
oThis contributes to pH balance in body fluids like the blood
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