Download File

Biology 12
13.2 Anatomy of the Kidney and Excretion
The kidney’s blood supply is supported by many branches of the renal vein and renal artery.
A kidney has three regions.
• Renal cortex: outer, granulated layer
• Renal medulla: contains cone-shaped tissue masses called renal pyramids
• Renal pelvis: central space that is continuous with the ureter
The kidney is composed of over 1 million nephrons, also known
as renal or kidney tubules.
Blood Supply in a Nephron
Each nephron has it own blood supply.
• From the renal artery, an afferent arteriole leads to the
glomerulus, a knot of capillaries inside the glomerular
capsule
• Blood leaving the glomerulus enters the efferent
arteriole, which takes blood to the peritubular capillary
network, before exiting through the renal vein
Biology 12
Parts of a Nephron
Glomerular capsule (Bowman’s capsule)
• Inner layer composed of cells called podocytes
• Spaces between podocytes allow small molecules from the glomerulus to enter the
glomerular capsule
Proximal convoluted tubule (PCT)
• Lined with cuboidal epithelial cells that have packed microvilli to increase the surface
area for reabsorption
o Also contain several mitochondria to carry out active reabsorption of substances
Loop of Henle
• Also known as the loop of the nephron
• Consists of a descending limb and an ascending limb
• Lined with simple squamous epithelium
Distal convoluted tubule (DCT)
• Composed of cuboidal epithelial cells that lack microvilli but have many mitochondria
• Helps move molecules from the blood into the tubule (tubular
secretion)
• The DCTs of many nephrons enter one collecting duct
• Collecting ducts carry urine to the renal pelvis
Urine Formation
Urine formation is divided into the following processes:
• Pressure filtration: water, salts, nutrients, and wastes move from
the glomerulus to the inside of the glomerular capsule
• Selective reabsorption: nutrient and salt molecules are actively
reabsorbed from the convoluted tubules into the blood of the
peritubular capillary network
• Tubular excretion: certain molecules are actively secreted from
the blood in the peritubular capillary network into the
convoluted tubules
Pressure filtration occurs when blood enters the afferent arteriole and
the glomerulus.
• Due to blood pressure, water and small molecules move from the glomerulus to the
inside of the glomerular capsule
o These filterable blood components form the glomerular filtrate
Biology 12
Selective Reabsorption
In selective reabsorption, nutrients and ions from the glomerular filtrate in the convoluted
tubules are reabsorbed into the blood of the peritubular capillary network.
Examples:
• Na+ ions are actively reabsorbed into the blood, with Cl- ions following passively
o This reabsorption of salt (NaCl) results in water moving passively from the tubule
into the blood
• Glucose and amino acids are actively reabsorbed into the blood almost exclusively at the
proximal convoluted tubule
The glomerular filtrate that enters the proximal convoluted tubule is divided into two portions.
• Reabsorbed filtrate components: reabsorbed from the tubule into blood
• Nonreabsorbed filtrate components: continue to pass through the nephron to be
processed into urine (become the tubular fluid that enters the loop of Henle)
Tubular Excretion
Tubular excretion is another way substances are removed from blood in the peritubular
capillary network and added to the tubular fluid.
• Hydrogen ions, potassium ions, creatinine, and certain drugs are removed by active
transport from the blood in the peritubular capillary network into the convoluted
tubules
• The resulting urine contains:
o Substances that have undergone glomerular filtration but have not been
reabsorbed
o Substances that have undergone tubular excretion