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 NephronEach 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

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Parts of a NephronGlomerular 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 reabsorptiono 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 FormationUrine 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 capsuleo These filterable blood components form the glomerular filtrate

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Selective ReabsorptionIn 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 ExcretionTubular 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

reabsorbedo Substances that have undergone tubular excretion