1. Chapter 16: Urinary System and Excretion

2. THE NEPHRON
Each kidney contains about one million microscopic filtering tubules called NEPHRONS

3. Anatomy of a Nephron Each nephron has its own blood supply.
An afferent arteriole approaches the glomerular capsule and divides to become the glomerulus, a knot of capillaries.
The efferent arteriole leaves the capsule and branches into the peritubular capillary network.
Afferent = Going Toward, Arriving.
Efferent = Leaving, Exiting
Blood pressure is higher in the glomerulus because the efferent arteriole is narrower than the afferent arteriole.
Peritubular = Around the Tubule

4. Nephron anatomy
A nephron is made up of a glomerular capsule, the proximal convoluted tubule, the loop of the nephron, the distal convoluted tubule, and the collecting ducts. You can trace the path of blood about the nephron by following the arrows.

5. Parts of a Nephron
The closed end of the nephron is a cuplike Glomerular/Bowman’s capsule.
Spaces between podocytes of the glomerular capsule allow small molecules to enter the lumen from the glomerulus via glomerular/pressure filtration.
Microvilli make up the inner brush border of the proximal convoluted tubule; these microvilli increase the surface area available for absorption.

6. Proximal convoluted tubule
The cuboidal epithelial cells of the proximal convoluted tubule have many mitochondria and microvilli to carry out active transport (following passive transport) from the tubule to blood.
The photomicrograph (left) shows that the cells lining the proximal convoluted tubule have a brushlike border composed of microvilli, which greatly increase the surface area exposed to the lumen. The peritubular capillary network surrounds the cells. The diagrammatic representation (right) shows that each cell has many mitochondria, which supply the energy needed for active transport, the process that moves molecules (green) from the lumen of the tubule to the peritubular capillary, as indicated by the arrows.

7. The descending loop of the nephron (Loop of Henle) allows water to leave and the ascending portion extrudes salt.
Active transport is used in the upper portion of the ascending limb to drive out Sodium ions, Chloride ions passively follow.
Each part of a nephron is anatomically suited to its specific function in urine formation.

8. The cuboidal epithelial cells of the distal convoluted tubule have numerous mitochondria but lack microvilli.
They carry out active transport from the blood to the tubule or tubular secretion.
Collecting ducts gather in the renal medulla and form the renal pyramids.

10. 1. Glomerular Filtration
Urine Formation
1. Glomerular Filtration
During glomerular filtration, small molecules including water, wastes, and nutrients are forced from the blood inside the glomerulus to the inside of the glomerular capsule.
Blood cells, platelets, and large proteins do not move across.
About 180 liters of water are filtered daily.
Urine formation is divided into three steps: glomerular filtration, tubular reabsorption, and tubular secretion.

11. Reabsorption from Nephrons
Substance
Amount Filtered
Amount Excreted
Reabsorp-tion (%)
Water, L
180
1.8
99.0
Sodium, g
630
3.2
99.5
Glucose, g
0.0
100.0
Urea, g 54
30.0
44.0
Amount filtered and amount excreted are amounts per day.

12. 2. Tubular Reabsorption
During tubular reabsorption, certain nutrients, water and some urea moves from the proximal convoluted tubule into the blood of the peritubular capillary network.
Tubular reabsorption is a selective process because only molecules recognized by carrier molecules are actively reabsorbed.
The rate of this process is limited by the number of carriers.

13. 3. Tubular Secretion
During tubular secretion, specific substances such as hydrogen ions, creatinine, and drugs such as penicillin move from the blood into the distal convoluted tubule.
In the end, urine contains substances that have undergone glomerular filtration but have not been reabsorbed, and substances that have undergone tubular secretion.

14. 4. Water Reabsorption
Water is reabsorbed all along the Nephron, as tubular reabsorption of salts, glucose, amino acids occurs at the proximal convoluted tubule, water will passively follow.
As filtrate moves down the Descending Limb of Henle, salt and urea molecules suck water out of the filtrate.
As filtrate moves down the Collecting Ducts, cells are permeable to water, this allows the salts and urea outside the collecting duct to suck water into the tissue fluid.

15. 5. Excretion of Urine

16. Steps in urine formation
The three main steps in urine formation are glomerular filtration, tubular reabsorption, and tubular secretion.
During glomerular filtration, water, salts, nutrient molecules, and waste molecules move from the glomerulus to the inside of the glomerular capsule. These small molecules are called the glomerular filtrate.
During tubular reabsorption, nutrient and salt molecules are actively reabsorbed from the proximal convoluted tubule into the peritubular capillary network, and water flows passively.
During tubular secretion, certain molecules are actively secreted from the peritubular capillary network into the distal convoluted tubule.
In the end, urine is composed of the substances within the collecting duct (water, salts, urea, uric acid, ammonia, and creatinine.