KIDNEY FUNCTIONS URINE FORMATION

Learning Outcomes:

• Describe the function of each section of the nephron

• Describe the process of urine formation

• Describe how the kidneys maintain blood pH

Urine Composition

• Water

• Nitrogenous wastes (urea etc.)

• Salts

• May contain H+ and other ions

• May contain drugs, vitamins and other substances

Urine Formation

3 major steps:

1. Glomerular filtration - in the glomerular capsule

2. Tubular reabsorption - in the proximal tubule and loop of Henle

3. Tubular secretion - distal tubule

• Different parts of the nephron have different permeability characteristics

• Substances are filtered out of the blood by the nephron and excreted in urine

• Some substances are reabsorbed back into the blood from the nephron

Pressure Filtration

• Blood enters the glomerulus from the afferent arteriole

• Blood pressure forces small molecules out of the capillaries into the capsule

• Substances that leave the blood:

Water, salts, nutrients (glucose, amino acids), nitrogenous wastes (urea)

• Filtrate resembles plasma

Selective Reabsorption

• At the proximal tubule and loop of Henle

• Some substances must be reabsorbed back into the blood

• Examples:

• Water, nutrients (glucose, amino acids), salts

• Na+ ions, glucose, amino acids - move back into the peritubular capillaries by active transport (why active transport?)

• Cl- ions follow passively

• Water is reabsorbed by osmosis

Absorption of Water at the Loop of Henle

• Function of the loop of Henle is to remove as much water as possible from the filtrate and return it to the blood

• Descending limb is permeable to water but not salt

• Ascending limb is permeable to salt but not water

• Sodium is actively pumped out of the ascending limb

• Medulla is hypertonic to the filtrate (contains high [Na+])

• Water leaves the tubule by osmosis and enters the medulla, then the capillaries

Fig. 16.7

Fig. 16.6

Tubular Secretion

• At the distal tubule• Excess H+ ions, drugs,

and other substances move from the blood into the nephron by active transport

• Ions secreted or kept in the blood as needed, to maintain correct pH

Reabsorption of Water

• More water is reabsorbed into the blood from the collecting duct

• Collecting duct is in the hypertonic medulla so water is drawn out by osmosis

Fig. 16.6

• Urine is now ready to be excreted from the body

• Collects in the renal pelvis before going to the bladder

Path of Blood

1. Renal artery & arterioles

2. Afferent arteriole

3. Glomerulus

4. Efferent arteriole

5. Peritubular capillary network

6. Venules to renal vein

Questions

• How is the composition of blood in the renal artery different from that in the renal vein?

• What is the importance of the hypertonic condition of the renal medulla?

• Why is glucose transported actively instead of passively at the proximal tubule?

Fig. 16.6

Fig. 16.6