The Excretory System: Kidneys and the Nephron

Your body's water treatment plant

Toronto's water treatment plants take in water from Lake Ontario, remove what shouldn't be there and send clean water back out. Your kidneys do something similar for your blood, every minute of your life. About a fifth of the blood your heart pumps at rest goes straight to your kidneys. They filter it, keep everything useful, and send the rest out as urine.

In this lesson you will learn what the kidneys remove, how they are built, and the three processes (filtration, reabsorption and secretion) that happen in each of their millions of tiny filtering units, the nephrons. In the next two lessons, you will see how the same machinery controls the body's water, salt and pH.

Why excrete? The problem with nitrogen

When your cells break down proteins and nucleic acids, they release nitrogen. In the liver, amino acids that are not needed are deaminated: their amino group (–NH₂) is removed and becomes ammonia (NH₃). The rest of the molecule can be used for energy, as you saw in Metabolic Processes.

Ammonia is very toxic, even in small amounts, because it disturbs pH and harms nerve cells. Animals deal with it in three ways:

• Ammonia: fish and many aquatic invertebrates release ammonia directly into the water, which dilutes it right away. It costs little energy, but needs lots of water.
• Urea: mammals (including us) and adult amphibians convert ammonia to urea in the liver, using the urea cycle. Urea is far less toxic than ammonia, so it can be carried safely in the blood to the kidneys. Making it costs some ATP.
• Uric acid: birds, reptiles and insects make uric acid, which is not very soluble and can be excreted as a paste with very little water. This is a big advantage for animals that lay shelled eggs or live in dry places, but it costs the most energy.

Other organs also excrete wastes: the lungs remove CO₂, the skin loses some water, salts and urea in sweat, and the liver breaks down old red blood cells and many drugs. But the kidneys do the most to control the makeup of the blood.

The urinary system

• Kidneys: two bean-shaped organs, each about the size of a fist, against the back wall of the abdomen just above the waist.
• Renal arteries bring blood from the aorta; renal veins return filtered blood to the inferior vena cava.
• Ureters: tubes that carry urine from each kidney to the bladder, moved along by waves of smooth muscle contraction (peristalsis).
• Urinary bladder: a stretchy muscular bag that stores urine. Stretch receptors in its wall signal when it is filling.
• Urethra: the tube that carries urine out of the body. Sphincter muscles control its opening.

If you cut a kidney lengthwise, you see three regions: the outer renal cortex, the inner renal medulla (arranged in cone-shaped renal pyramids), and the hollow renal pelvis, which collects urine and funnels it into the ureter.

The nephron: about a million filters per kidney

Each nephron is a long, thin tube with a filter at one end. Follow it in order:

1. Glomerulus: a tight ball of capillaries. Blood enters through the afferent arteriole and leaves through the narrower efferent arteriole. Because the exit is narrower than the entrance, blood pressure in the glomerulus is high.
2. Bowman's capsule: a cup that surrounds the glomerulus and catches what is filtered.
3. Proximal convoluted tubule (PCT): a twisted section in the cortex. Its cells are covered with microvilli and packed with mitochondria, because they do a lot of active transport.
4. Loop of Henle: a U-shaped section that dips down into the medulla. It has a descending limb and an ascending limb.
5. Distal convoluted tubule (DCT): a second twisted section back in the cortex.
6. Collecting duct: many nephrons empty into one collecting duct, which runs down through the medulla to the renal pelvis.

After the efferent arteriole leaves the glomerulus, it branches into the peritubular capillaries, which wrap around the tubule. These capillaries pick up everything the tubule reabsorbs.

Three processes: filtration, reabsorption, secretion

1. Glomerular filtration. The high pressure in the glomerulus pushes fluid through the capillary walls and into Bowman's capsule. The filter has tiny pores, so water, glucose, amino acids, ions, urea and vitamins get through, but blood cells and large proteins (such as albumin) stay in the blood. Filtration is not selective by usefulness, only by size. Your kidneys make about 180 L of filtrate per day, yet you produce only about 1 to 2 L of urine. So most of the filtrate must be taken back.

2. Tubular reabsorption. Useful substances move from the filtrate back into the blood:

• In the PCT, carrier proteins actively transport Na⁺ out of the tubule. Glucose and amino acids are brought in along with Na⁺ (a kind of co-transport you met in Biochemistry). Normally all of the glucose and amino acids are reabsorbed here, along with about two-thirds of the water and Na⁺. Negative ions follow the positive Na⁺, and water follows by osmosis.
• In the loop of Henle, the descending limb is permeable to water (water leaves), while the ascending limb is not permeable to water but pumps out salt. This makes the medulla very salty, which later lets the collecting duct pull water out of the urine.
• In the DCT and collecting duct, the amount of Na⁺ and water reabsorbed is adjusted by hormones (aldosterone and ADH), depending on what the body needs.

Reabsorption has limits. Carriers can only move so much at once. If blood glucose rises above about 10 mmol/L, the carriers in the PCT are saturated and glucose appears in the urine, a classic sign of untreated diabetes.

3. Tubular secretion. Cells of the tubule (mostly in the DCT) actively move substances from the blood into the filtrate: H⁺ ions (to control pH), K⁺ ions, and some drugs (such as penicillin) and toxins. This removes substances that were not filtered enough or that are present in excess.

The result is urine: water, urea, excess salts, H⁺ and other wastes. It flows through the collecting ducts to the renal pelvis, down the ureter and into the bladder.