Chapter 11.9: Excretory System

Table of Contents

• Osmoregulation
• The Kidney
• The Nephron
• Filtration
• Reabsorption
• Secretion
• Excretion
• Hormones
• General Pathway
• Optional: Chapter 11.9 YouTube Video Playlist

Preview

This chapter will talk about human excretion. Excretory systems of other animals will be discussed in the Diversity of Life chapter. The excretory system is necessary for humans to filter out metabolic wastes from the body’s fluids and eliminate them as urine, all while making sure the body retains the water and solutes it needs to survive.

Osmoregulation

Osmoregulation is the absorption and excretion of water and solutes, so an organism can maintain a proper water balance. The difference between marine fish and freshwater fish is the most commonly used example:

1. Marine (saltwater) fish. Marine fish are hypoosmotic to their environment. They are less salty than the surrounding saltwater. Therefore, they’re constantly losing water to their environment. As a result, marine fish:
   1. Constantly drink water
   2. Rarely urinate
   3. Secrete accumulated salts through their gills
2. Freshwater fish. Freshwater fish are hyperosmotic to their environment. They are more salty than the surrounding freshwater. Therefore, they’re constantly absorbing too much water. As a result, freshwater fish:
   1. Rarely drink water
   2. Constantly urinate
   3. Absorb salts through their gills

The Kidney

Although there are several major players in the excretory system, the most significant and prominent one is the kidney. Humans have two kidneys. The top, outer portion of the kidney is called the cortex (this is where the blood enters the kidneys). The middle portion is called the medulla. The very inner portion is called the pelvis (this is where the filtrate exits the kidneys).