Unit 15: The Digestive and Excretory Systems
The path food takes from your first bite to the nutrients your cells actually use, and how your kidneys clean your blood.
6 topics · 18 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
How Digestion Breaks Food Into Usable Molecules
- How it worksHow do mechanical and chemical digestion work together to break large food molecules into small ones?
- Common mistakeWhy isn't digestion just physically grinding food into smaller pieces?
- Going furtherWhat happens to a protein-rich meal once digestive enzymes start breaking its peptide bonds?
Your turn1 of 2
A bite of steak is made of large protein molecules that your small intestine cells can't take in directly, no matter how thoroughly you chew it. Why can't your cells absorb a bite of steak directly without breaking it down first?The Journey of Food Through the Digestive Tract
- How it worksWalk me through how food moves from the mouth through the esophagus, stomach, and intestines.
- How it worksHow do rhythmic muscle contractions called peristalsis push food along the digestive tract?
- Common mistakeWhy doesn't gravity alone move food down your esophagus into your stomach?
Your turn1 of 2
Food travels from mouth to the end of the digestive tract along what is anatomically just one long continuous tube, yet a full meal can take many hours to make that trip. Why does food take hours to travel a path that's really just one continuous tube?The Role of the Liver and Pancreas in Digestion
- How it worksHow does bile from the liver break large fat globules into smaller droplets for digestion?
- How it worksHow does the pancreas release enzymes that continue breaking down carbohydrates, proteins, and fats?
- Common mistakeWhy isn't bile itself an enzyme that chemically breaks down fat molecules?
Your turn1 of 2
The liver and pancreas sit beside the digestive tract rather than being part of the actual tube food travels through, yet both organs are essential for fully digesting a meal. Why does a high-fat meal need help from an organ that isn't even part of the digestive tract itself?Absorption in the Small Intestine
- How it worksHow do villi and microvilli increase the surface area available for nutrient absorption?
- Worked exampleFind how much more surface area a folded, villi-covered intestine provides compared to a smooth tube of the same length.
- Common mistakeWhy doesn't nutrient absorption happen mainly in the stomach instead of the small intestine?
Your turn1 of 2
Folding the small intestine's lining into villi and microvilli increases its absorptive surface area by a factor of about 424 compared to a smooth tube of the same length. If a smooth tube of that length would have 2.18 square meters of surface area, what is the folded intestine's actual surface area?The Kidneys and Filtering Blood
- How it worksHow does a nephron filter blood plasma and then reabsorb the substances the body still needs?
- Common mistakeWhy doesn't the kidney simply filter out and discard everything in the blood?
- Going furtherWhat happens to waste filtration when kidney function starts to fail?
Your turn1 of 2
Blood circulates through the kidneys and gets filtered roughly 339 times over the course of a day. If each individual pass through the kidneys filters about 0.247 liters of blood plasma, how many liters of plasma pass through kidney filtration over that day?How the Kidneys Balance Water and Salt
- How it worksHow does antidiuretic hormone signal the kidneys to reabsorb more or less water based on blood concentration?
- Common mistakeWhy doesn't your body just get rid of excess salt through sweat alone?
- Ties togetherHow does the kidney's water and salt balance connect to the negative feedback loops covered earlier?
Your turn1 of 2
Eating a large bag of salty chips often leaves you noticeably thirstier within the hour, well before you've had any chance to actually lose extra water. Why does eating a lot of salty food make you feel thirsty afterward?