Question

In humans, digestion is completed and most nutrients are absorbed in the ________. a. mouth b. stomach c. small intestine d. rectum

Short answer

The small intestine.

Step-by-step solution

Understanding Digestion Phases

First, we need to understand where the major phases of digestion occur. Digestion starts in the mouth, where food is broken down mechanically and mixed with saliva. However, only minimal absorption of nutrients occurs in the mouth.

Roles of the Stomach and Rectum

Next, we consider the stomach, where food is mixed with gastric juices, further breaking it down. Nevertheless, the stomach primarily deals with digestion, not absorption. The rectum is part of the large intestine involved in waste storage and elimination, not nutrient absorption.

Identifying the Small Intestine Function

The small intestine is the main site for nutrient absorption. After partial digestion in the stomach, food moves to the small intestine, where enzymes complete digestion, and absorption of nutrients into the bloodstream occurs.

Conclusion About Digestion and Absorption

Considering all options, digestion is completed and most nutrients are absorbed in the small intestine. This makes the small intestine the correct answer for where these processes predominantly occur.

Key concepts

Nutrient Absorption

Nutrient absorption is a vital component of human digestion that occurs after food is broken down into smaller, absorbable molecules. These nutrients include carbohydrates, proteins, fats, vitamins, and minerals.
During digestion, food is broken down into molecules such as glucose, amino acids, and fatty acids. These smaller molecules are absorbed from the digestive tract into the bloodstream, where they are then transported to cells throughout the body. This process ensures that the body receives necessary energy and building blocks for growth and repair.
Effective nutrient absorption is influenced by several factors including the health of the digestive tract, the presence of digestive enzymes, and the duration food remains in the digestive system. Disorders or diseases affecting the digestive system can impair nutrient absorption, leading to potential nutritional deficiencies.

Small Intestine

The small intestine is a long, coiled tube that plays a central role in digestion and nutrient absorption. It can be broken down into three parts: the duodenum, jejunum, and ileum, each with specific functions.
1. **Duodenum** - This is the first segment of the small intestine. Here, food mixes with digestive enzymes and bile to continue the breakdown process. 2. **Jejunum** - The middle segment, where the majority of nutrient absorption occurs. Its lining is covered in tiny, finger-like projections called villi, which increase surface area to maximize absorption.
3. **Ileum** - The final part of the small intestine, responsible for absorbing any remaining nutrients not absorbed by the jejunum.
In total, the small intestine accounts for about 90% of nutrient absorption, making it an essential component for sustaining life.

Digestive System Phases

The digestive system works in phases to efficiently break down food and absorb nutrients. Each phase involves different organs and processes.

• **Ingestion** - The phase where you take food into your mouth. Includes the mechanical breakdown of food by chewing.
• **Digestion** - Starts in the mouth and continues in the stomach and small intestine, involving the breakdown of food with enzymes and stomach acids.
• **Absorption** - Primarily takes place in the small intestine, where nutrients pass into the bloodstream.
• **Egestion** - The removal of waste from the body. This happens as feces are eliminated through the rectum and anus.

Understanding these phases helps in recognizing how nutrients are processed and why the small intestine is so important for absorption.

Enzymes in Digestion

Enzymes are catalysts that significantly speed up the chemical reactions during digestion, breaking down large macromolecules into their component parts.
Different types of enzymes target specific types of nutrients:

• **Amylase** - Found in saliva and the pancreas, breaks down carbohydrates into sugars.
• **Proteases** - Like pepsin in the stomach, break down proteins into amino acids.
• **Lipase** - Secreted by the pancreas, helps in breaking down fats into fatty acids and glycerol.
• **Lactase, sucrase, maltase** - Found in the small intestine, help in breaking down sugars like lactose, sucrose, and maltose.

The role of enzymes is crucial as they allow the digestive system to efficiently process and absorb nutrients. Without enzymes, digestion would be incredibly slow or ineffective, leading to malnutrition and other health issues.