Human Digestive and Respiratory Systems: A Detailed Look

The Human Digestive System

The digestive process begins in the **mouth**. The first processes are chewing, conducted by teeth (*incisors, canines, molars, and premolars*), and salivation, managed by a muscular organ called the tongue. The **tongue** moves food during chewing and contains **taste buds** for sensing flavor. During salivation, food is broken down by teeth and moistened by **saliva**, a watery secretion produced by the **salivary glands**. Saliva production can be triggered by the sight or smell of food. Saliva contains **mucin**, which gives it viscosity, and enzymes such as **salivary amylase** and **lingual lipase** (which hydrolyzes milk fat).

The ground food and saliva form the **bolus**, which is pushed into the **esophagus** through the **pharynx** during swallowing. The nasal cavities close off from the pharynx, the **trachea** rises and is closed by the **epiglottis**. When the bolus reaches the esophagus, **peristaltic movements** facilitate its transition. No enzymes are involved, but mucus acts as a lubricant for the bolus.

The bolus reaches the **stomach** when the **cardia** opens. The cardia connects the esophagus and stomach. The stomach is a sac that stores food and secretes **gastric juice**, containing **hydrochloric acid (HCl)**, mucin, and **pepsin**. HCl maintains a pH of around 2, killing bacteria. Pepsin hydrolyzes proteins. Mucin prevents damage to the stomach walls by forming a protective layer. In infants, a protein ensures that milk remains in the stomach for a longer period. The **pyloric sphincter** opens to release **chyme**, the product of this process, into the **duodenum**.

The **small intestine** is a tube approximately 8 meters long, consisting of the duodenum, **jejunum**, and **ileum**. It is lined by **microvilli** that increase the surface area for absorption. Most digestion occurs in the duodenum, where **pancreatic juice** and **bile** are introduced.

• The **liver**, located below the **diaphragm**, produces bile, a yellowish fluid composed of salts, water, pigments, and cholesterol. Bile emulsifies fats.
• The **pancreas**, located below the stomach, produces pancreatic juice, which contains **sodium bicarbonate**. This neutralizes the acidity of the chyme and facilitates enzyme action.

**Lipase** and **amylase** hydrolyze fats and polysaccharides, respectively. **Trypsin** digests proteins. The intestinal mucosa also produces intestinal juice. **Sucrase**, **maltase**, and **lactase** hydrolyze sucrose. When enzymes have acted on the chyme, it becomes **chyle**, a viscous, white substance.

Intestinal Absorption

Intestinal absorption is the passage of food through the walls of the digestive tract into the blood. Nutrients are absorbed into the blood through the **villi** of the small intestine. The small intestine transports blood to the liver via veins. Absorption occurs through osmosis or diffusion. Glucose and galactose are absorbed by active transport, as are amino acids. Fatty acids enter the mucosal cells and are converted back into fats. These fats combine with cholesterol and proteins to form **chylomicrons**.

The **large intestine** is about 2 meters long and has three parts:

• The **cecum**, a short, sac-like structure where the **appendix** is located. Inflammation of the appendix causes **appendicitis**.
• The **colon**, which has three parts: ascending, transverse, and descending.
• The **rectum**, the last centimeter of the intestine, leading to the **anus**.

Most nutrients from the chyme have been absorbed in the small intestine. The remaining bulk passes into the large intestine, where water and sodium are absorbed, along with the last remaining nutrients. Undigested food passes into the anal canal, water is absorbed, and the resulting solid waste is evacuated through the rectum and anus.

Pulmonary Ventilation

Pulmonary ventilation is the process of renewing air in the lungs through **inspiration** and **expiration**.

During inspiration, the diaphragm contracts, the ribs move upward, and the volume of the **thoracic cavity** increases. This causes the pressure to fall below atmospheric pressure, and air enters through the airways.

During expiration, the **intercostal muscles** and diaphragm relax and return to their original positions. The volume of the rib cage and lungs decreases, and air is expelled.