Human Respiratory and Digestive Systems Explained

Posted on Apr 5, 2025 in Biology

Respiratory System Anatomy and Function

Airways: Path of Air

These tubes conduct air containing oxygen into the lungs and carbon dioxide (CO2) out.

  • Nasal Cavity: Air enters through the nostrils. The inside walls have capillaries, which warm the incoming air.
  • Pharynx: A common cavity for the digestive and respiratory systems. The tonsils, which produce white blood cells, are located on the sides of the pharynx.
  • Larynx: Its entrance is controlled by the epiglottis, a flap that closes during swallowing to prevent food from entering the airway.
  • Trachea: A tube approximately 12cm long, supported by C-shaped rings of cartilage to keep it open.
  • Bronchi and Bronchioles: The trachea divides into two main bronchi, each leading to a lung. Inside the lungs, the bronchi split into progressively smaller tubes called bronchioles.

Lungs and Gas Exchange

The lungs are the primary organs where gases are exchanged between the blood and the air. This process occurs in tiny air sacs called alveoli at the end of the bronchioles.

Inhalation (Breathing In)

The diaphragm contracts and moves downward. The rib muscles contract, lifting the rib cage up and out. This increases the volume of the chest cavity and lungs, causing air to rush in.

Gas Exchange

Oxygen (O2) from the inhaled air passes from the alveoli into the surrounding capillaries (blood vessels). Carbon dioxide (CO2) passes from the blood into the alveoli to be exhaled.

Exhalation (Breathing Out)

The diaphragm relaxes and moves upward. The rib muscles relax, allowing the rib cage to get smaller. This decreases the volume of the chest cavity, forcing air out of the lungs.

Common Respiratory Diseases

  • Common Cold: An infection in the upper respiratory tract, typically caused by a virus.
  • Bronchiolitis: A common viral infection affecting the lining of the small airways (bronchioles), primarily in infants and young children.
  • Pneumonia: An infection that inflames the air sacs (alveoli) in one or both lungs. The air sacs may fill with fluid or pus.
  • Emphysema: A lung condition characterized by gradual damage to the alveoli, leading to shortness of breath. It’s often associated with smoking.
  • Sinusitis: Inflammation or swelling of the tissue lining the sinuses, often caused by infection (bacterial, viral, or fungal).
  • Asthma: A chronic condition where the airways narrow, swell, and may produce extra mucus, leading to difficulty breathing, coughing, and wheezing.

Digestive System: Organs and Processes

The Digestive Tract

A continuous tube, approximately 8 meters long, extending from the mouth to the anus. It varies in width and includes the following main parts:

  • Mouth: The cavity where food intake occurs. Food undergoes three initial processes here: insalivation (mixing with saliva), mastication (chewing), and swallowing.
    • Saliva: A watery substance secreted by salivary glands, containing enzymes to start digestion.
    • Tongue: A muscular organ that aids in manipulating food for chewing and swallowing; covered with taste buds.
    • Mastication: The process of breaking down food into smaller pieces using the teeth.
    • Teeth: Hard, bone-like structures used for cutting, tearing, and grinding food.
  • Pharynx: Connects the mouth to the esophagus; also part of the respiratory system.
  • Esophagus: A muscular tube connecting the pharynx to the stomach. Muscular contractions (peristalsis) move food downwards.
  • Stomach: A J-shaped organ that mixes food with digestive juices. The valve at the entrance is the cardia, and the valve at the exit is the pylorus.
  • Small Intestine: A long, coiled tube (6-7 meters) where most nutrient absorption occurs. It is divided into three sections: the duodenum, jejunum, and ileum.
  • Large Intestine: Absorbs water and electrolytes from remaining indigestible food matter and transmits the useless waste material from the body. Its parts include the caecum, colon, and rectum.
  • Anus: The opening at the end of the digestive tract where feces are eliminated.

Accessory Digestive Organs

These organs assist digestion but are not part of the continuous digestive tract.

  • Liver: The body’s largest gland, reddish-brown in color. It produces bile, essential for fat digestion, which is stored in the gallbladder.
  • Gallbladder: Stores and concentrates bile produced by the liver.
  • Pancreas: A glandular organ located behind the stomach. It produces digestive enzymes (pancreatic juice) and hormones like insulin and glucagon that regulate blood sugar.

Digestive Juices and Their Functions

  • Saliva: Secreted in the mouth, begins the digestion of starch (carbohydrates) and lubricates food. It also contains antibacterial compounds.
  • Gastric Juices: Produced in the stomach, contain enzymes like pepsin (for protein digestion) and strong hydrochloric acid to kill bacteria and aid digestion.
  • Bile: Produced by the liver and stored in the gallbladder, released into the small intestine. Contains bile salts that emulsify fats (break them into smaller droplets).
  • Pancreatic Juice: Produced by the pancreas and released into the small intestine. Contains enzymes that digest proteins, lipids (fats), and carbohydrates.
  • Intestinal Juice: Secreted by the walls of the small intestine. Contains enzymes that complete the digestion of proteins, lipids, and carbohydrates.

Common Digestive Diseases

  • Appendicitis: Inflammation and infection of the appendix (a small pouch attached to the large intestine). If untreated, it can rupture (burst), causing serious complications.
  • Cirrhosis: A late stage of scarring (fibrosis) of the liver caused by many forms of liver diseases and conditions, such as chronic alcoholism and hepatitis.
  • Diarrhea: Characterized by frequent, loose, watery bowel movements. Can lead to dehydration if severe or prolonged.
  • Constipation: Characterized by infrequent bowel movements or difficulty passing stools, often resulting in hard, dry stools.
  • Celiac Disease: An autoimmune disorder where ingesting gluten (a protein found in wheat, barley, and rye) leads to damage in the lining of the small intestine.
  • Dental Cavities (Caries): Breakdown and decay of teeth due to acids produced by bacteria feeding on sugars in the mouth.