Human Biology: Digestion, Respiration, Circulation, and Excretion

Digestive Function

Ingestion: Intake of food into the digestive tract through the mouth. Food is crushed by the teeth, mixed with saliva, and begins digestion.

Digestion: Processing of food into nutrients that can be utilized by cells. This occurs through two types of actions:

• Mechanical: Physically breaking down food through cutting, grinding, and churning. This occurs in the mouth and stomach.
• Chemical: Transforming food into simpler compounds using substances that cause chemical changes. This starts with saliva in the mouth.

Absorption: Small molecules pass into the bloodstream to be carried to cells. Most absorption occurs in the jejunum and ileum of the small intestine, where the inner walls are highly folded into villi containing blood vessels. These vessels recognize and distribute the digested substances throughout the body.

Egestion: Undigested waste accumulates in the rectum and is periodically eliminated through the anus as feces.

Saliva: Produced by the salivary glands and contains:

• Mucin: Acts as a lubricant and has antibacterial properties.
• Digestive Enzymes (Amylase): Begins the digestion of carbohydrates.

Respiration

Respiration is the process by which our body captures oxygen from the air and expels carbon dioxide produced by cells. This involves:

• Ventilation: Air movement in and out of the lungs.
• Gas Exchange: Oxygen enters the blood from the air in the lungs, and carbon dioxide moves from the blood to the air.

Respiratory Pathway

• Nasal Passages: Interior is coated with mucus to trap microorganisms and dust. The pituitary gland heats, humidifies, and cleans the air.
• Pharynx: Contains the tonsils, which have defensive cells. The epiglottis closes the entrance to the larynx to prevent food from entering the respiratory tract.
• Larynx: Short tube containing the vocal cords, which vibrate as air passes to produce sound.
• Trachea: Tube about 12cm long, reinforced with cartilage rings to keep it open. Lined with hair cells and mucus-producing cells to trap microorganisms.
• Bronchi: Two tubes branching from the trachea, one leading to each lung. These further branch into bronchioles and finally into tiny air sacs called alveoli.
• Lungs: Two spongy organs protected by the ribs and enclosed by two membranes called pleura. The lungs are divided into lobes.

Mechanics of Respiration

• Inspiration (Inhalation): An active process where the intercostal muscles contract, the diaphragm flattens, and abdominal muscles relax, increasing the volume of the thoracic cavity and drawing air into the lungs.
• Expiration (Exhalation): A passive process where the intercostal muscles and diaphragm relax, abdominal muscles contract, decreasing the volume of the thoracic cavity and expelling air from the lungs.

Circulatory System

The circulatory system transports substances throughout the body, including hormones and immune cells. It consists of the heart (pump), blood (liquid), and blood vessels (ducts).

Blood Composition:

• Plasma: Thick yellow liquid composed mainly of water and other substances.
• Red Blood Cells (Erythrocytes): Cells without a nucleus that carry oxygen.
• White Blood Cells (Leukocytes): Part of the immune system.
• Platelets (Thrombocytes): Involved in blood clotting.

Excretory System

Excretory Organs:

• Kidneys: Produce urine and perform other functions.

Elimination Pathway:

• Ureters: Carry urine from the kidneys to the bladder.
• Bladder: Stores urine.
• Urethra: Channel for urine to exit the body.

Blood is filtered by the nephrons in the kidneys, removing waste products and excess substances to form urine. Excretion is the process of eliminating metabolic waste products, maintaining homeostasis by regulating fluid and salt balance, and controlling the concentrations of body fluids.