Digestive, Circulatory, Respiratory and Nervous Systems — O/L Science

The human body is organised into organ systems that work together to maintain life. The circulatory system (heart, blood, blood vessels) transports oxygen, nutrients, hormones, and waste. The double circulation in humans has two loops: pulmonary (heart ↔ lungs) and systemic (heart ↔ body).

The respiratory system (lungs, trachea, bronchi, alveoli) exchanges gases — oxygen diffuses into blood; CO₂ diffuses out. The digestive system (mouth → stomach → small intestine → large intestine) breaks food into nutrients via enzymes. The nervous system (brain, spinal cord, nerves) receives stimuli and coordinates responses. The endocrine system uses hormones (chemical messengers) to regulate growth, metabolism, and reproduction.

All systems are interdependent. A respiratory infection raises the heart rate (circulatory system responds); digestive problems reduce nutrient supply to all systems. Health is the balanced functioning of all systems simultaneously.