O-Level Biology Human Body Systems: The Complete Revision Guide

Human body systems — digestive, circulatory, respiratory, nervous, and endocrine — collectively account for a large portion of O-Level Biology marks. Understanding how these systems work individually and how they interact with each other is essential for scoring A1.

The Digestive System

Key processes: ingestion → digestion (mechanical and chemical) → absorption → assimilation → egestion. Know the enzymes: amylase (starch → maltose), protease (proteins → amino acids), lipase (fats → fatty acids + glycerol). Know where each enzyme is produced and where it acts.

The Circulatory System

The double circulatory system: pulmonary circulation (heart → lungs → heart) and systemic circulation (heart → body → heart). Know the structure and function of arteries, veins, and capillaries. Know the composition of blood: plasma, red blood cells (haemoglobin, no nucleus), white blood cells (phagocytes and lymphocytes), platelets.

The Nervous System

Central nervous system (brain + spinal cord) and peripheral nervous system (sensory and motor neurons). Know the reflex arc: receptor → sensory neuron → relay neuron (in spinal cord) → motor neuron → effector. Understand the structure of a neuron and how nerve impulses are transmitted.

The Endocrine System

Key hormones: insulin (lowers blood glucose), glucagon (raises blood glucose), adrenaline (fight or flight), thyroxine (metabolic rate), ADH (water reabsorption in kidneys). Understand the negative feedback mechanism for blood glucose regulation.