In human physiology, chemical coordination and long-distance regulation of body functions are primarily brought about by which of the following?

Introduction:
The human body uses two main systems to coordinate its activities: the nervous system and the endocrine system. While nerves provide rapid electrical signalling, the endocrine system uses chemical messengers that travel through the blood to reach distant organs. This question focuses on identifying which substances are responsible for this chemical coordination over long distances within the body.

Given Data / Assumptions:
- The phrase chemical coordination refers to regulation of body functions through chemical signals. - The options include blood, lymph, enzymes and hormones. - We assume normal structure of the circulatory and endocrine systems. - The question targets the main agents that act as chemical messengers rather than the fluid that carries them.

Concept / Approach:
Hormones are chemical messengers secreted by endocrine glands directly into the bloodstream. They travel to distant target organs where they bind to specific receptors and regulate processes such as growth, metabolism, reproduction and stress responses. Blood and lymph are transport fluids that carry many substances, including hormones, but they are not themselves the coordinating chemicals. Enzymes are biological catalysts that speed up chemical reactions but do not typically serve as long distance signalling molecules. Therefore, hormones are the main agents of chemical coordination in the body.

Step-by-Step Solution:
Step 1: Define chemical coordination as regulation of body functions by chemical messengers that act on distant cells and organs. Step 2: Recall that endocrine glands such as pituitary, thyroid, adrenal and pancreas release hormones into the bloodstream. Step 3: Understand that hormones travel through blood to target tissues and adjust their activity by binding to specific receptors. Step 4: Recognise that blood and lymph are the transport mediums that carry many substances, including hormones, but do not themselves provide regulatory instructions. Step 5: Note that enzymes act locally as catalysts of biochemical reactions and are not typically released into blood for long distance signalling. Step 6: Conclude that hormones are the substances that directly bring about chemical coordination in the body.

Verification / Alternative check:
Endocrinology texts describe hormones as secretions of endocrine glands that affect distant cells, often summarised as chemical messengers. They detail hormone feedback loops, such as insulin and glucagon regulating blood glucose or thyroid hormones regulating metabolic rate. In contrast, circulatory system chapters present blood and lymph as fluids that transport gases, nutrients, wastes and hormones. Biochemistry chapters explain that enzymes accelerate specific reactions but are not primarily used as body wide signals. These descriptions confirm that hormones are the correct answer for chemical coordination.

Why Other Options Are Wrong:
Blood: A fluid connective tissue that carries hormones, nutrients and gases, but it is not itself the coordinating signal. Lymph: A fluid that returns tissue fluid to the blood and participates in immune function, but again it is a transport fluid rather than a chemical messenger. Enzymes: Catalysts that speed up biochemical reactions, usually within cells or digestive tracts; they do not serve as long range signalling molecules in the way hormones do.

Common Pitfalls:
Students sometimes confuse the transporter with the message, choosing blood because hormones travel in it. Others may think enzymes coordinate processes simply because they are important in metabolism. To avoid this, remember that coordination requires a signal that can be turned on or off and recognised specifically by target cells. Hormones fulfil this role, while blood and lymph only move substances around and enzymes just help reactions proceed faster.

Final Answer:
Chemical coordination in the human body is primarily brought about by Hormones released by endocrine glands and carried by the blood to target organs.