Which of the following usually accounts for the smallest percentage of leukocytes in a blood sample?

This is because anatomy and physiology are closely related branches of biology that study the structure and function of living organisms respectively.
Anatomy describes the shape, size, location, and relationships of body parts, while physiology explains how those parts work together to maintain life.
For example, the anatomy of the heart determines how it pumps blood, and the anatomy of the lungs determines how they exchange gases.
Choice B is wrong because our understanding of both anatomy and physiology is constantly changing as new discoveries are made in the field of biology.
Choice C is wrong because body parts take up space regardless of their physiological functions.
Choice D is wrong because physiological functions are not limited to an organism, but can also occur at the cellular, tissue, organ, and system levels.

The pancreas is an organ that has both endocrine and exocrine functions.
As an endocrine gland, it secretes hormones such as insulin and glucagon that regulate blood sugar levels.
As an exocrine gland, it releases substances that neutralize stomach acid and digestive enzymes that break down proteins, fats, and carbohydrates.
Choice A is wrong because the thyroid gland is only an endocrine gland.
It secretes hormones such as thyroxine and calcitonin that regulate metabolism and calcium levels.
Choice C is wrong because the anterior pituitary is only an endocrine gland.
It secretes hormones such as growth hormone, prolactin, and adrenocorticotropic hormone that regulate growth, lactation, and stress response.
Choice D is wrong because the liver is mainly an exocrine gland. It produces bile that helps digest fats and detoxifies the blood.
The liver also has some endocrine functions, such as producing insulin-like growth factor 1 and angiotensinogen, but these are not its primary roles.