Anatomy & Physiology › Section A · Introduction to the human body › Topic A.1
Anatomy, physiology, and levels of organisation
Every part of the human body can be studied in two ways: what it is built like, and how it works. This topic sets out those two sciences and the ladder of levels, from molecules to the whole person, that the rest of the course climbs step by step.
Two sciences, one body
Anatomy is the study of the body's structures: their shape, size, composition and position. The word comes from Greek roots meaning to cut apart, because early anatomy relied on dissection. Physiology is the study of how those structures work, from the chemistry inside a single cell to the coordination of whole organs. Structure and function are tightly linked. The air sacs of the lungs have walls far thinner than a sheet of paper, which lets oxygen cross into the blood quickly. Heart valves are flaps that open one way only, which keeps blood moving forward. Whenever you meet a new structure, ask what its shape lets it do.
How anatomists divide the work
Gross anatomy, also called macroscopic anatomy, studies structures large enough to see without magnification, such as a bone, the liver or a muscle. Microscopic anatomy studies structures that need a microscope. Its branches include histology, which examines tissues, and cytology, which examines cells. Anatomists also choose between two approaches. Regional anatomy studies everything in one area of the body together, such as all the bones, muscles, vessels and nerves of the hand. Systemic anatomy studies one body system at a time, such as every bone of the skeleton. This course mostly works system by system.
How physiologists divide the work
Physiologists usually specialise by system: neurophysiology studies the brain and nerves, cardiovascular physiology the heart and vessels, and renal physiology the kidneys. They work at every scale, from single molecules to the whole exercising body. Much of physiology rests on measurement and carefully designed experiments: recording blood pressure, sampling blood chemistry, or tracking how an organ responds when one condition changes. One idea runs through nearly all of it, the body's tendency to keep its internal conditions steady. That idea, homeostasis, gets its own topic, Homeostasis, later in this section.
The levels of organisation
The body is built in levels of increasing complexity, and each level is made of units from the level below. Anatomy and physiology texts usually name six. 1. Chemical level: atoms joined into molecules, such as water, glucose and proteins. 2. Cellular level: cells, the smallest units that can carry out the functions of life on their own. 3. Tissue level: groups of similar cells, with the material around them, working together. 4. Organ level: distinct structures made of two or more tissue types. 5. Organ system level: groups of organs that cooperate in a major function. 6. Organism level: the whole living person. Some schemes add finer steps, such as subatomic particles or organelles, the small working parts inside a cell.
Cells, tissues and organs
A typical human cell has a flexible membrane enclosing cytoplasm, a watery fluid holding organelles, and it does the work of life: taking in nutrients, releasing energy and building molecules. A tissue is a group of similar cells, sometimes of a few related types, together with the extracellular material they produce. There are four primary tissue types: epithelial, connective, muscle and nervous tissue, covered in Section D, The tissue level. An organ combines two or more tissues into a structure with a recognisable shape and job. The stomach is a good example: epithelium lines it and secretes digestive juices, smooth muscle churns its contents, connective tissue holds the layers together, and nervous tissue controls the timing.
The eleven organ systems: part 1
Introductory courses usually group the organs into eleven systems. Here are the first six. 1. Integumentary: skin, hair, nails and skin glands; protects the body, helps regulate temperature and helps make vitamin D. 2. Skeletal: bones, cartilage and ligaments; supports and protects, stores minerals and makes blood cells in red bone marrow. 3. Muscular: the skeletal muscles and their tendons; moves the body, maintains posture and produces heat. 4. Nervous: brain, spinal cord, nerves and sense organs; sends fast electrical signals that sense, interpret and respond. 5. Endocrine: glands such as the pituitary, thyroid and adrenal glands; release hormones that regulate slower, longer-lasting processes. 6. Cardiovascular: heart, blood vessels and blood; transports oxygen, nutrients, wastes and heat.
The eleven organ systems: part 2
The remaining five complete the list. 7. Lymphatic: lymph vessels, lymph nodes, spleen and thymus; returns excess tissue fluid to the blood and houses many immune cells. 8. Respiratory: nasal cavity, pharynx, larynx, trachea and lungs; takes in oxygen and removes carbon dioxide. 9. Digestive: the tube from mouth to anus, with the liver, gallbladder and pancreas; breaks food down, absorbs nutrients and eliminates solid waste. 10. Urinary: kidneys, ureters, urinary bladder and urethra; filters blood, removes wastes and regulates water and salts. 11. Reproductive: the gonads (testes or ovaries) and related organs; produces sex cells and hormones and, in females, supports pregnancy. The groupings overlap: the pancreas makes digestive enzymes and the hormones insulin and glucagon, so it belongs to two systems.
Worked example: placing structures on the ladder
Assign each item to its level of organisation: a glucose molecule, a skeletal muscle cell, blood, the heart and the urinary system. 1. Glucose is a molecule of carbon, hydrogen and oxygen atoms: chemical level. 2. A skeletal muscle cell is one cell, however long: cellular level. 3. Blood is cells suspended in a fluid matrix, so it is a tissue, a type of connective tissue, not an organ. 4. The heart contains cardiac muscle, connective tissue, epithelium lining its chambers and nervous tissue, so it is an organ. 5. The urinary system is a set of organs sharing one job: organ system level. Check: each answer is built from units of the level below it.
Worked example: one job, many systems
Trace how the energy in a slice of bread reaches a working leg muscle, naming each system involved. 1. Digestive system: the mouth and small intestine break the starch down to glucose, which is absorbed into the blood. 2. Cardiovascular system: the blood carries the glucose to the muscle. 3. Endocrine system: the pancreas releases insulin, which signals muscle cells to take up glucose. 4. Respiratory system: the lungs supply the oxygen the muscle cells use to release most of the energy from glucose. 5. Muscular system: the muscle cells use that energy to contract. 6. Respiratory system again: the lungs breathe out the carbon dioxide produced. One everyday activity needs at least five systems, so no system can be understood in isolation.
Common mistakes to avoid
First, treating anatomy and physiology as unrelated: knowing that a structure is thin or folded is only half the story until you ask why. Second, calling blood an organ; it is a tissue. Third, mixing up cytology (cells) and histology (tissues). Fourth, assuming each organ belongs to exactly one system: the pancreas serves the digestive and endocrine systems, and the male urethra carries both urine and semen. Fifth, stopping the ladder at the organ system: the organism level is the top, because only the whole body carries out every function of life at once.
Key takeaways
Anatomy studies structure and physiology studies function, and the two are linked because shape determines what a structure can do. Gross anatomy studies visible structures; microscopic anatomy includes cytology and histology. Regional anatomy studies one area, systemic anatomy one system. The six levels of organisation are chemical, cellular, tissue, organ, organ system and organism. Tissues come in four primary types, and an organ contains at least two of them. The eleven organ systems are integumentary, skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary and reproductive, and they constantly work together.
Flashcards
Flashcard
The study of the body's structures
Test your understanding
Question 1 of 5
A researcher measures how quickly the kidneys filter blood when a person is dehydrated. Which field is this work mainly part of?
Question 2 of 5
Which list puts the levels of organisation in order from simplest to most complex?
Question 3 of 5
Why is the heart classed as an organ rather than a tissue?
Question 4 of 5
A student says each organ belongs to exactly one organ system. Which organ best disproves this?
Question 5 of 5
A textbook chapter covers every bone, muscle, blood vessel and nerve of the forearm together. Which approach does it use?