Structure and Function

Core Concepts

Landmarks and reference lines

The thorax is a cage of bones: 12 pairs of ribs, the sternum, and the thoracic vertebrae. Key landmarks:

• Sternal notch: the dip at the top of the sternum.
• Angle of Louis: the ridge where the manubrium meets the body of the sternum; it lines up with the 2nd rib and 2nd costal cartilage and is the counting-off point for ribs and intercostal spaces (the spaces between ribs).
• Vertebra prominens: the prominent spinous process at the base of the neck (usually C7), the starting point for counting down the spine.
• Costal angle: the angle formed where the lower ribs meet at the front; normally about 90 degrees or less.

Reference lines are imaginary vertical lines used to name locations: midsternal (down the center of the sternum), midclavicular (through the midpoint of each collarbone), anterior axillary, midaxillary, and posterior axillary (from the armpit region), plus scapular and vertebral lines on the back. When you document "diminished breath sounds at the right base, midclavicular line," the provider knows exactly where you listened.

The lungs and their lobes

The right lung has three lobes (upper, middle, lower); the left lung has two (upper and lower) because the heart takes up space on the left. Lobes are separated by fissures. The lobes do not sit where you might guess from the outside: the upper lobes project anteriorly and superiorly, the lower lobes dominate the posterior chest, and the right middle lobe is best heard anteriorly and in the axilla. The trachea divides into the right and left main bronchi, which branch into smaller bronchi, bronchioles, and finally the alveoli — tiny air sacs where gas exchange happens. Each lung is wrapped in a double-layered membrane (the pleura); the thin pleural space between the layers holds a small amount of fluid that lets the lungs slide as they inflate and deflate. Surfactant, a substance lining the alveoli, lowers surface tension and keeps the air sacs from collapsing.

Mechanics of breathing

Breathing is a pressure game. At rest, inspiration is active: the diaphragm contracts and flattens, and the external intercostal muscles lift the ribs, enlarging the chest and lowering pressure inside, so air flows in. Expiration at rest is mostly passive — the muscles relax and the elastic lungs recoil. Accessory muscles (sternocleidomastoid, scalenes) can help when breathing is harder, but using them at rest is a sign of increased work of breathing — an important observation. Tidal volume is the amount of air moved in and out with each normal breath. Disease changes this machinery: stiff lungs, narrowed airways, fluid in the pleural space, or a collapsed lung all change what you see, feel, hear, and percuss — which is exactly why the physical exam works.

Breast structure

The breast sits on the chest wall over the pectoral muscles and extends into the axilla as the axillary tail. Each breast contains 15–20 lobes of glandular tissue that branch into ducts leading to the nipple; the nipple is surrounded by the areola. Fibrous bands called Cooper's ligaments support the breast tissue and attach it to the chest wall. Breast tissue changes across the lifespan and with hormones (development, menstrual cycle, pregnancy, lactation, menopause), and people of any gender can have breast tissue — assessment is performed respectfully, with consent, and according to facility policy.

The lymphatic system

The lymphatic system is a network of vessels that carries lymph (fluid from tissues) back toward the bloodstream and filters it through lymph nodes — small, bean-shaped structures that house immune cells. Nodes swell as part of the immune response: a clue, not a diagnosis. Key groups for this chapter: head and neck nodes (cervical chains), axillary nodes (central, pectoral, subscapular, lateral, and apical groups) that drain the arm and breast, epitrochlear nodes (just above the elbow), and inguinal nodes in the groin. Drainage patterns matter: knowing that the breast and arm drain to the axilla tells you where to palpate when a person has breast or arm symptoms, and why axillary nodes are important after breast surgery (educational context — specific post-operative care follows orders and policy).

Common Confusions