USMLE Anatomy Syllabus

In which mediastinum does the heart lie, and what covers it?

The heart lies in the middle mediastinum, enclosed by the pericardium (an outer fibrous pericardium and an inner serous pericardium with parietal and visceral/epicardial layers separated by the pericardial cavity).

Trace the path of blood through the four chambers and valves of the heart.

Right atrium → tricuspid valve → right ventricle → pulmonary valve → lungs → left atrium → mitral (bicuspid) valve → left ventricle → aortic valve → systemic circulation.

Which heart valves are the atrioventricular valves, and which are semilunar?

Atrioventricular: tricuspid (right) and mitral/bicuspid (left). Semilunar: pulmonary and aortic. AV valves are anchored by chordae tendineae to papillary muscles.

Describe the normal cardiac conduction pathway.

SA node (pacemaker, right atrium) → AV node → bundle of His (AV bundle) → right and left bundle branches → Purkinje fibers → ventricular myocardium.

Name the coronary arteries and the major regions they supply.

The right coronary artery (RCA) supplies the right atrium/ventricle and, in most people, the inferior LV and SA/AV nodes. The left main splits into the LAD (anterior interventricular septum, anterior LV) and the circumflex (lateral/posterior LV).

Write the formula for cardiac output and state normal resting values.

$$CO = HR \times SV$$ where HR is heart rate and SV is stroke volume. Typical resting values: $HR \approx 70\ \text{bpm}$, $SV \approx 70\ \text{mL}$, giving $CO \approx 5\ \text{L/min}$.

Give the formula for stroke volume in terms of ventricular volumes.

$$SV = EDV - ESV$$ where EDV is end-diastolic volume and ESV is end-systolic volume. Ejection fraction is $EF = \frac{SV}{EDV} \times 100\%$ (normal $\geq 55\%$).

What are the three layers of an artery or vein wall, from inner to outer?

Tunica intima (endothelium + basement membrane), tunica media (smooth muscle and elastic fibers), and tunica externa/adventitia (connective tissue). Arteries have a thicker tunica media than veins.

Structurally, how do arteries differ from veins?

Arteries have thick, muscular/elastic walls, small lumens, and no valves (high pressure). Veins have thinner walls, larger lumens, and valves to prevent backflow (low pressure). Capillaries are single endothelium for exchange.

Name the major branches of the aortic arch in order.

Brachiocephalic (innominate) trunk, left common carotid artery, and left subclavian artery. The brachiocephalic trunk then divides into the right common carotid and right subclavian arteries.

What are the three unpaired midline branches of the abdominal aorta supplying the gut?

Celiac trunk (foregut), superior mesenteric artery (midgut), and inferior mesenteric artery (hindgut).

Which two large veins return systemic blood to the right atrium?

The superior vena cava (drains head, neck, upper limbs, thorax) and the inferior vena cava (drains abdomen, pelvis, lower limbs). The coronary sinus also drains cardiac venous blood into the right atrium.

Write the simplified hydraulic (Ohm's law) relationship for blood flow.

$$Q = \frac{\Delta P}{R}$$ where $Q$ is flow, $\Delta P$ is the pressure gradient, and $R$ is resistance. By Poiseuille's law, $R \propto \frac{1}{r^{4}}$, so vessel radius is the dominant determinant of resistance.

How does the lymphatic system return fluid to the bloodstream?

Blind-ended lymphatic capillaries collect interstitial fluid (lymph), which passes through lymph nodes and progressively larger vessels, then drains into the venous system at the junction of the subclavian and internal jugular veins.

What are the two main lymphatic ducts and their drainage territories?

The thoracic duct drains the lower body plus the left head, neck, thorax, and left upper limb (roughly 3/4 of the body) into the left venous angle. The right lymphatic duct drains the right upper quadrant into the right venous angle.

Name the primary and secondary lymphoid organs.

Primary (where lymphocytes develop/mature): red bone marrow and thymus. Secondary (where immune responses occur): lymph nodes, spleen, tonsils, and MALT (e.g., Peyer's patches).

What are the main functions of the spleen?

The white pulp mounts immune responses (B and T lymphocytes), while the red pulp filters blood, removes old/damaged erythrocytes and platelets, and stores blood. It is the largest lymphoid organ.

Name the four lobes of the cerebral cortex and one primary function of each.

Frontal (motor control, executive function, Broca's area), parietal (somatosensory processing), temporal (auditory, memory, Wernicke's area), and occipital (vision).

What are the three main parts of the brainstem, and one key function of each?

Midbrain (visual/auditory reflexes, eye movement), pons (relay between cerebrum and cerebellum, respiratory centers), and medulla oblongata (cardiovascular and respiratory control, autonomic centers).

How many pairs of cranial nerves are there, and where do most emerge?

12 pairs of cranial nerves. CN I (olfactory) arises from the cerebrum and CN II (optic) from the diencephalon; the remaining 10 emerge from the brainstem (midbrain, pons, medulla).

Name the three meningeal layers from outermost to innermost.

Dura mater (tough outer layer), arachnoid mater (web-like middle layer), and pia mater (delicate inner layer adherent to the brain/cord). CSF flows in the subarachnoid space between arachnoid and pia.

Where is cerebrospinal fluid (CSF) produced, and trace its circulation.

Produced by the choroid plexus in the ventricles. Path: lateral ventricles → interventricular foramina (Monro) → third ventricle → cerebral aqueduct → fourth ventricle → subarachnoid space → reabsorbed by arachnoid granulations into the dural venous sinuses.

In the spinal cord, how is grey matter versus white matter organized?

Grey matter is centrally located (H/butterfly-shaped, containing neuron cell bodies) with dorsal, lateral, and ventral horns. White matter is peripheral, containing myelinated ascending and descending tracts (columns/funiculi).

Describe the functional difference between the dorsal and ventral roots of a spinal nerve.

The dorsal (posterior) root carries sensory (afferent) fibers and contains the dorsal root ganglion (sensory cell bodies). The ventral (anterior) root carries motor (efferent) fibers. They merge to form a mixed spinal nerve.

How many pairs of spinal nerves are there, and what are the regional counts?

31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

At what vertebral level does the spinal cord typically end in adults, and what structures continue below?

The spinal cord ends at the conus medullaris around the L1-L2 vertebral level. Below it, the cauda equina (lumbar and sacral nerve roots) and the filum terminale continue within the vertebral canal.

Contrast the function of the dorsal column and the spinothalamic tract.

The dorsal column-medial lemniscus pathway carries fine touch, vibration, and proprioception (crosses in the medulla). The spinothalamic tract carries pain and temperature (crosses at the level of entry in the spinal cord).

What is the structural difference between a nerve, a fascicle, and an axon covering in a peripheral nerve?

Endoneurium wraps individual axons, perineurium bundles axons into fascicles, and epineurium encloses the entire nerve. This parallels the connective tissue organization of skeletal muscle.

Name the major nerves arising from the brachial plexus.

The five terminal branches are the musculocutaneous, axillary, radial, median, and ulnar nerves. The brachial plexus arises from ventral rami C5-T1.

What are the major nerves of the lumbosacral plexus, and which is the largest?

Femoral and obturator nerves (lumbar plexus); sciatic nerve (sacral plexus), which is the largest nerve in the body and divides into the tibial and common fibular (peroneal) nerves.

What is the anatomical basis of the difference between sympathetic and parasympathetic outflow?

Sympathetic (thoracolumbar) outflow arises from spinal cord segments T1-L2 with short preganglionic and long postganglionic fibers. Parasympathetic (craniosacral) outflow arises from cranial nerves III, VII, IX, X and sacral segments S2-S4 with long preganglionic and short postganglionic fibers.