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Osteoblasts are the bone-forming cells; they secrete osteoid, which later mineralizes into mature bone. Once an osteoblast becomes enclosed in matrix it differentiates into an osteocyte that maintains the tissue.
The biceps brachii crosses both the shoulder and elbow and, because of its radial tuberosity attachment, supinates the forearm as it flexes the elbow. It is a prime mover for elbow flexion in the supinated position.
The dermis is the connective tissue layer beneath the epidermis and contains glands, follicles, blood vessels, and sensory nerve endings. The epidermis overlying it is avascular epithelium.
O negative red cells carry neither A nor B surface antigens nor the Rh D antigen, so recipient antibodies are unlikely to attack them. That is why O negative packed cells are used when crossmatching is not possible.
The typical thorax contains 12 pairs of ribs: 7 true pairs that attach directly to the sternum, 3 false pairs that attach via the costal cartilage above, and 2 floating pairs. The number of thoracic vertebrae matches this count.
The phrenic nerve arises from cervical roots C3 to C5 and provides the sole motor supply to the diaphragm. Its cervical origin explains why diaphragm control can persist with injury below that level.
Erythrocytes are packed with hemoglobin, which binds oxygen in the pulmonary capillaries and releases it in peripheral tissues. Their biconcave shape and lack of a nucleus maximize surface area and hemoglobin content.
The pancreatic islets contain beta cells that release insulin and alpha cells that release glucagon, the two main regulators of blood glucose. The pancreas therefore serves both endocrine and exocrine roles.
Calcium released from the sarcoplasmic reticulum binds troponin C, which shifts tropomyosin away from the myosin-binding sites on actin. Only then can myosin heads attach and form cross-bridges.
The mitral (bicuspid) valve sits between the left atrium and left ventricle and closes during ventricular systole. Its closure contributes to the first heart sound.
The subscapularis lies on the anterior scapula and is the only rotator cuff muscle that medially rotates the humerus. The other three cuff muscles contribute to abduction or lateral rotation.
The radial nerve winds around the spiral groove of the humerus, so a mid-shaft fracture can damage it. Loss of wrist extensors produces the characteristic wrist drop.
Residual volume is the air that remains in the airways and alveoli after a maximal forced exhalation, and it keeps the alveoli inflated between breaths. It cannot be measured by simple spirometry.
The jejunum has the most prominent circular folds, villi, and microvilli, giving it the greatest surface area for absorption. Most nutrient uptake occurs there before contents pass into the ileum.
Calcitonin is released by the thyroid's parafollicular cells when serum calcium rises and inhibits osteoclasts, reducing calcium release from bone. Its effect is the direct opposite of parathyroid hormone.
The glomerulus is a tuft of fenestrated capillaries inside Bowman's capsule where hydrostatic pressure drives filtration of plasma. The filtrate then travels through the tubules for reabsorption and secretion.
Immature lymphocytes from bone marrow migrate to the thymus, where they mature and are screened for self-tolerance before entering circulation. B lymphocytes, in contrast, mature in bone marrow.
The latissimus dorsi extends, adducts, and medially rotates the humerus, which is why it drives pulling motions such as swimming and climbing. It also assists in forceful expiration.
The pulmonary veins return oxygenated blood from the lungs to the left atrium, making them the exception to the rule that veins carry deoxygenated blood. The pulmonary artery carries deoxygenated blood away from the heart.
Simple squamous epithelium is a single layer of flat cells, creating the thinnest possible barrier for diffusion between alveolar air and capillary blood. Thinness, not thickness, is what makes gas exchange efficient.
The A band corresponds to the full length of the thick myosin filaments, which do not change length when thin filaments slide past them. Shortening occurs because the I band and H zone narrow as actin is pulled toward the center of the sarcomere.
The Golgi tendon organ sits at the muscle-tendon junction and monitors tension through Ib afferents, triggering reflexive inhibition when force becomes excessive. Muscle length is monitored instead by the muscle spindle.
The AV node has its own intrinsic firing rate of roughly 40 to 60 beats per minute and is the next most excitable pacemaker site in the conduction system. Purkinje fibers can fire, but far more slowly, and the bundle branches do not act as primary pacemakers.
Greater venous return stretches the ventricular myocardium, and within physiologic limits the extra stretch increases the force of contraction. The result is a larger stroke volume, so cardiac output rises to match the increased return.
Hyperventilation blows off carbon dioxide faster than it is produced, lowering the partial pressure of CO2 and shifting the carbonic acid buffer equation away from hydrogen ions. The resulting rise in blood pH is a respiratory alkalosis.
The epidermis renews itself from its deepest layer, the stratum basale, which rests on the basement membrane and contains the mitotically active stem cells. Daughter cells are pushed toward the surface, flattening and keratinizing as they ascend, so the more superficial layers contain older, less active cells.
Quiet expiration is a passive process: when the diaphragm and external intercostals relax, the stretched elastic tissue of the lungs and chest wall recoils and raises intrathoracic pressure, driving air out. Muscular effort is recruited only for forced expiration.
Aldosterone drives sodium reabsorption and potassium secretion in the distal nephron, and water follows the retained sodium. Losing this signal causes sodium loss with potassium retention, so serum sodium falls, serum potassium rises, and plasma volume drops.
The sagittal plane divides the body into left and right portions and is the plane in which forward-and-backward movements such as flexion and extension take place. Because elbow flexion and extension move the forearm forward and backward relative to the humerus, they are sagittal-plane motions.
A contraction in which the muscle develops tension while lengthening is eccentric. The muscle is still active and controlling the movement, which is why eccentric contractions are often called decelerating or braking contractions.
By convention the origin is the more stable or proximal attachment, while the insertion is the attachment that is moved toward the origin. The belly is the fleshy contractile portion between the two attachments.
An antagonist is the muscle that produces the opposite action to the prime mover and must lengthen or relax to permit the movement. Because the triceps extends the elbow while the biceps flexes it, the triceps is the antagonist during flexion.
A sarcomere is the segment of a myofibril between two Z lines and contains the actin and myosin filaments whose interaction produces shortening. It is considered the smallest functional unit of contraction.
Myosin heads bind actin and pull the thin filaments toward the center of the sarcomere, drawing the Z lines closer together while the filaments themselves keep their length. Shortening therefore results from filaments sliding past one another, not from filaments compressing.
Slow-twitch oxidative fibers have abundant mitochondria and capillaries and a high resistance to fatigue, which makes them ideal for prolonged low-intensity work such as maintaining posture. Fast-twitch fibers generate force quickly but fatigue sooner.
Levers are classified by the arrangement of fulcrum, load, and effort. In a second-class lever the load lies between the fulcrum and the effort, exactly as in a wheelbarrow.
Abduction means movement away from the midline in the frontal plane, while adduction means movement toward the midline. These terms apply to the limbs and to the digits.
Muscle spindles lie within the muscle belly and detect changes in length and the speed of that change. Sudden stretch excites the spindle, which drives alpha motor neurons to contract the stretched muscle.
Golgi tendon organs sit at the junction of muscle and tendon, where they sense the force transmitted through the tendon. High tension causes them to inhibit the muscle, which protects it from overload.
Walking differs from running in that there are short intervals when both feet are grounded. These intervals are termed double support and they disappear in running, where a float phase appears instead.
Fusiform muscles are spindle-shaped with fibers parallel to the line of pull, giving them a good range of shortening. Pennate and circular arrangements produce different force and excursion characteristics.
Pennation packs more short fibers into a given cross-sectional area, and force production depends on fiber number and cross-sectional area rather than on fiber length. The trade-off is a shorter range of shortening compared with parallel-fibered muscles.
The olecranon is the proximal projection of the ulna that forms the point of the elbow and fits into the olecranon fossa of the humerus. It is the key posterior landmark of the elbow joint.
The subscapularis arises from the anterior scapula and inserts on the lesser tubercle, giving it a line of pull for medial rotation. The other three cuff muscles act mainly as lateral rotators or abductors.
Tension depends on the number of myosin heads that can bind actin. At near-resting length the overlap is optimal, so the largest number of cross-bridges can form; at extremes of shortening or stretch, overlap is reduced and tension falls.
Torque equals force multiplied by the perpendicular distance from the axis of rotation, called the moment arm. Increasing the distance increases the moment arm and therefore increases torque for the same force.
Rotation occurs in the transverse plane around a vertical axis. Cervical rotation, such as turning the head to look over the shoulder, is the classic example of a transverse-plane motion.
A two-joint muscle loses force-generating capacity when it is simultaneously shortened at both of its joints, because the sarcomeres are already too short for effective cross-bridge formation. The opposite condition, being stretched over both joints, produces passive insufficiency instead.
Plane, or gliding, joints have flat or slightly curved surfaces that slide on one another, as between the carpal bones. They are nonaxial, allowing only small gliding motions rather than motion around a defined axis.
In a closed kinetic chain the distal segment is fixed against an external resistance, so the proximal segments move over the stationary distal segment. In an open chain the reverse occurs: the proximal segment is fixed and the distal segment travels.