Anatomy Terms Starting With R

Radius is the lateral long bone of the forearm that rotates around the ulna to produce pronation and supination and forms the primary articular surface of the wrist joint.

The radial head articulates with the capitulum of the humerus at the elbow, while the distal radius expands to form the major weight-bearing surface of the wrist, contacting the scaphoid and lunate carpal bones across roughly 80 percent of the load transmitted from hand to forearm. During pronation, the radius crosses diagonally over the ulna; during supination, it returns to a parallel position, a rotational range of approximately 150 degrees unique to the tetrapod forearm. At its lateral shaft, the radial tuberosity anchors the biceps brachii tendon, making the biceps a powerful supinator as well as a flexor.

Distal radius fractures, including the Colles fracture with dorsal displacement of the distal fragment, are the most common fractures in adults over 65 and typically result from a fall on an outstretched hand.

Renal cortex is the outer region of the kidney, housing all glomeruli, proximal convoluted tubules, and distal convoluted tubules, where blood filtration and the bulk of tubular reabsorption take place.

Its granular appearance in cross-section reflects the dense packing of renal corpuscles and tubular coils; cortical columns of Bertin extend inward between the medullary pyramids, dividing the kidney into lobes. Approximately 90 percent of total renal blood flow perfuses the cortex, delivered by interlobular arteries that branch from the arcuate arteries at the corticomedullary junction. This disproportionate blood supply sustains the high metabolic rate of active tubular transport, which consumes more oxygen per gram of tissue than almost any other organ in the body.

Cortical ischemia, as occurs during renal artery occlusion or prolonged hypotension, causes acute tubular necrosis and a rapid fall in glomerular filtration rate.

Renal medulla is the inner region of the kidney, organized into pyramid-shaped lobes containing the loops of Henle, vasa recta, and collecting ducts that together establish and exploit an osmotic concentration gradient to produce concentrated urine.

Medullary pyramids taper to papillary tips that project into the minor calyces, where collecting ducts discharge final urine. The countercurrent arrangement of the loops of Henle and the vasa recta builds a rising solute gradient from approximately 300 mOsm at the corticomedullary junction to over 1,200 mOsm at the papillary tip in a well-hydrated human. Medullary blood flow through the vasa recta is deliberately slow, roughly one-tenth the cortical flow rate, preserving this gradient rather than washing it away.

Antidiuretic hormone increases the water permeability of collecting duct cells, allowing water to move down the gradient and producing concentrated urine.

Renal tubule is the tubular portion of the nephron, comprising the proximal tubule, loop of Henle, and distal tubule, that transforms glomerular filtrate into final urine through selective reabsorption and secretion.

Each segment carries out distinct transport tasks. The proximal tubule reabsorbs approximately 65 percent of filtered water, sodium, glucose, and amino acids through active transport and osmosis. Descending into the medulla, the loop of Henle builds an osmotic gradient through countercurrent multiplication: its descending limb is freely permeable to water, while the ascending limb actively pumps sodium and chloride into the surrounding interstitium without allowing water to follow.

At the distal tubule and collecting duct, antidiuretic hormone governs additional water reabsorption, and aldosterone stimulates sodium retention while potassium and hydrogen ions are secreted to regulate electrolyte and acid-base balance.

Respiratory tract is the continuous airway system through which air passes between the external environment and the gas-exchange surfaces of the lungs, encompassing the nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, and alveoli.

The upper respiratory tract, from the nasal cavity through the larynx, warms and humidifies inhaled air to near body temperature and full saturation before it reaches the lungs, and coarse nasal hairs trap large particles at the entrance. Below the larynx, the trachea and bronchi are lined with pseudostratified ciliated epithelium interspersed with goblet cells that secrete mucus; cilia beat at roughly 1,000 strokes per minute to sweep trapped particles and pathogens upward toward the pharynx in a process called mucociliary clearance. Branching continues through approximately 23 generations of airways, ending at the alveoli, where the air-blood barrier is only about 0.5 micrometers thick, thin enough for rapid diffusion of oxygen and carbon dioxide.

Chronic exposure to cigarette smoke paralyzes cilia and destroys alveolar walls, impairing both clearance and gas exchange.

Retina is the light-sensitive neural tissue lining the inner posterior surface of the eye, containing rod and cone photoreceptors along with several layers of interneurons and ganglion cells whose axons converge to form the optic nerve.

Cones concentrate in the fovea centralis, a small pit roughly 1.5 millimeters in diameter at the center of the macula, where each cone connects to its own ganglion cell to support the highest spatial acuity and color discrimination in bright light. Rods, numbering about 120 million per eye, spread across the peripheral retina and respond to single photons, providing high-sensitivity monochromatic vision in dim conditions. Behind the photoreceptors, the retinal pigment epithelium recycles bleached visual pigments, phagocytoses shed photoreceptor outer segments daily, and maintains the outer blood-retinal barrier.

Diabetic retinopathy and age-related macular degeneration are the two leading causes of irreversible blindness worldwide, both targeting retinal vasculature and photoreceptor integrity.