Anatomy Terms Starting With L

Large Intestine large intestine is the final 1.5-meter segment of the alimentary canal comprising the cecum, colon, rectum, and anus that absorbs water and electrolytes from indigestible residue and compacts feces for elimination.

The colon houses the densest microbial community in the body approximately 1011 bacteria per mL of luminal contents that ferment indigestible polysaccharides, producing short-chain fatty acids that nourish colonic epithelium. Its ascending segment absorbs the bulk of remaining water; the transverse and descending colon consolidate feces; the rectum stores feces before defecation. Colorectal cancer is the third most common cancer globally, arising primarily from adenomatous polyps that develop in the colonic mucosa.

Larynx is the cartilaginous structure at the top of the trachea that houses the vocal folds, protects the lower airway during swallowing, and regulates airflow between the pharynx and trachea.

Its cartilaginous framework includes the thyroid, cricoid, and arytenoid cartilages; the epiglottis folds over the glottis during swallowing to prevent aspiration. Sound production occurs as exhaled air vibrates the vocal folds, whose tension is adjusted by laryngeal muscles to control pitch. During human infancy, the larynx descends in the neck, enlarging the pharyngeal resonating chamber and enabling the complex speech articulation unique to humans among primates.

This descent also increases the risk of choking, a trade-off not seen in other mammals whose larynx remains high and interlocked with the soft palate.

Leukocyte is any nucleated, non-hemoglobin-containing blood cell that forms the cellular arm of the immune system, including granulocytes, monocytes, and lymphocytes.

Granulocytes, which include neutrophils, eosinophils, and basophils, are the most numerous white blood cells and respond rapidly to infection; monocytes differentiate into tissue macrophages and dendritic cells; lymphocytes mediate both adaptive and innate immune responses. All leukocytes are produced in the bone marrow and lymphoid organs, and their numbers rise dramatically during infection or inflammation. A healthy adult maintains roughly 4,500 to 11,000 leukocytes per microliter of blood, a range that clinicians use to detect infection, autoimmune activity, or malignancy.

Leukemia represents malignant proliferation of leukocyte precursors, while leukopenia, an abnormally low count, severely compromises immune defense.

Ligament is a short band of dense fibrous connective tissue that connects bone to bone across a joint, providing passive mechanical stability while permitting normal joint motion.

Ligaments consist predominantly of parallel type I collagen fibers arranged along lines of tensile stress, with a small proportion of elastin that allows slight elongation under load. Their poor vascularization slows healing after rupture; the anterior cruciate ligament of the knee, one of the most commonly injured ligaments in athletes, typically requires 9 to 12 months to recover after surgical reconstruction. Proprioceptive nerve endings embedded within ligaments contribute to joint position sense, so rupture impairs coordination as well as mechanical stability.

This sensory loss partly explains why athletes who return too early after ligament repair face elevated re-injury risk.

Loop of Henle is the U-shaped tubular segment of the nephron that descends into the renal medulla and returns to the cortex, generating the osmotic gradient in the medullary interstitium that enables urinary concentration.

The descending limb is permeable to water but not to solutes, so tubular fluid becomes progressively concentrated as it descends; the ascending limb is impermeable to water but actively transports sodium chloride outward, diluting the tubular fluid as it rises. Countercurrent multiplication between the two limbs builds a steep osmotic gradient in the medulla, rising from roughly 300 mOsm/kg at the corticomedullary junction to over 1,200 mOsm/kg at the papillary tip. This gradient allows the collecting duct, under the influence of antidiuretic hormone, to reabsorb water and produce concentrated urine.

Longer loops extend deeper into the medulla and generate steeper gradients, directly linking loop length to a species’ capacity to conserve water.

Lumbar refers to the five large vertebrae of the lower back, designated L1 through L5, that bear the greatest compressive load of the vertebral column and permit significant flexion and extension but limited rotation.

Lumbar vertebrae are distinguished by their large kidney-shaped bodies, robust transverse processes, and broad spinous processes; their facet joints are oriented in the sagittal plane, permitting flexion and extension while resisting axial rotation. Supporting the weight of the head, trunk, and upper limbs makes this the most mechanically stressed segment of the spine. Intervertebral disc herniation, particularly at L4 to L5 and L5 to S1, is one of the most common causes of low back pain and sciatica worldwide, affecting an estimated 5 to 10 percent of adults with low back pain.

Lumbar spinal stenosis, a narrowing of the spinal canal at these levels, is increasingly prevalent with age and can compress nerve roots, causing pain and weakness in the legs.

Lungs are a pair of spongy, lobed organs in the thoracic cavity where gas exchange between air and blood occurs across the thin alveolar-capillary membrane, delivering oxygen to the bloodstream and removing carbon dioxide.

The right lung has three lobes and the left lung two, the smaller left accommodating the cardiac notch where the heart sits. Each lung is enclosed by a pleural sac whose thin fluid layer allows frictionless expansion against the thoracic wall during breathing. Ventilation is driven by the diaphragm and intercostal muscles, which change thoracic volume and create pressure gradients that move air in and out.

At rest, an adult human moves roughly 500 milliliters of air per breath, but the total lung capacity reaches approximately 6 liters, with only a fraction exchanged during quiet breathing.