Anatomy Terms Starting With A

Adipose tissue is a connective tissue composed of lipid-storing cells called adipocytes that store energy, insulate the body, cushion organs, and secrete hormones that regulate metabolism and appetite.

White adipose tissue stores triglycerides in a single large lipid droplet and predominates in adults, while brown adipose tissue generates heat by burning fat directly through a process called non-shivering thermogenesis. Brown fat is densely packed with mitochondria containing uncoupling protein 1, which diverts energy from ATP synthesis into heat production. Beyond energy storage, adipose tissue secretes leptin, adiponectin, and other signaling molecules that influence insulin sensitivity, inflammation, and hunger.

Excess visceral adipose tissue, the fat deposited around abdominal organs, correlates with elevated risk of type 2 diabetes and cardiovascular disease because it secretes higher levels of pro-inflammatory cytokines than subcutaneous fat.

Adrenal Gland is a small, triangular endocrine gland that sits atop each kidney and secretes hormones that regulate the stress response, metabolism, salt balance, and blood pressure.

Each gland has two structurally and functionally distinct regions. The outer cortex synthesizes steroid hormones from cholesterol, including cortisol, which mobilizes glucose during stress, and aldosterone, which controls sodium retention in the kidneys. Its inner medulla consists of modified nervous tissue that releases epinephrine and norepinephrine directly into the bloodstream within seconds of a perceived threat, raising heart rate and redirecting blood to skeletal muscle.

Chronic overactivation of the cortex, as seen in Cushing’s syndrome, produces sustained high cortisol levels that cause muscle wasting, weight gain, and immune suppression.

Alveoli are microscopic air sacs clustered at the ends of the bronchioles in the lungs, where oxygen and carbon dioxide exchange between inhaled air and the pulmonary capillaries by diffusion across an extremely thin epithelial barrier.

Each alveolus is lined by type I pneumocytes, flat cells only 0.1 to 0.2 micrometers thick that form the diffusion barrier, and by type II pneumocytes, which secrete pulmonary surfactant. Surfactant is a phospholipid and protein mixture that reduces surface tension at the air-liquid interface, preventing alveolar collapse during exhalation. Adult human lungs contain approximately 300 million alveoli, producing a total gas-exchange surface area of roughly 70 square meters, comparable to a singles tennis court.

Premature infants born before 28 weeks gestation lack sufficient surfactant, causing respiratory distress syndrome because their alveoli collapse with each breath.

Aorta is the largest artery in the human body, originating from the left ventricle of the heart and carrying oxygenated blood to all systemic tissues except the lungs.

The aortic wall contains concentric layers of elastic fibers that stretch during systole when the left ventricle ejects blood at pressures around 120 mmHg, then recoil during diastole to sustain pressure near 80 mmHg and maintain continuous flow between heartbeats. Curving upward and posteriorly, the aortic arch gives rise to the brachiocephalic artery, the left common carotid artery, and the left subclavian artery, supplying the head, neck, and upper limbs. Below the arch, the vessel descends as the thoracic aorta and then the abdominal aorta, branching into arteries that supply the digestive organs, kidneys, and lower limbs before dividing into the common iliac arteries at the level of the fourth lumbar vertebra.

At its root, the aorta in adult humans measures approximately 2.5 centimeters in diameter.

Appendicular Skeleton is the subdivision of the skeleton consisting of the bones of the upper and lower limbs together with the pectoral and pelvic girdles that attach those limbs to the axial skeleton.

Comprising 126 bones in adult humans, the appendicular skeleton includes the clavicles, scapulae, and bones of the arms and hands in the upper division, and the hip bones, femora, and bones of the legs and feet in the lower division. Each hand alone contains 27 bones, an arrangement of small joints that permits the precise, independent finger movements needed for grasping and tool use. The femur, the longest and strongest bone in the body, transmits the full weight of the upper body to the lower limb during standing and walking.

Evolutionary modifications of the same basic limb plan produce the flipper of a whale, the wing of a bird, and the digging forelimb of a mole, illustrating how a conserved skeletal template can be reshaped for radically different functions.

Arteriole is a small-diameter blood vessel that branches from an artery and delivers blood to capillary beds, regulating local blood flow and systemic blood pressure by contracting or relaxing the smooth muscle in its wall.

Arterioles typically measure 10 to 100 micrometers in diameter and are distinguished from larger arteries by having a wall composed predominantly of smooth muscle relative to their lumen size. Contraction of this smooth muscle, called vasoconstriction, narrows the lumen and increases resistance, reducing flow into the downstream capillary bed. Relaxation, called vasodilation, widens the lumen and increases flow to meet local metabolic demand, as occurs in skeletal muscle during exercise.

Because resistance to flow changes with the fourth power of vessel radius, even a small change in arteriole diameter produces a large change in blood flow, making arterioles the primary site of vascular resistance regulation.

Artery is a blood vessel with a thick, muscular, elastic wall that carries blood away from the heart and toward the tissues of the body.

Arterial walls consist of three concentric layers: the tunica intima, a smooth endothelial lining that minimizes friction; the tunica media, a thick layer of smooth muscle and elastic fibers that withstands and moderates high pressure; and the tunica adventitia, a connective tissue sheath that anchors the vessel to surrounding structures. Most arteries carry oxygenated blood, but the pulmonary arteries carry deoxygenated blood from the right ventricle to the lungs, demonstrating that the defining feature of an artery is the direction of flow away from the heart, not oxygen content. Elastic arteries close to the heart, such as the aorta, stretch during systole and recoil during diastole to smooth pulsatile flow.

Muscular arteries farther from the heart regulate distribution of blood to specific organs by adjusting the tone of their thick smooth muscle layer.

Atrium is either of the two thin-walled upper chambers of the heart that receive blood returning from the veins and contract to deliver it into the ventricles below.

The right atrium receives deoxygenated blood from the body through the superior and inferior venae cavae, while the left atrium receives oxygenated blood from the lungs through the four pulmonary veins. Both atria contract simultaneously, pushing blood through the tricuspid and mitral valves into their respective ventricles just before ventricular contraction begins, a contribution called the atrial kick that adds roughly 20 percent to ventricular filling at rest. Atrial walls are considerably thinner than ventricular walls because atria pump blood only a short distance at low pressure into the adjacent ventricles.

When the atria beat in a rapid, disorganized pattern, a condition called atrial fibrillation, the atrial kick is lost and blood can pool and clot, raising the risk of stroke.

Axial Skeleton is the central bony framework of the human body, consisting of 80 bones that form the skull, vertebral column, thoracic cage, and the ossicles of the middle ear.

The vertebral column contains 33 vertebrae arranged in five regions: cervical, thoracic, lumbar, sacral, and coccygeal. Each vertebra encloses the spinal cord within its vertebral foramen while articulating with adjacent vertebrae through intervertebral discs and facet joints that permit flexion, extension, and rotation of the trunk. Twelve pairs of ribs articulate with the thoracic vertebrae posteriorly and most connect to the sternum anteriorly, forming a cage that protects the heart and lungs while moving rhythmically during breathing.

The skull, comprising 22 bones fused at immovable joints called sutures, encloses and protects the brain and houses the sensory organs of sight, hearing, smell, and taste.