Anatomy Terms Starting With C

Capillary capillary is the smallest type of blood vessel, formed by a single layer of endothelial cells, where oxygen, carbon dioxide, nutrients, and waste products move between the blood and surrounding tissues.

Capillaries form dense networks called capillary beds that permeate all metabolically active tissues; total capillary length in the adult human body is estimated at 60,000 to 100,000 kilometers. Their thin walls allow small molecules to diffuse freely, while plasma proteins such as albumin generate oncotic pressure that retains fluid within the vessel and prevents excessive leakage into surrounding tissue. Three structural types exist: continuous capillaries with tight junctions, fenestrated capillaries with pore-like openings suited to filtration and secretion, and sinusoidal capillaries with large gaps that permit whole cells to pass, as in the bone marrow and liver.

Precapillary sphincters regulate blood flow into individual capillary beds, directing circulation toward tissues with the greatest metabolic demand at any given moment.

Cardiac Cycle cardiac cycle is the complete sequence of electrical and mechanical events that constitutes one heartbeat, encompassing the filling of the ventricles during diastole and their ejection of blood during systole.

Each cycle begins with spontaneous depolarization of the sinoatrial node, which propagates through the atrioventricular node and the His-Purkinje conduction system to trigger sequential atrial and then ventricular contraction. Closure of the atrioventricular valves at the onset of ventricular systole produces the first heart sound, S1, while closure of the semilunar valves at the end of ejection produces S2. At a resting rate of 75 beats per minute, each cycle lasts approximately 0.8 seconds, with diastole occupying roughly two-thirds of that interval, giving the ventricles adequate time to fill.

Stroke volume, the amount of blood ejected per beat, averages about 70 mL in a resting adult, yielding a cardiac output near 5 liters per minute.

Cardiac muscle is the involuntary, striated muscle tissue forming the myocardium of the heart that contracts rhythmically and continuously throughout life.

Unlike skeletal muscle fibers, cardiac muscle cells are short, branched, and typically uninucleate, connected end to end by intercalated discs that contain gap junctions for rapid electrical coupling and desmosomes for mechanical strength. This coupling causes the ventricular myocardium to depolarize and contract almost simultaneously, functioning as a coordinated unit called a functional syncytium. Cardiac muscle resists fatigue because its cells are densely packed with mitochondria, which can occupy up to 35% of cell volume, and the tissue depends almost entirely on aerobic oxidative metabolism fueled by fatty acids and glucose.

When a coronary artery occlusion kills cardiomyocytes during a myocardial infarction, the dead cells are replaced by fibrous scar tissue rather than new muscle, because adult cardiac muscle has negligible regenerative capacity.

Cartilage is a resilient, avascular connective tissue produced by chondrocytes and embedded in a matrix rich in collagen and proteoglycans, providing flexible structural support at joints and throughout the skeleton.

Hyaline cartilage covers articular joint surfaces and forms the cartilaginous skeleton of the embryo; fibrocartilage provides tensile strength in intervertebral discs and the menisci; elastic cartilage maintains the shape of the ear and epiglottis. Because cartilage lacks blood vessels and nerves, it repairs poorly after injury, and articular cartilage damage can accumulate into osteoarthritis. The embryonic skeleton is initially mostly cartilaginous and progressively replaced by bone through endochondral ossification.

Cerebral Spinal Fluid is the clear, colorless fluid produced by the choroid plexus that fills the ventricular system of the brain and the subarachnoid space, cushioning and nourishing the central nervous system.

CSF is produced at a rate of approximately 500 mL per day but only 150 mL is present at any time, as it is continuously reabsorbed into the venous sinuses via arachnoid granulations. It provides buoyancy that reduces the effective weight of the brain from 1,400 g to about 50 g, protecting it from injury. CSF analysis by lumbar puncture is a critical diagnostic tool for meningitis, subarachnoid hemorrhage, multiple sclerosis, and other neurological disorders.

Chemosynthesis is the process by which some microorganisms make their own food using energy from chemical reactions rather than from sunlight, typically in places where there is no light at all.

While most life on Earth gets its energy from the sun via photosynthesis, chemosynthetic microorganisms can live in complete darkness. They use energy released by reacting chemicals such as hydrogen sulfide, ammonia, or iron with oxygen to build sugars from carbon dioxide. This process supports entire ecosystems at deep-sea hydrothermal vents and cold seeps on the ocean floor, where sunlight never reaches.

Collecting Duct collecting duct is the final tubular segment of the nephron that receives urine from the distal convoluted tubule and conducts it to the renal pelvis while regulating water and ion balance under hormonal control.

The collecting duct is the primary site of action of antidiuretic hormone, which inserts aquaporin-2 water channels into principal cell membranes to increase water reabsorption and concentrate the urine. Aldosterone acts on principal cells to increase sodium reabsorption and potassium secretion, fine-tuning extracellular fluid volume and composition. The collecting duct determines final urine concentration, which can range from 50 to 1,200 mOsm/kg depending on hydration state.

Compact bone is the dense outer layer of bone tissue composed of tightly packed cylindrical osteons, also called Haversian systems, that provides mechanical strength and rigidity to the skeleton.

Each osteon consists of concentric lamellae of mineralized collagen surrounding a central Haversian canal containing blood vessels and nerves; connecting Volkmann’s canals run perpendicular to link adjacent osteons. Osteocytes embedded in lacunae within the lamellae maintain the bone matrix and communicate with each other through a canalicular network. Compact bone constitutes about 80% of total bone mass and bears the compressive and tensile loads experienced during locomotion.

Connective tissue is the most abundant tissue type in the body, composed of cells scattered within an extracellular matrix of fibers and ground substance that binds, supports, and protects other tissues.

The four main connective tissue types are loose connective tissue, dense connective tissue, cartilage, and bone, each varying in cell composition, fiber density, and matrix consistency. Common cell types include fibroblasts that secrete collagen and elastin, macrophages for immune surveillance, and mast cells that mediate inflammation. The extracellular matrix, which can be fluid, gel, or mineralized, determines the mechanical properties of each connective tissue subtype.

Cornea cornea is the transparent, dome-shaped anterior surface of the eye that refracts incoming light and accounts for approximately two-thirds of the eye's total focusing power.

The cornea has five layers: epithelium, Bowman’s layer, stroma, Descemet’s membrane, and endothelium. It is one of the most densely innervated tissues in the body, making it exquisitely sensitive to touch. Unlike most tissues, the cornea is avascular, receiving oxygen by diffusion from tears and aqueous humor, a property that makes contact lens wear a significant oxygen-limiting factor.

Cortisol is a hormone made by the adrenal glands that helps the body manage stress, control blood sugar, reduce swelling, and keep the immune system working properly.

When something stressful happens, the brain signals the adrenal glands to release cortisol into the blood. Cortisol raises blood sugar so muscles have quick energy, slows down body processes that are not immediately needed, and reduces inflammation. After the stressful event is over, cortisol levels should fall back to normal, but when stress never stops, cortisol stays high and can cause weight gain, poor sleep, and a weakened immune system.