Heart

The heart is a muscular organ in humans and other animals, which pumps blood through the blood vessels of the circulatory system. Blood provides the body with oxygen and nutrients, and also assists in the removal of metabolic wastes. The heart is located in the middle compartment of the mediastinum in the chest.

In humans, other mammals, and birds, the heart is divided into four chambers: upper left and right atria; and lower left and right ventricles. Commonly the right atrium and ventricle are referred together as the right heart and their left counterparts as the left heart. Fish in contrast have two chambers, an atrium and a ventricle, while reptiles have three chambers. In a healthy heart blood flows one way through the heart due to heart valves, which prevent backflow. The heart is enclosed in a protective sac, the pericardium, which also contains a small amount of fluid. The wall of the heart is made up of three layers: epicardium, myocardium, and endocardium.

Keep

A keep (from the Middle English kype) is a type of fortified tower built within castles during the Middle Ages by European nobility. Scholars have debated the scope of the word keep, but usually consider it to refer to large towers in castles that were fortified residences, used as a refuge of last resort should the rest of the castle fall to an adversary. The first keeps were made of timber and formed a key part of the motte-and-bailey castles that emerged in Normandy and Anjou during the 10th century; the design spread to England as a result of the Norman invasion of 1066, and in turn spread into Wales during the second half of the 11th century and into Ireland in the 1170s. The Anglo-Normans and French rulers began to build stone keeps during the 10th and 11th centuries; these included Norman keeps, with a square or rectangular design, and circular shell keeps. Stone keeps carried considerable political as well as military importance and could take up to a decade to build.

Captain

Captain and Chief officer are overlapping terms, formal or informal, for the commander of a military unit, the commander of a ship, airplane, spacecraft, or other vessel, or the commander of a port, fire department or police department, election precinct, etc. Captain is a military rank in armies, navies, coast guards, etc., typically at the level of an officer commanding a company of infantry, a ship, or a battery of artillery, or similar distinct unit. Chief officer may be used interchangeably with captain in some situations, as when a Captain-ranked Navy officer is serving as the commander of a ship. The terms also may be used as an informal or honorary title for persons in similar commanding roles. In mining (esp. Cornish), it is an honorific given the superintendent or manager of a mine.

The term "captain" derives from katepánō (Greek: κατεπάνω, lit. "[the one] placed at the top", or " the topmost") which was used as title for a senior Byzantine military rank and office. The word was Latinized as capetanus/catepan, and its meaning seems to have merged with that of the Italian "capitaneus" (which derives from the Latin word "caput", meaning head). This hybridized term gave rise to the English language term captain and its equivalents in other languages (Capitan, Kapitan, Kapitän, El Capitan, Il Capitano, Kapudan Pasha etc.)