Module 3 definitions

• ADHESION: Water molecules stick to the walls of xylem vessels. This helps to maintain the column of water in position even when transpiration is not taking place e.g. at night. Without adhesion, the force of gravity could cause the water column to drop.
• AFFINITY: A natural attraction. In evolution terms, it can mean a resemblance between species that suggests a common origin.
• ALVEOLI: [singular alveolus]. Known as air sacs. Arranged in groups at the end of the smallest bronchioles. The alveoli are lined with a single layer of epithelium [which acts as a gas exchange surface] but there is also an extracellular matrix of collagen and elastic fibres between the alveoli. The elastic fibres allow the alveoli to expand during inspiration and recoil during expiration.
• AORTA: The main artery which divides into three above the heart. Aorta carries oxygenated blood from the left ventricle under high pressure to all parts of the body except the lungs.
• APOPLAST PATHWAY: The movement of water through the cell walls of plant cells. The cell walls are readily permeable and the pathway offers little resistance to the flow of water.
• ARTERIOLE: As blood flows further from the heart, arteries branch into smaller blood vessels called arterioles. Arterioles regulate blood flow. They have a relatively thicker layer of smooth muscle than arteries and when this contracts it narrows the arteriole and restricts blood flow. The smooth muscle is controlled by the autonomic nervous system. Arterioles have a relatively thinner layer of elastic tissue because blood pressure is lower.
• ARTERY: A blood vessel which usually carries oxygenated blood away from the heart. They have a narrow lumen and blood is under high pressure. The outer tunica adventitia/externa of collagen is relatively thinner in arteries than veins. Arteries have a thick tunica media of smooth muscle and elastic tissue and a thin smooth inner lining [endothelium] which reduces friction to allow blood to flow freely. The overall thickness of the artery wall is greater than in veins, to resist the vessel bursting under pressure.
• ATRIOVENTRICULAR NODE [AVN]: A group of cells which lie between the atria of the heart. After a short delay, the AVN conveys a wave of electrical excitation between the ventricles along Purkyne fibres.
• ATRIUM: [pleural atria]. A top thin walled chamber of the heart. The right atrium receives deoxygenated blood from the superior and inferior vena cavae. Thr left atrium receives oxygenated blood from the pulmonary veins.
• BARORECEPTOR: A type of sensory receptor which detects blood pressure.
• BICUSPID VALVE: The valve between the left atrium and left ventricle. Sometimes known as the left atrioventricular valve.
• BOHR EFFECT: A decrease in pH and an increase in carbon dioxide concentration lowers the affinity of haemoglobin for oxygen. The oxygen dissociation curve for haemoglobin shifts to the right at any given partial pressure of oxygen.
• BRADICARDIA: A condition which results in a slow heart rate of less than 60 beats per minute
• BREATHING RATE: The number of breaths per minute [a breath is taking air in and breathing it out]
• BRONCHIOLES: These are numerous, thin, narrow branches of the bronchi. They do not have cartilage rings. The walls are lined with smooth muscle so that larger bronchioles can control the flow of air in and out of the alveoli [air sacs]. Smaller bronchioles have no goblet cells or cilia on their epithelium.
• BRONCHUS: [plural bronchi]. Bronchi are two divisions of the trachea, each leading to one lung. They are similar in structure to the trachea although they have a smaller diameter and thinner walls. The bronchi have complete rings of cartilage to support them.
• BUCCAL CAVITY: Known as the oral cavity or mouth in animal anatomy.
• BUNDLE OF HIS: A collection of Purkyne fibres located in the inner ventricular walls, which convey a wave of electrical excitation from the AVN to the apex of the ventricles.
• CAPILLARY: Small, numerous blood vessels responsible for the exchange of materials with cells. They have a small narrow lumen which slows blood flow to enable a longer time for materials to be exchanged. There are spaces [intercellular junctions] between endothelial cells to allow white blood cells to escape to combat infections within tissues.
• CARBONIC ANHYDRASE: An enzyme located in red blood cells which catalyses the combination of carbon dioxide and water into carbonic acid. Carbonic acid then dissociates into H+ ions and HCO3- ions.
• CARDIAC CYCLE: The contraction and relaxation of heart muscle in a regular cycle. The cycle consists of sequences of systole followed by diastole. The atria contract followed by the ventricles and then there is a short period of relaxation before the cycle starts again.
• CARDIAC OUTPUT: "The volume of blood pumped by one ventricle of the heart in one minute. It is usually measured in dm3 min-1.
• Cardiac output = heart rate x stroke volume"
• CAROTID ARTERIES: Main arteries which supply blood from the heart to the head. Chemoreceptors located in the walls of the carotid arteries are sensitive to the pH of the blood resulting from changes in blood carbon dioxide concentration.
• CARTILAGE: A form of connective tissue that provides strengthening and support. It is composed of cells surrounded by a matrix of complex polysaccharides and amino groups. Cartilage is resistant to tension and compression. It is not as strong as bone but is more flexible than bone.
• CASPARIAN STRIP: A continuous band which goes around the wall of endodermal cells in the root. It is made of suberin, a waxy, waterproof substance.
• CHEMORECEPTOR: A type of receptor which responds to chemicals e.g blood carbon dioxide concentration.
• CHITIN: A large structural polysaccharide made from chains of modified glucose. Chitin is found in the exoskeletons of insects, as rings around the outside of insect tracheae and in the hyphal walls of many fungi species.
• CHLORIDE SHIFT: HCO3- ions diffuse out of red blood cells [to form sodium hydrogen carbonate in the plasma]. This is coupled with the diffusion of chloride ions into the red blood cell to maintain electrochemical neutrality.
• CILIATED EPITHELIUM: A layer of cells which forms a covering or lining which posses tiny hairs or cilia on its surface. The cilia of ciliated epithelium beat constantly to remove mucus towards the top of the trachea.
• CLOSED CIRCULATORY SYSTEM: A circulatory system in which blood circulates entirely within blood vessels.
• COHESION: Water molecules are polar and form hydrogen bonds between one another and hence tend to stick together. Cohesion forces help to maintain a continuous column of water in xylem vessels.
• COLLAGEN: A structural fibrous protein which on boiling yields gelatin. Found in skin, tendons; cartilage; teeth and the walls of blood vessels. A collagen molecule consists of three polypeptide chains in the shape of a helix, wound around each other to form a three stranded 'rope'. Collagen has a high tensile strength.
• COMPANION CELL: A cell closely associated with a phloem sieve tube element, directly linked via many plasmodesmata. Companion cells appear to regulate the activity of sieve tube elements and play a role in the loading and unloading of sucrose.
• CORONARY ARTERY: Blood vessel which supplies oxygenated blood to cardiac muscle.
• CORONARY VEIN: Blood vessel which returns deoxygenated blood from heart muscle [myocardium] to the right atrium.
• COUNTERCURRENT FLOW: The flow of blood through the gill lamellae is in the opposite direction to the flow of water over the gills. This maintains a steep diffusion gradient between the water and the blood, so maximising the oxygen that can be extracted from water.
• DEOXYHAEMOGLOBIN


  The form of haemoglobin without oxygen.
• DIASTOLE: The period of time when the heart muscle is relaxed.
• DOUBLE CIRCULATORY SYSTEM: Blood passes through the heart twice during each circuit of the body.