Tissues, Bone and Homeostasis

Levels of organisation: cell, then tissue, then organ, then organ system.

• A tissue is a group of similar cells (with the material between them) that carry out one function. Blood, cartilage, bone and endothelium are tissues.
• An organ is made of two or more tissues. The trachea, the stomach and the skin are organs.
• There are four basic tissue types: epithelial, connective, muscle and nervous.
• Endothelium is the single layer of flat (squamous) cells lining every blood vessel and the heart.

Bone is a connective tissue: a matrix of collagen (gives flexibility) hardened by calcium phosphate (gives strength), with living cells inside.

• Red bone marrow makes all the blood cells.
• Parathyroid hormone raises blood $\mathrm{Ca^{2+}}$ (it increases bone breakdown); calcitonin from the thyroid lowers it. Vitamin D helps the gut absorb calcium.
• Cartilage is firm but flexible and has no blood vessels, so it heals slowly. It covers joint surfaces and forms the epiglottis, the rings of the trachea, the outer ear and the end of the nose.
• Joints: fibrous (fixed, the skull sutures), cartilaginous (slightly movable, between vertebrae) and synovial (freely movable: the knee is a hinge joint, the hip and shoulder are ball-and-socket joints).
• Muscles work in antagonistic pairs: the biceps bends the elbow, the triceps straightens it.

Homeostasis keeps the internal environment (the tissue fluid around cells) within narrow limits.

• A control loop has a receptor (detects the change), a control centre (compares it with the set point) and an effector (a muscle or gland that responds).
• Negative feedback: the response reverses the change. Almost all homeostasis works this way.
• Positive feedback: the response increases the change until an end point is reached. Examples: oxytocin and contractions in labour, blood clotting, the LH surge before ovulation, sodium entry during an action potential.
• Values held by homeostasis: blood glucose, core temperature, blood water content, blood pH and carbon dioxide, blood pressure, ion concentrations. Body mass is not held at a set point by such a loop; it changes over weeks with diet and growth.
• Blood glucose control does not need the brain: the islet cells of the pancreas both detect the change and respond to it.

Core temperature is held near 37 °C by the hypothalamus.

• Heat is lost by radiation, conduction, convection and evaporation of sweat.
• Sweat cools because water has a high latent heat of vaporisation: each gram that evaporates takes about 2.4 kJ of heat from the skin. Its high specific heat capacity is a different property: it stops body temperature changing quickly.
• In humid air sweat evaporates slowly, so it cools poorly.
• The arterioles supplying skin capillaries widen or narrow; the capillaries themselves do not move.