Energy is necessary to circulate blood, lymph and tissue fluid throughout the body; it is necessary for breathing and taking in oxygen; it is necessary for making new cells for carrying out growth and repair; it is used to transmit nerve impulses so that it can respond to changes in the environment; and it is needed to build different complex molecules such as enzymes and hormones from the simple molecules produced after digestion of food.
The heart is a muscular pump which forces blood around the body through a system of blood vessels, namely arteries, veins and capillaries. Blood carries dissolved oxygen to the body cells and at the same time removes the waste products of respiration, carbon dioxide and water. However, blood is also important in distributing heat around the body, along with hormones, nutrients, salts, enzymes and urea. It is important that the blood flows in only one direction through the heart so it is supplied with special valves to ensure that this happens.
The force blood exerts on the walls of the blood vessels it is passing through is known as the blood pressure (BP). It can be measured using a special piece of equipment called a sphygmomanometer.
Arteries and arterioles Arteries leave the heart and supply smaller vessels known as arterioles which, in turn, supply the smallest blood vessels, the capillaries. Arteries usually carry oxygenated blood. The exceptions are the pulmonary and umbilical arteries carrying, respectively, blood to the lungs and placenta in pregnancy for oxygenation.
The digestive system
The alimentary canal is a tube that extends from the mouth to the anus. It is dilated, folded and puckered in various places along its length. Many glands are associated with the alimentary canal, and have important roles to play in digestion. When food is taken into the mouth it is mixed with saliva, chewed or masticated by the action of the tongue and teeth, rolled into a small ball known as a bolus, and swallowed. This process is called mechanical digestion and is an important part of physically breaking the food down at an early stage.
The salivary glands
Three pairs of salivary glands pour their secretions known as saliva into the mouth. Saliva, a digestive juice, contains an enzyme known as salivary amylase, which begins the digestion of carbohydrates as well as lubricating the mouth and helping bolus formation.
The stomach is the widest part of the alimentary canal. Food can stay in the stomach for up to three hours, with a protein meal remaining the longest and food not containing protein passing through relatively quickly. During this time, the strong stomach walls roll and churn the food around and pour on secretions from the gastric glands
The remainder of the small intestine, known as the ileum, is mainly concerned with the absorption of the now fully digested food. It is specially adapted for this by:
• long length.
• folded interior.
• lining covered in many thousands of tiny projections called villi.
• epithelial cells of villi covered in microvilli, projections so small that they can only be detected using an electron microscope.
The liver is a large dark-red organ. It has a multitude of vital functions in the body, one of which is to produce bile. Bile contains no enzymes at all, but it provides important bile salts that cause the emulsification of fats (lipids) in the duodenum
The pancreas is a slim, leaf-shaped gland, located between the intestines and the stomach, close to the duodenum. It secretes enzyme-rich pancreatic juice as well as alkaline salts needed to neutralise the acidic secretions from the stomach. Pancreatic enzymes go to work on all three macronutrients (protein, fat and carbohydrate) and are important agents for the complete breakdown of the complex food molecules into amino acids, glucose and similar simple sugars, fatty acids and glycerol. Major products of digestion
-Peptides and amino acids are nitrogenous compounds.
-Glycerol and fatty acids – glycerol is used for energy or reconverting fatty acids into a form of fat which can be stored. -Fat is stored under the skin and around organs where it forms a long-term energy store to be used after glycogen stores are depleted.