yr 8 science - bodily functions

The digestive system

Digestion defintion: Digestion is the breaking down of food so that the nutrients it contains can be absorbed into your blood and carried to each cell in your body.

Physical and chemical digestion

• There are two main types of digestion:

  1. Physical (mechanical) digestion: larger pieces of food are broken down into smaller pieces through the action of structures (Eg. teeth chewing, stomach churning)

  2. Chemical digestion: enzymes break down complex substances into their simplest forms (Eg. stomach acid, saliva in the mouth)

• Enzymes increase the rate of chemical reactions. Without enzymes, a single meal could take years to break down

Digestive system and tract

• Your digestive system is all the organs that are involved in digestion, even if food doesn’t directly pass through.

• The digestive tract is the organs that form a tube travelling from your mouth to your anus where food passes.

Mouth

• Teeth cut, chew and grind food into smaller pieces increasing its surface area

• The tongue is a large muscular organ that helps to manipulate the food into a round ball to be swallowed

• Saliva is secreted from the salivary glands

  • The salivary glands make around 1.5 litres of saliva everyday – this is the start of chemical digestion.

Oesophagus

• A tubular muscle joining the mouth and stomach

• Peristalsis is the muscular movement that causes the food to move down the oesphagus

• The oesophagus is very close to the trachea (windpipe). The epiglottis, a small flap of tissue, closes off the trachea preventing food from entering the lungs

Stomach

• An elastic, muscular J-shaped bag

• It is a temporary food storage area, which can expand to hold between 2 and 4 litres of food

• Food in the stomach is physically digested by the walls of the stomach contracting and relaxing, churning the food

• Food is also chemically digested by gastric juice which contains mucus, water, hydrochloric acid and enzymes

• Stomach acid has a pH of 1-3 to help in digestion and kill bacteria.

• Cells lining the inside of the stomach produce mucus to stop the acid burning the stomach walls

• Sphincters are circular muscles which regulate the movement of food into and out of the stomach, like drawstrings on a sleeping bag.

• Food remains in the stomach for 3-6 hours until it no longer resembles food; it is runny, warm and smelly. This mixture is called chyme

Small intestine

• The small intestine is narrow

• If you laid a small intestine out in a straight line, it would be 5-7m long

• Food moves through the tube by peristalsis

• Nutrients that are required by all cells are absorbed from food in the small intestine

• Villi are finger like structures along the inner wall of the intestine that have a rich supply of blood vessels and increased surface area for effective absorption of nutrients.

• Bacteria in the small intestine also help with digestion

• Chyme takes about 5 or 6 hours to pass through the small intestine

Pancreas

• Pancreatic juice from the pancreas is secreted onto food in the small intestine

• Pancreatic juice neutralises the stomach acid mixed chyme so that the small intestine is not damaged

• Food does not travel through the pancreas

Liver

• Bile is made in the liver, then stored in the gall bladder until food reaches the small intestine.

• Bile is then released into the small intestine through a tube called the bile duct

• Bile from the liver acts as a detergent which breaks down fats into smaller droplets so that enzymes can work more effectively

• Food does not travel through the liver

• The liver also performs many other functions as the largest internal organ. For example, it takes toxins out of your blood, controls blood sugar and stores vitamins

Large intestine

• The large intestine (colon) is wider but shorter than the small intestine. It is approximately 1.5 m long

• Water is absorbed into the bloodstream from the large intestine Material is pushed through by peristalsis

• Chyme stays in the large intestine for up to 14 hours, or sometimes longer

Rectum and anus

• The rectum is the final part of the journey for solid, undigested food, or faeces

• The rectum stores faeces until it starts to become full.

• Then, messages are sent to the brain to make you realise that you need to go to the toilet

• Rectal muscles push the faeces out of the ring of muscle at the end of the rectum called the anus

The circulatory system

Characteristics of the heart

• The heart is a muscle that pumps blood around the body. This delivers oxygen to cells and removes waste

• The heart is made of cardiac muscle which doesn’t tire

• Nerves connected regulate the speed that the muscle contracts

• Imagine the force needed to squeeze a tennis ball. That’s how much force the heart uses to pump blood around the body!

• The average adult heart is the size of a fist

Chambers of the heart

• The human heart has 4 chambers:

  • right atrium

  • right ventricle

  • left atrium

  • left ventricle

• NOTE: The left side of the heart is on the right side of a page and the right side of the heart is on the left side of a page.

• Atria have thinner walls as they are receiving chambers.

• Ventricles are thick and muscular because they pump the blood to the lungs and the body.

• The left ventricle is thicker than the right ventricle as it pumps blood around most of the body while the right pumps blood to just the lungs which are close by

• Valves in the heart make sure the blood flows the correct way

Movement of blood

• The left side of the heart pumps oxygen-rich blood to the rest of the body where it collects carbon dioxide

• Oxygen-poor blood then travels to the right side of the heart. The right ventricle pumps blood through the pulmonary artery to the lungs.

• There, the blood becomes oxygenated and carbon dioxide is removed. Blood is returned via the pulmonary vein to the left side of the heart

Measuring a beating heart

• You can measure how fast your heart is beating by taking your pulse

• Place the fingertips of one hand on the opposite wrist, where an artery passes near the surface of the skin

• When we are stressed the heart can beat at 200 beats per minute. At rest, a healthy athlete's heart beats at around 60 beats per minute

Blood Vessels

Blood vessels

• Blood travels through the body in tubes called blood vessels

• There are three types of blood vessels

• Veins

• Arteries

• Capillaries

Arteries

• Carry blood away from the heart

• Have very thick, elastic, muscular walls that can cope with blood travelling at high pressure

• A severed artery is life threatening

• Almost arteries are located deep under your skin

• The largest blood vessel is the aorta in the heart.

Veins

• Carry blood to the heart

• Have thin walls as the blood travelling in them is at low pressure

• To assist blood to flow back to the heart, veins have valves that allow blood to flow in the direction of the heart only

• Veins can be visible on the surface of skin

Capillaries

• Only one cell thick to allow efficient movement of nutrients and wastes

• Every cell in the body is only millimetres from a capillary

Components of blood

Blood

• The most important role of blood is to transport cell requirements and wastes around the body.

• The average human has about 5 litres or blood

Plasma

• Plasma is a pale yellow fluid containing dissolved gases, proteins, enzymes, hormones, vitamins, minerals and waste.

• About 55% of blood is plams

• Plasma is mostly water

Red blood cells

• Delivers oxygen to cells and removes carbon dioxide

• Produced in bone marrow

• When they mature their nucleus breaks down, allowing more oxygen to be transported

• Are elastic cells that can move through tiny capillaries

• The average life cycle of a red blood cell is 120 days

• Contains hemoglobins which gives them a red colour

White blood cells

• Part of the immune system that fights harmful substances that enter the body

• numbers will increases if the body is fighting an infection

• Slightly larger then red blood cells

• Have a life spend of minutes to years

Platelets

• Clump together to help form a clot and plug a damaged blood vessel

• Fragments of bone marrow cells (have no nucleus)

• Smaller than red blood cells

• Have a lifespan of 1-2 weeks