Circulation, respiration, and digestion
Three systems work together to deliver oxygen and nutrients to every cell and remove waste.
The heart
The heart is a muscular pump with four chambers. The right side pumps oxygen-poor blood to the lungs; the left side pumps oxygen-rich blood to the body, so its wall is thicker. Valves stop blood flowing backward; their closing makes the 'lub-dub' sound. A resting heart beats about 60-100 times per minute.
Blood vessels
Arteries carry blood away from the heart under high pressure and have thick, elastic walls. Veins carry blood back, have valves, and rely partly on surrounding muscle to push blood along. Capillaries, one cell thick, are where oxygen, nutrients, and wastes are exchanged with tissues.
Blood
Blood is plasma (mostly water carrying nutrients, hormones, and wastes) plus cells. Red blood cells carry oxygen bound to haemoglobin; white blood cells fight infection; platelets help clotting. An adult has about 5 litres of blood.
The respiratory system
Air passes through the nose, trachea, and bronchi into about 300-500 million tiny air sacs called alveoli. Their huge total surface and thin walls let oxygen diffuse into the blood and carbon dioxide diffuse out. The diaphragm contracts to pull air in and relaxes to push it out.
Control of breathing
Breathing rate is controlled mainly by carbon dioxide levels, not oxygen. Rising carbon dioxide makes blood more acidic, which sensors detect, triggering faster, deeper breathing. This is why holding your breath becomes uncomfortable as carbon dioxide builds up.
The digestive system
Digestion breaks food into small molecules. Chewing and salivary amylase begin breaking starch in the mouth; the stomach's acid and pepsin start protein digestion; the small intestine, with enzymes from the pancreas and bile from the liver, finishes digestion and absorbs nutrients through villi, finger-like folds that increase its surface area.
The large intestine and liver
The large intestine absorbs water and houses trillions of bacteria that produce some vitamins and influence health. The liver processes absorbed nutrients, stores glycogen, makes bile, and detoxifies drugs and alcohol. It is one of the few organs that can regenerate after damage.
Excretion
The kidneys filter about 180 litres of blood plasma per day, reabsorbing most water and useful substances and producing about 1.5 litres of urine. They remove urea, a nitrogen waste from protein breakdown, and regulate water, salt, and pH balance.
Key takeaways
The four-chambered heart pumps blood to the lungs and body. Arteries, veins, and capillaries carry and exchange materials. Alveoli provide a huge surface for gas exchange, and carbon dioxide levels drive breathing. Digestion breaks food down for absorption in the small intestine. The liver processes nutrients; the kidneys filter blood and make urine.
Flashcard
Right side to lungs, left side to body
Check your understanding
Why is the wall of the left ventricle thicker than the right?