Animal Tissues, Organs and Organ Systems¶

Part of Organisation.

This topic explains how organ systems keep animal cells supplied with oxygen, nutrients and waste removal. Digestion and circulation are the main examples, with disease showing what happens when systems fail.

What You Need to Learn¶

Animal cells depend on digestion, gas exchange and circulation working together. Connect enzyme action, blood components and organ structure to those tasks, then examine how disease disrupts them and how treatments help. Learn to compare treatment trade-offs and interpret evidence about health, cancer and lifestyle risks.

The Human Digestive System¶

• Food is broken down mechanically and chemically so soluble molecules can pass into the blood from the small intestine.
• The stomach uses acid and protease enzymes, the pancreas and small intestine add digestive enzymes, and the liver produces bile to help fat digestion.
• The small intestine is well adapted for absorption because villi provide a large surface area, a short diffusion distance and a good blood supply.

Enzymes and Digestion¶

An enzyme is a protein that speeds up a chemical reaction without being used up. Its active site fits a particular substrate: in the simplified lock-and-key model, only the matching molecule fits, explaining specificity. These catalysts control both digestive breakdown and the reactions that build new molecules in metabolism.

Follow substrate binding and product release in the lock-and-key model below. Its shapes are simplified symbols for molecules. Explain why the same enzyme can act again after its products leave. Open full interactive.

Warming increases molecular movement and collisions up to an enzyme’s optimum temperature. Excessive heat alters its shape: denaturation means the substrate no longer fits the active site. An enzyme also has an optimum pH; large departures can change the active site and reduce activity. Stomach protease works in acid, whereas many intestinal enzymes work best in alkaline conditions.

Compare the temperature and pH curves below, then predict what happens to amylase activity outside its optimum conditions. The curves illustrate patterns rather than measurements for every enzyme; an optimum depends on the particular enzyme. Open full interactive.

Small soluble digestion products can be absorbed. Cells use them to build their own proteins, lipids and carbohydrates, while glucose also supplies respiration. The mouth mixes food with saliva; the oesophagus carries it to the stomach. Most absorption happens in the small intestine, and the large intestine absorbs water from undigested material before faeces leave the body.

Bile is made in the liver and stored in the gall bladder. It neutralises stomach acid arriving in the small intestine and emulsifies lipids into smaller droplets. This increases the surface area available to lipase; bile itself is not an enzyme.

Reaction rate = amount of product formed ÷ time. In an amylase investigation, when each trial begins with the same amount of starch, 1 ÷ time taken for starch to disappear can compare rates. Test samples with iodine at regular intervals, use buffers to vary pH, and keep temperature constant.

The Heart and Blood Vessels¶

• The heart pumps blood in a double circulatory system: one circuit to the lungs and one to the rest of the body.
• Arteries carry blood away from the heart under high pressure, veins return blood at lower pressure and contain valves, and capillaries have thin walls for exchange with tissues.
• Coronary heart disease happens when the coronary arteries are narrowed by fatty deposits, reducing the oxygen supply to the heart muscle.

Blood Route and Gas Exchange¶

Blood returns from the body through the vena cava into the right atrium, then the right ventricle pumps it through the pulmonary artery to the lungs. Oxygenated blood returns through the pulmonary vein to the left atrium; the left ventricle pumps it into the aorta and around the body. The left ventricular wall is thicker because the body circuit requires greater pressure. Valves prevent backflow. Coronary arteries supply the heart muscle itself.

Trace blood from the body through the right heart to the lungs, then through the left heart back to the body. Select the chambers, vessels and valves in the model below. The drawing is schematic and its animation illustrates filling and pumping; explain how a leaking valve would disrupt one-way flow. Open full interactive.

Air passes through the trachea and bronchi to the alveoli. Many alveoli give a large surface area; their thin walls and surrounding capillaries create a short diffusion route. Ventilation and blood flow maintain gradients so oxygen enters the blood and carbon dioxide leaves it.

Artery walls contain thick muscle and elastic tissue to withstand and smooth pressure changes. Veins have a wider lumen and thinner walls, with valves helping return blood at lower pressure. Capillary walls are one cell thick, allowing efficient exchange. Blood-flow rate is volume ÷ time: 120 cm³ passing in 30 s gives 4 cm³/s.

Compare an artery, capillary and vein below and select their features. The cross-sections exaggerate proportions to make differences visible. Explain why the wall suited to exchange would be unsuitable for carrying high-pressure blood away from the heart. Open full interactive.

A group of cells in the right atrium sets the heart’s natural rhythm. An artificial pacemaker supplies electrical impulses when the rhythm is irregular.

Cardiovascular Treatments¶

A stent holds a narrowed coronary artery open, restoring oxygen supply quickly, but insertion carries surgical and clotting risks. Statins lower cholesterol and slow further fatty deposits; they avoid surgery but require ongoing use and may have side effects.

A faulty valve may obstruct flow or leak, allowing backflow and making pumping less effective. Replacement mechanical valves are durable but can require medicines to reduce clotting; biological valves may need replacing sooner. A donor heart, sometimes with lungs, can replace a failing organ, but donors are limited and immune rejection must be controlled. An artificial heart can support circulation while a patient awaits transplant or recovers, although infection, clots and the need for power are limitations. Compare the benefit, risk and suitability for the particular patient.

Blood, Health and Cancer¶

• Red blood cells transport oxygen using haemoglobin, white blood cells defend against pathogens, platelets help clotting and plasma carries dissolved substances.
• Lifestyle factors such as smoking, poor diet, alcohol and limited exercise can increase the risk of non-communicable disease, although some risk is also influenced by genetics.
• Benign tumours remain in one place, whereas malignant tumours invade tissues and can spread to other parts of the body.

Blood Components and Disease¶

In a blood image, red cells are numerous biconcave discs with no nucleus. Their shape increases exchange area, haemoglobin binds oxygen, and lack of a nucleus leaves more space for haemoglobin. White cells have nuclei and can engulf pathogens or produce antibodies. Platelets are small cell fragments that help form clots to prevent blood loss and pathogen entry. Plasma is the liquid carrying these components, dissolved food molecules, carbon dioxide and urea.

Health includes mental and physical well-being. Communicable diseases spread between organisms; non-communicable diseases do not. Diet, stress and living conditions affect health too. Diseases can interact: a weakened immune system increases infection risk, some viruses can trigger cancers, immune responses can produce allergies, and prolonged physical illness can contribute to depression.

Cancer results from changes in cells that disrupt control of division. Benign growths stay contained, while malignant cells invade nearby tissue and can travel in blood to produce secondary tumours. Genetic inheritance and environmental exposures can both influence risk.

Risk Factors and Evidence¶

A risk factor increases the likelihood of disease without guaranteeing it. Smoking damages lungs and exposes cells to cancer-causing chemicals. Diet, smoking and limited exercise contribute to cardiovascular risk; obesity increases the risk of Type 2 diabetes. Alcohol can damage liver cells and impair brain function. Smoking can restrict oxygen supply to an unborn baby, and alcohol can disrupt its development. Carcinogens, including ionising radiation, can cause cell changes leading to cancer. Several factors often act together.

These diseases can reduce quality of life, require care and lead to loss of earnings. Families and communities may provide support, while health services and national economies bear treatment and productivity costs.

To compare disease frequency fairly, use representative samples and compare similar groups rather than selecting only people already ill. Record category counts in frequency tables and use bar charts for categories or histograms for continuous measurements grouped into intervals. Label axes with units and choose a scale that makes differences visible. A scatter plot can reveal a correlation between two measured variables; it does not establish that one caused the other, because a third factor could influence both. Mechanistic evidence strengthens a causal explanation.

Common Confusions¶

• Arteries vs. veins: Arteries carry blood away from the heart under high pressure (thick walls); veins return blood under low pressure (thin walls, with valves to prevent backflow).
• Coronary arteries: These supply the heart muscle itself, not the rest of the body. Blockage here causes a heart attack by starving the muscle of oxygen.
• Red vs. white blood cells: Red cells carry oxygen (haemoglobin-based); white cells attack pathogens (defensive role). They have very different structures and functions.
• Benign vs. malignant: Benign tumours stay localized; malignant tumours invade surrounding tissues and can metastasize (spread). Benign is not "safe" medically, just less aggressive.

Key Terms¶

• Villus: a finger-like projection in the small intestine that increases the surface area for absorption.
• Artery: a blood vessel that carries blood away from the heart.
• Vein: a blood vessel that returns blood to the heart.
• Capillary: a tiny blood vessel with thin walls for exchange of substances.
• Haemoglobin: the iron-containing protein in red blood cells that binds oxygen.
• Coronary artery: blood vessel that supplies oxygen to the heart muscle itself.
• Coronary heart disease: disease caused by fatty deposits in the coronary arteries reducing blood flow to the heart muscle.
• Atherosclerosis: the buildup of fatty deposits (plaque) in arteries, narrowing them.
• Benign tumour: a non-cancerous tumour that stays in one place.
• Malignant tumour: a cancerous tumour that can invade tissues and spread in the body.
• Metastasis: the spread of cancer from one part of the body to another.
• Double circulatory system: separate circuits to the lungs and body, allowing higher blood pressure and more efficient oxygen distribution.

Connected Pages¶

• Cell Structure
• Principles of Organisation
• Plant Tissues, Organs and Systems
• Hormonal Coordination in Humans
• Biology Key Terms Glossary