Why Do Multicellular Organisms Need a Circulatory System?

1. Multicellular organisms need a circulatory system because diffusion alone can't supply all their cells with essential substances.
2. As organisms grow larger, most cells become too far from the external environment for oxygen, nutrients, or waste products to move efficiently by diffusion.
3. A transport system overcomes these limits by moving substances rapidly throughout the body.

How Do Simple Organisms Survive Without a Circulatory System?

1. Simple organisms survive without a circulatory system because every cell is close enough to the environment for diffusion to meet its needs.
2. Unicellular organisms like amoeba rely entirely on diffusion across their membrane.
3. Slightly larger but simple organisms like hydra have only two cell layers and live in water, allowing oxygen and nutrients to diffuse directly into their cells.

How Does the Circulatory System Work?

1. The circulatory system moves blood around the body to deliver essential substances and remove waste.
2. It operates as a coordinated transport network made of three key components:
   1. The heart, which provides the force to pump blood.
   2. Blood vessels, which act as pathways for blood flow.
   3. Blood, which carries oxygen, nutrients, hormones, and waste.

How Do the Heart and Blood Vessels Keep Blood Moving?

1. The heart maintains circulation by contracting rhythmically to push blood into the vessels.
2. This pumping action creates a directional flow supported by a hierarchy of vessels:
   1. Arteries
      1. Carry blood away from the heart under high pressure.
      2. Have thick, elastic walls to withstand this pressure.
   2. Veins
      1. Return blood to the heart under lower pressure.
      2. Contain valves that prevent backflow and help move blood effectively.
   3. Capillaries
      1. Microscopic vessels where substances diffuse between the blood and surrounding cells.
      2. Thin, one-cell-thick walls maximize exchange efficiency.
3. This vessel hierarchy ensures that blood can be moved rapidly across large distances and then slowed down for efficient exchange at the tissues.

What Is Transported in the Blood?

1. Blood transports oxygen, nutrients, hormones, and waste products between cells and the organs that exchange them.
2. Its components each carry out a specific role:
   1. Plasma transports water, dissolved nutrients, hormones, and waste.
   2. Red blood cells carry oxygen using haemoglobin.
   3. White blood cells provide immune defence against pathogens.
   4. Platelets help form clots to prevent blood loss when vessels are damaged.

How Do Plants Transport Substances?

1. Plants use xylem and phloem to transport water, minerals, and sugars throughout the organism.
2. Although plants lack blood, they rely on specialized tissues that move substances efficiently over long distances.

What Types of Circulatory Systems Exist in Animals?

1. Animals may have open or closed circulatory systems depending on their size and metabolic needs.
2. These systems differ in how blood moves and how efficiently substances are delivered.
   1. Open Circulatory System: Blood is not confined to vessels and flows through body cavities,.
   2. Closed Circulatory System: Blood remains inside vessels, allowing higher pressure and faster transport.

How Do Animals Adapt Their Circulatory Systems?

1. Animals adapt their transport systems to meet their metabolic needs and environment.
2. Fish possess a single circulation suited to aquatic life, while mammals have a double circulation that supports high oxygen demand.

What Are the Key Differences Between Diffusion and Transport Systems?

Diffusion is passive and works only over small distances, while transport systems move substances rapidly across the whole organism.