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6.1: Planning

Difficulty Level: At Grade Created by: CK-12
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Key Ideas

  • Veins and venules are the counterparts of arteries and arterioles. They carry blood away from the capillaries back to the heart.
  • The walls of veins are thinner than those of arteries and can expand and collapse according to how much blood is in them.
  • The pressure in veins is lower than the pressure in capillaries. So blood flows in one direction—from capillaries to venules to veins. One-way valves and the action of muscles help carry blood back to the heart.


Students explored the exchange of materials in the capillary beds in Section 4. In this section, students investigate the structure and function of the veins and venules. They explore the effect of gravity on the blood flow in veins, which are the vessels that return blood from the capillaries back to the heart. Students observe the valves in the veins of the hand. They investigate the one-way flow of blood by recreating and extending some experiments conducted by William Harvey. They construct a graph representing the distribution of blood within the circulatory system.



investigate the one-way flow of blood in the veins.

compare the structure and function of veins and venules to those of arteries and arterioles.

describe the differences in pressure in veins and capillaries.

explain how blood gets back to the heart from the feet when walking.

make a circle graph showing the percentages of blood found in different parts of the circulatory system.

observe the valves in veins of the hand.

explain how valves help to maintain the one-way flow of blood in the veins.


veins, venules

Student Materials

Activity 5-1: The Direction of Blood Flow

  • Activity Report
  • Clock or watch with a second hand

Teacher Materials

Activity 5-1: The Direction of Blood Flow

  • Activity Report Answer Key
  • Model from Activity 1-1: The Pathway of Blood through Your Body
  • Siphon pump from Activity 2-2: The Siphon Pump
  • Heart drawings from Activity 2-1: Exploring the Heart
  • Models and charts of the heart and circulatory system

Advance Preparation

See Activity 5-1 in the Student Edition.

Make sure students have their models from Activity 1-1: Pathway of Blood through Your Body and their heart drawings from Activity 2-1: Exploring the Heart. Display several siphon pumps.

Background Information

Blood pressure in veins is too low to propel the blood back to the heart. Veins above the heart return blood to the heart with the help of gravity. But the veins below the heart must work against gravity. The blood in these veins is propelled by the milking action of skeletal muscle contractions. One-way valves insure that the blood is propelled in the correct direction—toward the heart.

Vein walls are expandable, and blood tends to accumulate in veins. Up to 80% of the blood's volume can be found in the veins at anyone time. Occasionally veins become so stretched that these one-way valves no longer prevent backflow of blood. This backflow can result in varicose veins. Raising the feet above the head or using support hose can help drain these veins.

Problems can occur if a person returns too little blood to the heart and there is not enough blood in circulation. In these instances, blood accumulates in the lower portion of the body, and the person can faint (a result of too little oxygen going to the brain). Fainting causes a person to fall, which corrects the blood accumulation problem by reducing the affect of gravity on the flow of blood toward the heart.

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