4.9.6Additional lessons
Changes in Blood Supply to Organs — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 84 of 101
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Why Blood Supply Must Change
Different demands
- Different demands The need of each organ for oxygen and nutrients changes with its activity, so a fixed share of blood flow would waste resources.
Exercise
- Exercise Muscles use much more oxygen and glucose and release more carbon dioxide and heat, so they need more blood.
Digestion
- Digestion After a meal the digestive system needs a greater blood supply to carry absorbed nutrients away.
Priority organs
- Priority organs The brain and heart keep a good blood supply under almost all conditions, since they cannot function without a continuous supply.
Part 2 of 5How the Supply Is Changed
How the Supply Is Changed
Arterioles
- Arterioles The walls of arterioles contain smooth muscle, and its contraction or relaxation changes the diameter of the vessel.
Vasodilation
- Vasodilation Relaxation of the muscle widens the arteriole, which lets more blood enter the capillary bed of that organ.
Vasoconstriction
- Vasoconstriction Contraction of the muscle narrows the arteriole and reduces flow to organs that are less active.
Control signals
- Control signals Nerves of the sympathetic nervous system and the hormone adrenaline cause vasoconstriction in some organs and vasodilation in muscles.
Local factors
- Local factors Rising carbon dioxide and lactate in active muscle tissue also cause local vasodilation.
| Organ during exercise | Arteriole response | Blood flow |
|---|---|---|
| Skeletal muscle | Dilate | Increases |
| Skin (when hot) | Dilate | Increases |
| Digestive system | Constrict | Decreases |
| Kidneys | Constrict | Decreases |
Part 3 of 5Cardiac Output and Exercise
The heart increases the total volume of blood pumped per minute.
1
Cardiac output is the volume of blood the heart pumps per minute, and equals heart rate multiplied by stroke volume.
2
The cardiovascular control centre in the medulla oblongata increases heart rate through sympathetic nerves when exercise begins.
3
Adrenaline and stronger contraction of the heart muscle raise the stroke volume as well.
4
Venous return rises because contracting skeletal muscles squeeze the veins, which fills the heart and increases stroke volume.
5
Together these changes raise the total blood flow, and the arteriole changes direct that increase mainly to the working muscles.
Part 4 of 5Worked Example
Question
- At rest a person has a heart rate of 70 beats min⁻¹ and a stroke volume of 70 cm³.
- During exercise heart rate is 160 beats min⁻¹ and stroke volume is 110 cm³.
- Calculate the percentage increase in cardiac output.
- Resting cardiac output = 70 × 70 = 4900 cm³ min⁻¹.
- Exercise cardiac output = 160 × 110 = 17 600 cm³ min⁻¹.
- Increase = 17 600 − 4900 = 12 700 cm³ min⁻¹.
- Percentage increase = 12 700 ÷ 4900 × 100 = 259%.
Answer Cardiac output increases from 4900 to 17 600 cm³ min⁻¹, a rise of about 259%.
Part 5 of 5Exam Checklist & Tips
4.9.6 – Key Points for the Exam
- Vasodilation Widening of arterioles that increases blood flow.
- Vasoconstriction Narrowing of arterioles that reduces blood flow.
- Cardiac output Volume of blood pumped by the heart each minute.
- Stroke volume Volume of blood pumped by one ventricle in a single beat.
Common Mistakes
- Saying that arterioles 'move' toward muscles;
- the vessels change width and the blood flows according to resistance.
- Stating that total blood volume increases in exercise, when it is the distribution and the cardiac output that change.
- Forgetting that the brain's blood supply is maintained.
- Using heart rate alone as a measure of cardiac output.
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