6.1 Essential ideas

6.1.2 The blood system

  • The circulatory system consists of the heart and the blood vessels – arteries, veins and capillaries.
  • Blood circulates to and from the heart in two separate circulation loops, as shown in Figure 6.1.2a.

Figure 6.1.2a - Double circulationFigure 6.1.2a – Double circulation
The blood from the right and left sides of the heart do not mix.

Skill: Recognising structures of the heart

Figure 6.1.2b - Structure of the heart

Figure 6.1.2b – Structure of the heart

  • Trace the path of blood flow through the heart. Identify the structures as you go along. Click the image to see the labels.

The heartbeat is myogenic

  • The heart is a four-chambered muscular organ that can beat without input from the central nervous system.
  • The heartbeat is myogenic, meaning it originates from muscles in the heart. Click Figure 6.1.2c to review how a heartbeat is generated.

Figure 6.1.2c - Structures involved with heartbeat

Figure 6.1.2c – Structures involved with the heartbeat
Only the four chambers (no major blood vessels) of the heart are shown.

Heart rate is affected by the nervous system

  • Heart rate can be increased or decreased by impulses brought to the SAN or AVN by nerves originating in the medulla oblongata of the brain. See Figure 6.1.2g.
Name Nervous system Effect on heart rate
Vagus nerve Parasympathetic Decelerates
Cardiac accelerator nerve Sympathetic Accelerates

  • Epinephrine is a hormone and a neurotransmitter that is released by the adrenal glands as well as by sympathetic nerve endings. It increases the heart rate and prepares other body functions for vigorous physical activity.

Comparison of blood vessels

  Artery Capillary Vein
Diameter of lumen (mm) 10–5000 Less than 10 10–5000
Wall thickness (mm) Up to 1000 (relatively thick) About 0.5 (one cell thick) Up to 500 (relatively thin)
Wall composition (proportion of tissue layers) Figure 6.1.2d - Wall composition of artery Figure 6.1.2e - Wall composition of capillary Figure 6.1.2f - Wall composition of vein
Valves present No No Yes
Blood pressure (against walls of vessel) High(Muscular and elastic walls maintain high pressure between pulse cycles) Decreases as blood flows from artery towards vein Low(Backward flow is prevented by valves)
Direction of blood flow Away from the heart (ventricles), towards tissues From arteriole towards venuleExchange of materials between blood and tissues occurs here From tissues, towards the heart (atria)

Key concept

The blood system continuously transports substances to cells and simultaneously collects waste products.

Key questions

  • Identify the chambers, valves and major blood vessels of the heart.
  • Outline the heartbeat.
  • Describe how the heartbeat is increased or decreased.
  • Identify the structure of different types of blood vessels.
  • Describe how structure and function are related in the heart and blood vessels.

Study tip

  • Many students learn that arteries carry oxygenated blood and veins carry de-oxygenated blood. This is not always true (i.e. pulmonary artery/pulmonary vein).
  • It is always true that arteries carry blood away from the heart and veins carry blood towards the heart.

Course links

Hormones are carried through the blood to target tissues. The water content of blood, as well as the menstrual cycle, are controlled by hormones. See Page 6.1.6.

  • Gas exchange occurs at tissues. Learn how carbon dioxide and oxygen are exchanged at alveoli of the lungs. See Page 6.1.4.
  • 92% of blood plasma – the liquid part of the blood – is water. See Page 2.1.2.


Emotions are now understood to be the product of activity in the brain rather than the heart. Is knowledge based on intuition less valid than science?

Figure 6.1.2g - Nervous connection to the heartFigure 6.1.2g – Nervous connection to the heart
The (parasympathetic) vagus nerve and the sympathetic cardiac nerve originate in the medulla oblongata.

Skill: Identification of blood vessels from wall structure

  • Compare the micrograph in Figure 6.1.2h with the information in the table 'Comparison of blood vessels'. Can you distinguish between arteries and veins?
  • Use the scale bar to help you determine the thickness of the wall and lumen. Click the image for answers.

Figure 6.1.2h - Cross-section of blood vesselsFigure 6.1.2h – Cross-section of blood vessels