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.
Skill: Recognising structures 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 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|
|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
|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)|
|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)|
The blood system continuously transports substances to cells and simultaneously collects waste products.
- 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.
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?
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.