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Circulatory system


What are we learning?

How the heart pumps blood through the body

How blood cells get from one part of the body to another

Why pathways for blood need to be in parallel, rather than in series.

Two pumps

Our heart is not a single pump – it really is two separate pumps:

Blood from the right side doesn’t mix with blood from the left side.

One side receives low-oxygen blood from veins, and then sends it to the lungs to get oxygen

Other side receives oxygenated blood, and sends it out through arteries to the rest of the body

Also: Why is the left side of this picture labeled “right”? And vice versa? Because medical diagrams are drawn from a doctor’s point of view: Imagine that you are looking at a patient, lying down on a table.  Your left is the patient’s right side. 🙂

Normal blood flow thru heart

from medicinenet.com


Flow of blood in the heart

Right atrium receives low-oxygen blood from veins.

Blood pumped to right ventricle

then to the lungs – to get oxygen

Left atrium receives oxygenated blood.

Pumped to Left ventricle.

Then to aorta, out to the rest of the body.

Normal blood flow thru heart

from medicinenet.com

Pacemaker cells control speed of heart

There’s a group of pacemaker cells in the wall of the right atrium.

sinoatrial node Heart pacemaker

Image found at apsubiology.org, Anatomy Exam Reviews

They produces electrical impulses

Impulses travel along nerves

these impulses stimulate cardiac muscle to contract

This controls the speed at which heart pumps blood.

When body needs more O2/second, the pacemaker cells fire more quickly.

When body needs less O2 then pacemaker cells fire less quickly

Controlling blood pressure

Baroreceptors are biological pressure sensors

They sense blood pressure, and relay this info to the brain

This information is used to change heart rate

They use negative feedback: If they sense blood pressure rising, they cause a decrease in blood pressure (and vice versa)

Baroreceptor reflex Heart 2

from interactive-biology.com, Blood Pressure control measures

Blood cells flowing through an arteriole

Here is a visualization of red blood cells flowing through an arteriole.

Notice the blood clot on the right wall.

How is this image oversimplified? Most blood cells are not shown to make it easier to see the details.

Blood flowing in series

Here we see water being pumped uphill, and then going down a waterfall.

There’s only one pathway.

We say that all the parts of this path are in series.

Advantage: Very simple to set up.

Disadvantage: There’s a lot of resistance (friction between the water and the walls)

Water Flow Analogy Simple circuit 2

Blood flowing in parallel

Here we see water pumped uphill and then coming to a branch:
It now can go thru either of 2 different paths. Each path goes down a waterfall.

We say that the different possible pathways are in parallel.

Disadvantage: A little more complicated to set up.

Advantage: There’s much less resistance (friction between the water and the walls)


parallel circuit waterfall

Why does bloodflow need to be in series?

“If you took all the blood vessels out of an average child and laid them out in one line, the line would stretch over 60,000 miles. An adult’s would be closer to 100,000 miles long” – The Franklin Institute

Also see Are there 60,000 miles of blood vessels in the human body? Yes. Skeptics Stackexchange


Human circulatory in parallel by CMGlee on Wikicommons (license CC BY-SA 3.0)

by CMGlee on Wikicommons (license CC BY-SA 3.0)

Making connections

How blood flows from arteries, to capillaries, and then to veins


Heart-lung system

Anatomy of the heart


heart blood flow chambers

from pediatricheartspecialists.com

Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

HS-LS1-2. Develop and use a model to illustrate the key functions of animal body systems: Emphasis is on the primary function of the following body systems (and structures): digestive (mouth, stomach, small intestine [villi], large intestine, pancreas), respiratory (lungs, alveoli, diaphragm), circulatory (heart, veins, arteries, capillaries), excretory (kidneys, liver, skin), and nervous (neurons, brain, spinal cord).

College Board Science Standards

LSM-PE.1.1.3 Observe the anatomical structures of a variety of organisms, and describe the similarities and differences among them. Organize the organisms into groups based on their similarities and differences.

LSH-PE.2.1.1 Describe the structure and function of at least one organ located in a plant and the analogous organ located in an animal (e.g., organs used for food storage, movement, reproduction, etc.). Description includes the types of cells, the structure of these cells, and the processes they perform to support the function of both the organ and the organism as a whole.

LSH-PE.2.1.3 Describe, using information gathered from print and electronic resources, the structure and function of at least two organs that are part of a human body system (e.g., circulatory, digestive, gas exchange). Description includes how the two organs differ regarding the types of cells that make up each organ. Explain, using knowledge of systems of cells, how the cells and organs coordinate and contribute to the overall essential functions of the organism.

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