The Cardiovascular System



The Circulatory System: Structure and Function

• The circulatory system is responsible for many functions within our body:

o Transport of hormones, nutrients, vitamins, minerals, gases, and wastes to and from tissues

o It helps us fight disease

o It maintains homeostasis by stabilizing body temperature and pH

The Heart

• The heart is a powerful organ that weighs approximately 250 – 300 grams and is about the size of a clenched fist

• Located in the thoracic cavity

• a two-layered sac called the pericardium surrounds the heart

• The layers of the pericardium are separated by fluid that reduces friction as the heart beats.

Internal Anatomy of the Heart

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• The heart is divided into 4 chambers:

o Atria are the upper chambers of the heart

▪ left atrium - collects oxygen rich blood returning from the lungs via the pulmonary veins

▪ right atrium – collects oxygen poor blood returning from the body via the superior (anterior) and inferior (posterior) vena cava

o Ventricles (lower chambers) have thick specialized muscular walls for pumping blood out to the lungs and the rest of the body

▪ left ventricle – pumps oxygen rich blood out the aorta to the body

▪ right ventricle- pumps oxygen poor blood out the pulmonary arteries to

the lungs

• A muscular wall called the septum, divides the heart into its right and left sides

Heart Valves: there are two main types

i) The Atrioventricular (AV) Valves: between the atria & ventricles

1) Tricuspid Valve – the tricuspid valve prevents backflow into the right atrium.

2) Mitral Valve (bicuspid) – prevents backflow into the left atrium.

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ii) The Semilunar Valves: between the ventricles and the arteries.

3) Pulmonary Valve (semilunar) – prevents backflow from the pulmonary artery into the right ventricle.

4) Aortic Valve (semilunar) – prevents backflow from the aorta into the left ventricle.

How heart valves work:

• Strong fibrous strings called chordae tendinae prevent the A/V valves from inverting

To see the heart in action go to “How the Heart Works 3D”

The Systemic & Pulmonary Circuits

• The heart is actually 2 side-by-side pumps and is often called a double pump

• The right side of the heart is the pulmonary circuit pump

o Vessels in the pulmonary circuit carry blood to the lungs to oxygenate the blood.

• The left side of the heart is the systemic circuit pump

o Vessels in the systemic circuit carry oxygenated blood to all body tissues and organs.

• There is NO MIXING of blood between the right and left sides of the heart in adults

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NOTE: The pulmonary ciruit isn’t only for picking up oxygen but for expelling carbon dioxide waste. Further, the systemic circuit doesn’t only deliver oxygen but picks up the carbon dioxide waste produced from our cells as a product of cellular respiration.

The Cardiac Cycle: (see the diagram below for the steps)

• The chambers of the heart contract (systole) and relax (diastole) in a rhythmic cycle.

• Both atria contract (systole) to fill the ventricles with blood. Then both ventricles contract (systole), and blood is pumped into the arteries of the systemic and pulmonary circuits.

How the cardiac cycle produces the “Lub – Dub” sound of the heartbeat

When the ventricles become filled with blood the pressure on the atrioventricular valves causes them to “slam” shut, creating a "lub" sound.

Following a brief delay, the ventricles contract (systole) pushing the blood through the semilunar valves into both the aorta (left side) and the pulmonary artery (right side). As the blood leaves the heart the semilunar valves “slam” shut behind it to prevent backflow. This creates the "dub" sound.

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Source: Boundless. “The Cardiac Cycle.” Boundless Biology. Boundless, 21 Jul. 2015. Retrieved 03 Nov. 2015 from 

ASSIGNMENT: The Circulatory System Structures and Function Worksheet

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Did You Know?

• The average human heart beats 72 times per minute, approx 100,000 times per day.

• In a single day, the heart circulates the body's blood supply about 1,000 times.

• During our lifetime the heart circulates 20,000 L of blood through 96,000 km of vessels.

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