The Human Circulatory System

The Human Circulatory System

The Human Circulatory System

The circulatory system is the system that transports materials around the body to and from the cells. 

Why do humans need a circulatory system whereas bacteria and simple organisms do not? 

Because the cells of a complex organism such as a human have many cells that are far from the outside environment where nutrients would come from. The system brings the materials to the cells that would not normally receive them.

Humans have a closed circulatory system: This means that the blood is always contained in tubes and vessels. The human circulatory system is composed of the following.

  1. Blood vessels
  2. Heart
  3. Blood

Blood vessels
Humans have three types of blood vessels.  They are: Arteries

The Human Circulatory System

Structure: Thick, elasti
Contain layers of connective, and smooth muscle tissues. Do not contain valves    

Function:  Carry Blood away from the heart. Arteries divide to form very small arteries called arterioles.


Structure: Thin and slightly elastic.
Contain valves for one way flow of blood.

Function:  return blood to the heart Veins divide to become venules.

Medical Alert: 

Varicose Veins. This is a condition where the valves in the veins of a person are not working properly and blood seeps back into the vein causing the vein to become stretched and lose their elasticity.
Result: Sagging veins and lack of blood flow to the heart.


Structure:   Microscopic blood vessels that connect arterioles and venules. Thin walled and narrow blood cells pass through them in single file

Function: Allows material and gas exchange between the body cells and the blood. 

The heart   

Structure:  A four chambered muscular organ located in the chest cavity of a human. Made of Cardiac muscle. It is Covered by a Pericardium that protects it
Pericardium: A tough membrane that surrounds the heart.

Functions of Heart Structure

1.   Inferior/Superior Vena Cava:  Returns deoxygenated blood to the right atrium from the body.
2.   Right Atrium: Thin walled chamber of the heart that receives deoxygenated blood from the body
3.   Tricuspid valve: Controls the flow of blood entering the right ventricle from the right atrium
4.    Right Ventricle: Muscular chamber that pumps blood to the lungs
5.    Semilunar Valves: Valves that control the flow of blood out of the heart
6.    Pulmonary Arteries: Arteries that carry blood to the lungs
7.    Pulmonary Veins: Veins that bring blood to the heart from the lungs
8.    Left Atrium: Thin walled chamber that receives oxygenated blood from the lungs
9.   Left Ventricle: Thick walled chamber that pumps blood out of the heart and to the body
10.  Aorta: Large artery that carries blood away from the heart and to all parts of the body.
11.   Septum: A wall of muscle that separates the left side of the heart from the right side. This  prevents the mixing of oxygenated and deoxygenated blood.
12.  Chordae Tendonae: Control the opening and closing of the Tricuspid and Bicuspid (Mitral) valves
13. Bicuspid Valve: A valve that controls the flow of blood from the left atrium to the left ventricle.


LubDup sound of the heart

The “LubDub” sound of the heartbeat is caused by the closing of the heart’s valves. Lub Sound is caused by the closing of the A-V valves (tricuspid, bicuspid). Dub Sound is caused by the closing of the semilunar valves. 

Control of heart beat

The heart is caused to beat regularly by a structure called the Sinoatrial Node (S – A node) or the


How it happens

An electrical impulse from the brain is received by the S-A node (pacemaker) in the right atrium. The SA node sends a signal to the A-V node (atrioventricular node) in the right ventricle.  This electrical impulse causes the heart (ventricles) to contract.
The pacemaker controls the heartbeat for a human from the time they are born until they die or the pacemaker gives out. 

What happens if the pacemaker gives out?

The person’s heart will stop beating because the ventricles are not receiving electrical impulses causing them to contract. A person whose pacemaker gives out can get an artificial one inserted into their chest.

Control of the heart rate

The heart rate (speed) at which the heart beats is controlled by two nerves. Medulla Oblongata (Sometimes called the Cardioaccelerator nerve): Nerve in the brain that causes the heart to speed up when needed.

Vagus nerve: Nerve in the brain that causes the heart to slow down when needed. The medulla sends a message to the SA node to cause an impulse to be sent to the AV node causing the heart to contract more or less in an attempt to set the heart rate.

How is blood pressure measured?  

Blood pressure is measured using a blood pressure cuff or Sphygmomanometer. It measures the pressure in an artery while the heart is contracting (systolic pressure) and the pressure while the heart is resting (diastolic pressure). 

A simple fraction is calculated using the following formula: Blood Pressure =    Systolic Pressure/ Diastolic Pressure

For example: A person with a pressure 120/80 means that the person has a pressure of 120 while the heart is contracting and 80 when the heart is relaxing. P.S.   Normal blood pressure is different for each person but is usually around 120/80. 

Divisions of circulation

There are two types of circulation that happen in the human organism: Pulmonary Circulation and Systemic circulation

Pulmonary circulation: This is circulation of blood from the heart and to the lungs and vice versa. This type of circulation adds oxygen and removes carbon dioxide from the blood.

Systemic circulation: This is circulation of blood between the heart and the body. This type of circulation brings blood to the cells and from the cells. Systemic circulation has three subdivisions. They are: Coronary Circulation, Hepatic-portal circulation and renal circulation Coronary circulation is circulation that supplies blood and nutrients directly to the heart muscle.

Hepatic – portal circulation is circulation that carries nutrients and blood from the digestive system to the liver to maintain glucose levels in the body. Renal Circulation is circulation that carries blood to and from the kidneys so that nitrogenous wastes may be removed from the blood and excreted by the kidneys.

Major circulatory system disorders and treatment

Hypertension: High Blood Pressure

Causes:  diet, stress, inactivity.

Effects on body:  Leads to heart disease and possible failure

Arteriosclerosis: Hardening of the Arteries 

Causes: Diet – High in Cholesterol (LDL) and Fats.

Effect on body: Causes arteries to become inelastic which can reduce the amount of blood flow in them. This can lead to a heart attack and/or stroke.       

Atherosclerosis: Narrowing of Arteries

Causes: Fatty deposits within the artery walls from poor diet/fat intake etc. Effect on body: Narrowing of arteries reduces blood flow to heart and brain which may lead to heart attack and/or stroke.

 Stroke: Loss of blood flow to brain tissue causing cell death

Causes: Any one of the above and others.

Effect on Body:  loss of brain function and/or motor control (paralysis), death.

Coronary-blockage:  A blockage in the coronary arteries of the heart.

Causes: Diet, lack of exercise.

Effect on Body: Heart attack, death

Treatments for Circulatory System Disorders


Thrombolytics are a class of drugs known as “Clot busting” drugs. These drugs help to bust up blood clots that have formed. They help to clear blocked passageways etc., example;  Streptokinase or t – PA: A Clot busting drug that converts plasminogen to plasmin. Plasmin dissolves the clot.


Procedure in which a small catheter (tube) with a balloon attached is inserted into an artery and then inflated.  The inflation helps to stretch the artery in an attempt to increase blood flow to the heart.  Sometimes a Stent (small mesh netting) is put in place to keep the artery open after the balloon is removed

Coronary Bypass Surgery

Surgery in which a healthy blood vessel from another area in the body is used to create a new pathway around a blockage in a blood vessel near the heart, usually a coronary artery.




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