The Human Circulatory System

• nutrients
• gases
• hormones
• excretory products

blood pumped around body by contraction of the heart

blood passes through system of blood vessels:

• arteries
• veins
• capillaries

exchange occurs at the capillaries

mammals have a double circulatory system

• blood passes through heart twice during each circuit of the body

##################DRAW diagram of double circulation############

• route is:
• pulmonary circulation
• heart to lungs
• lungs to heart
• systemic circulation
• heart to rest of body
• rest of body to heart

The Heart

• middle of thorax
• pointing down and to left
 
• outer covering of heart wall is epicardium = layer of flattened epithelial cells and connective tissue
• inner covering is endocardium = layer of flattened epithelial cells and connective tissue
• most of heart wall is myocardium (made up of cardiac muscle)

Heart is divided into 4 chambers:

• 2 atria (aka auricles)
• 2 ventricles
 
• blood passes:
• from body.......(in vena cavae)
• to right atrium.....
• to right ventricle.............
• to lungs...........(in pulmonary artery)
• to left atrium..........(in pulmonary vein)
• to left ventricle............
• to rest of body (via aorta)

• Ventricle walls are thicker than atrial walls as blood travels further
• left ventricle wall thicker than right ventricle wall as blood travels further


• the left ventricle pumps blood into the aorta
• the first of several branches off the aorta leads to the left and right coronary arteries which supply blood to the cardiac muscle

Heart Beat

Cardiac Cycle:

• atrial systole - contraction of atria
• ventricular systole - contraction of ventricles
• diastole - relaxation of atria and ventricles

Blood flows through the heart in one direction

Backflow is prevented by one-way valves

• atrioventricular (AV) valves prevent blood flowing back from ventricles to atria
• right AV valve = tricuspid
• left AV valve = bicuspid (=mitral)



• when blood flowing from atrium to ventricle they are forced open
• when ventricle contracts they are forced shut by the pressure. Attachments to the papillary muscle via the chordae tendinae ("heart strings) prevent the valves being blown through



• semilunar valves
• two valves prevent flow back from the vessels into the ventricles:
• pulmonary semilunar valve stops backflow from pulmonary artery
• aortic semilunar valve prevents backflow from aorta



• semilunars are pocket-shaped. When blood is forced past under pressure they are forced open. When the blood pressure drops they fill up with blood thus closing.


• AV valve closure = lub
• semilunar closure = dup



• Cardiac muscle does not need to be stimulated it beats on its own (=myogenic).
• The beating is rhythymic
• The heart beat can be modified by hormonal or nervous stimulation



• The contraction begins at the sinuatrial node (SAN) (older books call this the sinoatrial node) in the wall of the right atrium. This is the pacemaker
• The contraction spreads across the right atrium and across the left atrium
• But does not spread directly into the ventricles



• The atrioventricular node (AV node) picks up the electrical impulse and conducts it down to the bottom of the ventricles via the Bundle of His and into the Purkinje fibres.
• From the Purkinje fibres the electrical excitation spreads upwards across the ventricles resulting in both ventricles contracting together from the bottom up forcing the blood into the aorta and pulmonary artery.

ECG

• The electrical changes in the heart can be monitored and produce an electrocardiogram (ECG)

• The normal ECG has a characteristic shape showing:
• P wave = excitation of the atria
• QTS complex = excitation of the ventricles
• T wave = recovery (depolarisation) of the ventricles

Pressure Changes

• At diastole the pressure in the ventricles drops to zero
• but the pressure in the aorta and the pulmonary artery remain high because of the semilunar valves and the elastic recoil of the arteries

Control of Heart Beat

Central nervous system (CNS) = the brain and the spinal cord, where processing takes place

Peripheral nervous system (PNS) which is the system of nerves which carries information to and from the CNS

voluntary nervous system

autonomic nervous system which controls subconscious (i.e. automatic) activity

Organs supplied by the autonomic nervous system receive signals from the two systems which make up the ANS:

sympathetic nervous system (generally stimulatory)

parasympathetic nervous system (generally inhibitory)

In the heart the parasympathetic nerve is the vagus nerve (decreases heart rate) while the sympathetic nerve is the sympathetic nerve (increases rate and force of contraction)

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