Higher Human Biology
Unit 2
Transport Mechanisms
Conducting System of the Heart
1.The pacemaker or sino-atrial (SA) node initiates
electrical impulses which make heart muscle
cells contract at a certain rate.
2.A wave of excitation (from the SA node) spreads
through the muscle cells of the two atria making
them contract (atrial systole).
3.The impulse is picked up by the atrio-ventricular
node (AV) node located near the base of the
atria.
Conducting System of the Heart
4. The impulse passes from the AV node to
the bundle of His. This bundle of
conducting fibres divides into right and left
branches which are continuous with the
Purkinjie fibres in the ventricular walls.
5. This causes contraction of the two
ventricles.
The Heart
Blood Pressure
• During ventricular systole the pressure of
the blood in the aorta rises to a maximum
e.g. 120 mmHg
• During ventricular diastole it drops to a
minimum e.g. 80 mm Hg
• Blood pressure is measured with a
sphygmomanometer
Diagram of Blood Pressure with
Time
Decreasing Blood Pressure
• There is a progressive decrease in
pressure as blood travels around the
circulatory system dropping almost to zero
by the time it reaches the right atrium
again.
• This is mainly due to peripheral resistance
(the resistance to blood flow caused by
friction between the blood and the walls of
the vessels)
Graph of Decrease in Blood
Pressure
The Lymphatic System
• The walls of the tiny lymphatic vessels are
porous enabling them to absorb the
excess tissue fluid filtered out of the
bloodstream in capillary beds.
• This fluid is called lymph and is collected
by a vast network of lymph capillaries
which unite to form larger lymphatic
vessels.
The Lymphatic System
The Lymphatic System
• Flow of lymph occurs due to muscle
contraction during breathing and moving.
• Larger lymph vessels connect to lymphatic
ducts which return the lymph fluid back to
the circulatory system.
• Thus, tissue fluid that was once part of
blood plasma is returned to the blood
circulatory system.
Absorption of Lipids
• Each villus in the
small intestine
possesses a lacteal.
• Droplets of lipid enter
the lacteals and pass
into the lymphatic
system where they
become a component
of lymph.
Lymph Nodes
• Lymph enters a node by several lymphatic
vessels and percolates through the spaces
between the nodules.
• This enables the macrophages that line these
channels an opportunity to remove any microorganisms present in the lymph by phagocytosis.
• The lymph leaves by a lymphatic vessel.
• N.B. Lymph nodes produce lymphocytes which
make anti-bodies.
Lymph node structure