Higher Human Biology
Unit 2 Physiology and Health
Course notes
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CURRICULUM FOR EXCELLENCE
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Reproduction
Gametes are produced from germline cells. Sperm (the male gametes) are
produced in the testes in the seminiferous tubules shown in Figure 1 below.
Figure 1 Location of seminiferous tubules in testes
Figure 2 Location of prostrate gland and seminal vesicle
The male sex hormone testosterone, is produced in the interstitial cells.
These testosterone-secreting cells are located in clusters in the between the
seminiferous tubules. Figure 2 above shows the location of the prostrate gland
and the seminal vesicles. Both secrete nourishing fluids that maintain
the mobility (ability to swim) and viability (fertilising potential) of the sperm.
The fluid in which the sperm are released is called semen.
Web site
http://www.youtube.com/watch?v=rlkqYm_LxgU
http://www.youtube.com/watch?v=2Gva_lAUrAg
Ova, the female gametes are produced from germiline cells in the ovaries. The
ovaries contain many immature ova in various stages of development. Ova
therefore mature in the ovaries. Each ovum is surrounded by a follicle that
protects the developing ovum as shown in Figure 3 below.
oviduct
ovary
uterus
Figure 3 Female reproductive system
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The follicle also secretes hormones. Mature ova are released into an oviduct
where they may be fertilised by a sperm to form a single cell called a zygote.
Hormonal control of reproduction
Hormones are chemical messengers produced by the body’s endocrine glands.
They are secreted directly into the bloodstream. When a hormone reaches its
target cells, it brings about a specific effect.
Hormones control the following events:
 the onset of puberty
 sperm production
 the menstrual cycle
1.
The onset of puberty
At puberty, the hypothalamus produces a releaser hormone. This
releaser hormone then stimulates the pituitary gland
which, in turn, then produces two hormones:
 FSH
follicle stimulating hormone
 ICSH
interstitial cell stimulating hormone (in
men) or LH - luteinising hormone (in women)
The release of these hormones at puberty, triggers
the onset of sperm production in men and the menstrual cycle in women.
2.
Hormonal control of sperm production
When FSH reaches the testes, it promotes sperm production in the
seminiferous tubules. ICSH stimulates the interstitial cells to produce
the male sex hormone testosterone.
Testosterone also stimulates the production of sperm in the
seminiferous tubules. It also activates the prostate gland and seminal
vesicles to produce their secretions.
Negative feedback control of testosterone by FSH and ICSH.
The regulation of testosterone production is tightly controlled to
maintain normal levels in the blood. Like many hormones, this is achieved
through negative feedback control.
Both the hypothalamus and the pituitary gland are important at
controlling the amount of testosterone produced by the testes. As the
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concentration of testosterone builds up in the bloodstream, it reaches a level
where it then starts to inhibit the secretion of FSH and ICSH by the pituitary
gland. Since this, in turn, leads to a decrease in testosterone concentration, it
is followed by the pituitary gland becoming active again and it starts producing
FSH and ICSH again, and so on. This type of self-regulating mechanism is called
negative feedback control and is shown in Figure 4 below.
hypothalamus
releaser
hormone
pituitary gland
testosterone
ICSH
FSH
testes
Figure 4 Negative feedback control of testosterone
3.
Hormonal control of the menstrual cycle
The menstrual cycle lasts for approximately 28 days. The first day of
menstruation is regarded as day one of the cycle. Four hormones are
involved in the menstrual cycle. Two of these hormones are:
1
FSH (follicle stimulating hormone)
2
LH
(luteinising hormone)
Both of these hormones are produced by the pituitary gland and both of
them affect the ovaries.
FSH stimulates the development and maturation of each follicle.
Ovarian hormones:
The follicle produces
OESTROGEN.
The corpus luteum produces
PROGESTERONE.
Figure 5 Development and maturation of follicle in an ovary
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FSH also stimulates the production of the hormone oestrogen by the follicle.
LH triggers the process of ovulation (the release of an ovum from the ovary).
LH is also responsible for the development of the corpus luteum from the
follicle - see Figure 5 on page 3. LH also stimulates the corpus luteum to
secrete the sex hormone progesterone.
The ovarian hormones
Oestrogen and progesterone are known as the ovarian hormones and they are
also involved in the menstrual cycle.
1.
Oestrogen
During the first half of the menstrual cycle which is known as the
follicular phase, oestrogen, which is produced by the follicle surrounding
an ovum, stimulates cell division (proliferation) in the inner layer of the
uterus which is called the endometrium. This helps to either:
1.
prepare the uterus for implantation of an ovum following
fertilisation or
2.
if the ovum is not fertilised, it helps in the repair of the
endometrium after menstruation
When the level of oestrogen peaks, this stimulates a surge in the
secretion of the hormone LH (and FSH) by the pituitary gland around day
14 of the cycle. (This surge in LH is the direct cause of ovulation.)
Oestrogen also has an effect on the consistency (thickness) of cervical
mucus, which in turn, makes it easier for sperm to swim through it.
Web site
http://www.youtube.com/watch?v=nLmg4wSHdxQ
2.
Progesterone
In the second half of the menstrual cycle, known as the luteal phase, LH
causes the follicle to become the corpus luteum. The corpus leteum
secretes the hormone progesterone. The rise in progesterone
concentration stimulates the further development and the
vascualarisation of the endometrium. This helps to prepare the
endometrium for the implantation of the blastocyst (see Figure 6 below)
if an ovum has been fertilised.
Web site
http://www.youtube.com/watch?v=P1h611sNji8
http://www.youtube.com/watch?v=WGJsrGmWeKE
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Figure 6 Development of a blastocyst
Negative feedback effect of ovarian hormones
The combined high concentrations of oestrogen and progesterone during the
luteal phase of the menstrual cycle inhibits the secretion of FSH and LH by the
pituitary gland. The level of both FSH and LH therefore drops and, as a
result, no new follicles develop at this time. As with testosterone, this is
achieved via negative feedback control as shown in Figure 7 below.
Oestrogen and
progesterone negative
feedback
Oestrogen and progesterone
Proliferation (oestrogen) AND
Vascualarisation (progesterone)
Figure 7 Negative feedback effect of ovarian hormones
 A releaser hormone from hypothalamus stimulates the pituitary gland to
secrete FSH, and later on in the cycle, LH into the bloodstream.
 FSH stimulates follicle growth. The follicle produces oestrogen which is
then secreted into the bloodstream. Oestrogen stimulates the
proliferation of the endometrium.
 LH causes the follicle to become the corpus luteum (after ovulation).
 The corpus luteum secretes progesterone into the bloodstream.
 If fertilisation does not occur, a lack of LH causes the degeneration
(breakdown) of the corpus lutem by about day 22 of the cycle.
 The levels of both progesterone and oestrogen then drops rapidly.
 By day 28, these two ovarian hormones are at such a low level that the
endometrium can no longer be maintained and menstruation occurs and
the cycle begins again.
Web site
http://www.google.co.uk/url?sa=t&rct=j&q=effects+of+LH+on+ovary+and+oestrogen+on+uterus&source=web&cd=5&cad=rja&uact=8&ved=0CEcQFjAE&url=http%3A%2F%2Fwww.s
umanasinc.com%2Fwebcontent%2Fanimations%2Fcontent%2Fovarianuterine.html&ei=Vp98U7fzI6ep0QWs1oHwAw&usg=AFQjCNFR8Dwk23cjifE_KKqUcbHXl4uKrg
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Figure 8 below outlines how these four hormones interact and synchronise the
events that occur during the menstrual cycle.
Pituitary hormones
Oestrogen
Ovarian hormones
Oestrogen
Menstruation
Proliferation phase
Menstruation
Figure 8 Menstrual Cycle
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Summary
The table summarises the functions of the hormones that are involved in the
menstrual cycle.
Sex Hormone
Site of production
testosterone
interstitial cells (in testes)
FSH
pituitary gland
ICSH - in men
pituitary gland
Function
stimulates sperm production; activates
prostrate gland and seminal vesicles
 stimulates sperm production (in
men)
 stimulates the development and
maturation of a follicle and the
production of oestrogen (in women)
 stimulates the production of
testosterone (in men)
 triggers ovulation and it is also
responsible for the development of
the corpus luteum (in women)
 stimulates the corpus
 stimulates proliferation of the
endometrium
 stimulates the secretion of LH
promotes further development and
vascularisation of the endometrium and
maintains it during the middle part of the
menstrual cycle and during pregnancy.
LH – in women
oestrogen
follicle (surrounding ovum)
progesterone
corpus luteum
The Biology of controlling fertility
Treatments for infertility and contraceptives are based on the biological
knowledge that has been acquired about fertilisation/fertility.
1.
Fertile periods
Fertility in females is cyclical and involves a fertile period as opposed to
fertility in men which is continuous.
a)
Fertility in men
Testosterone maintains a relatively constant level of the pituitary
hormones FSH and ICSH in
the blood. This, in turn, leads to a
fairly steady level of testosterone being secreted (by the
interstitial cells in the testes), and testosterone stimulates sperm
production.
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b)
Fertility in women
The interaction of the pituitary hormones (FSH and LH) and
ovarian hormones (oestrogen and progesterone) results in a short
period of fertility each month (1 to 2 days only) immediately
following ovulation.
Fertility periods and be calculated and then used by a woman to help to prevent
pregnancy or to work out when she is most fertile.
2.
Treatments for infertility
a)
Stimulating ovulation
A woman might not ovulate because her pituitary gland fails to
secrete an adequate amount of FSH or LH. In such cases, ovulation
can be stimulated by drugs. These drugs help to stimulate
ovulation by either:
b)
1.
Preventing the negative feedback effect that oestrogen has
on the production of FSH during the luteal phase of the
menstrual cycle OR
2.
Mimicing the normal action of the pituitary hormones FSH
and LH. These drugs are so effective that they can
stimulate “super ovulation”. This is when many ova are
released at the same time resulting in multiple births or the
ova can be collected for IVF (in vitro fertilisation)
treatment at a later date.
Artificial insemination
This treatment is useful for those men who have a low sperm count.
It involves collecting several samples of semen over a period of
time to ensure enough viable sperm are collected. The semen is
then preserved by freezing it until required. It is then
defrosted before being injected into their partner (as shown in
Figure 9 below) at the time when she is most likely to be fertile.
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semen injected
into uterus
uterus
Semen (containing
sperm) in syringe
Figure 9 Artificial insemination
c)
If a partner is sterile, a donor may be used.
Intracytoplasmic sperm injection (ICSI)
If mature sperm are defective or very low in number, ICSI can be used.
In this technique, the head of the sperm is drawn into a needle and is
then injected directly into an ovum to achieve fertilisation.
Web site
d)
http://www.youtube.com/watch?v=IQujLI-ArMY
In vitro fertilisation (IVF)
This involves the surgical removal of ova from the ovaries after hormone
therapy is used to stimulate ova production. The ova are then mixed with
sperm in a culture dish. The fertilised eggs are incubated until they have
formed at least 8 cells. They are then transferred to the uterus for
implantation as shown in Figure 10 below.
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Figure 10 In vitro fertilisation
Before being transferred to the uterus, pre-implantation genetic
screening can be used in order to identify genetic disorders and
chromosome abnormalities.
Web site
http://www.google.co.uk/url?sa=t&rct=j&q=IVF+site%3Ayoutube.com&source=web&cd=1&cad=rja&uact=8&ved=0CEUQtwIwAA&url=http%3A%2F%2Fw
ww.youtube.com%2Fwatch%3Fv%3DGeigYib39Rs&ei=swB-U9rYNJD70gWMtoHoDw&usg=AFQjCNGVtNQP0Be1wCOlZpmgniYriEto7g
3.
Contraception
There are both physical and chemical methods of contraception.
a)
Physical methods
This includes barrier methods such as:

using a cervical cap, condom or diaphragm

avoiding fertile periods

intra uterine devices e.g. a coil shown opposite (Figure 11)

sterilisation procedures (e.g.
vasectomy)
Figure 11 Intra uterine device (“coil”)
Although physical methods of contraception are very effective, they are not as
successful as chemical methods.
b)
Chemical methods
Chemical contraceptives are based on combinations of synthetic
hormones that mimic negative feedback thus preventing the release of
the hormones FSH and LH. This in turn, prevents implantation (“morning
after pills”) or causes a thickening of the cervical mucus (“mini pill”).
4.
Ante- and postnatal screening
A variety of techniques can be used to monitor the health of the mother
and the developing fetus. Antenatal screening identifies the risk of a
disorder so that further tests and a prenatal diagnosis can be offered.
Web site
http://www.google.co.uk/url?sa=t&rct=j&q=antenatal+screening+site%3Ayoutube.com&source=web&cd=2&cad=rja&uact=8&ved=0CDkQtwIwAQ&url=ht
tp%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3D2nNxpAyH89k&ei=25WEU6TsIIWfO9zwgfgL&usg=AFQjCNGt0WicEaFl9r2h1sDp9iSQyLlQuw
Antenatal care
Antenatal care includes:
a)
Checking the mother’s blood pressure, blood type and general health. It
also includes routine blood and urine tests.
b)
Ultrasound imaging (see Figure 12 below) is a dating scan (at 8 – 14
weeks) is used to determine the stage of pregnancy and the due date.
These are used along with tests for marker chemicals which vary
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normally during pregnancy. Measuring a substance at the wrong time
could lead to a false positive result. An anomaly scan (at 18-20 weeks)
may detect serious physical problems.
Figure 12 Ultra sound scan
c)
d)
Biochemical tests are used to monitor the health of the mother and the
fetus. These tests detect the normal physiological changes that occur
during pregnancy. Medical conditions can be detected by a range of
marker chemicals that indicate a condition but need not necessarily be
part of the condition.
Diagnostic testing are further tests that may be offered as a result of
routine screening or for individuals in high risk categories. The element
of risk needs to be assessed before deciding to proceed with these tests.
The individuals concerned are then left to decide what to do if a test is
positive. Cells from an amniocentesis sample (as shown in Figure 13
below) can be cultured in order to get enough cells to produce a
karyotype (as shown in Figure 14 below). Karyotypes are used to detect
chromosome abnormalities that cause certain conditions e.g. Down’s
syndrome.
Figure 14 A karyotype (male)
Figure 13 Amniocentesis
Chorionic villus sampling (CVS) from the placenta can be carried out
earlier in a pregnancy than amniocentesis. Although it has a higher risk
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of miscarriage, CVS karyotyping can be performed on the cells of a
fetus immediately.
e)
2.
Rhesus antibody testing – mothers generally do not show any immune
responses to their fetus (i.e. their immune system does not attack it).
However, when a Rhesus-negative mother is pregnant with a Rhesuspositive fetus, a potential problem arises. The problem is sensitisation
to Rhesus antigens. Antigens are “foreign” proteins. Rhesus antigens on
the surface of the fetus’s red blood cells are regarded as foreign by the
mother’s immune system if she comes into contact with them at a
sensitising event e.g. during birth.
After the birth, anti-Rhesus antibodies
are given to Rhesus-negative mothers in
order to destroy any Rhesus antigens left
behind by the baby before the mother’s
immune system has had time to respond to
them. (See Figure 15 opposite).
Postnatal screening
a)
Diagnostic testing for metabolic disorders such as phenylketonuria
(PKU) can be carried out. PKU is
Figure 15 Rhesus-negative mother and Rhesus positive fetus
an inborn error of metabolism
which is the result of a gene mutation. Babies are tested
immediately after birth for PKU. If they are found to have this
condition, they are put on a phenylalanine-free diet. If PKU is not
detected soon after birth, the baby’s mental development is
greatly affected.
Web site
http://www.google.co.uk/url?sa=t&rct=j&q=testing+for+PKU+site%3Ayoutube.com&source=web&cd=3&cad=rja&uact=8&ved=0CDwQtwIwAg&url=http%
3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3Dw3L2SPj7alQ&ei=qcqFU_6NBcmL0AXCsIGYAg&usg=AFQjCNGxPWwy3CZiVzWKAsP0C2rFAbtTmw
b)
Genetic screening and counselling
A pattern of inheritance can show up by collecting information
about a particular characteristic from members of a family and
then using it to make a family tree or pedigree chart like those
shown in Figure 16 and Figure 17 below.
Standardised nomenclature and symbols are used when
constructing pedigree charts. (Nomenclature is a set or system of
names or terms, as those used in a particular science e.g. genetics).
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Figure 16 Family pedigree chart showing inheritance of PKU
Figure 17 Family pedigree chart of Queen Victoria showing inheritance of haemophilia
Web site
http://www.youtube.com/watch?v=bmQwMllhCUM
c)
These pedigree charts are constructed and then used by a genetic
counsellor to inform and advise a couple who may be worried about
the possibility of passing on a genetic disorder which runs in their
family to their children. There are different patterns of
inheritance e.g. autosomal recessive, autosomal dominant,
incomplete dominance (also known as co-dominant) and sex-linked
gene disorders. (Refer to separate booklet).
Pre-implantation genetic diagnosis (PGD)
This is when IVF along with PGD is used to identify single gene disorders
and chromosomal abnormalities.
Objections to PGD are made on moral and ethical grounds because more
embryos are formed than will be used.
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The cardiovascular system
The cardiovascular consists of:
 blood (a fluid connective tissue)
 blood vessels (the tubes in which blood is confined)
 the heart (a muscular pump that keeps blood circulating)
Blood circulates from the heart through the arteries  capillaries  veins and
then back to the heart again as shown in Figure 18 below.
Direction of the flow of blood.
Figure 18
Direction of blood flow from the heart
Blood pressure decreases as the blood moves ways from the heart.
Web site
http://www.google.co.uk/url?sa=t&rct=j&q=blood+vessels+site%3Ayoutube.com&source=web&cd=2&cad=rja&uact=8&ved=0CDkQtwIwAQ&url=http%3A
%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DCjNKbL_-cwA&ei=1y6MU8PbJcXFOb_kgNAK&usg=AFQjCNHcgG6jWvYKjv76cHvPJPjILLUu8Q
1.
The structure and function of blood vessels
Blood vessels have a central lumen which is lined by a layer called the
endothelium.
lumen
muscular wall
endothelium
Figure 19 General structure of a blood vessel
The endothelium lining is surrounded by more layers of tissue shown in
Figure 19 above that differs between arteries, capillaries and veins.
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1.
Arteries
Arteries have an outer layer of connective tissue that contains elastic
fibres. The middle layer of an artery contains smooth muscle cells with
more elastic fibres as shown in Figures 20 and 21 below.
Figure 21 Cross section of an artery
Figure 20 Simplified structure of an artery
The elastic walls of the arteries stretch and recoil. This is to
accommodate the surge of blood that occurs with each contraction of
the heart. The smooth muscle can contract causing the artery to narrow
(vasoconstriction) or relax causing the artery to widen (vasodilation).
This narrowing or widening of the lumen, controls blood flow.
2.
Capillaries
Capillaries are thin-walled and are in close contact with cells. Both of
these features help to facilitate the exchange of substances between
our cells and our blood. Capillaries link arteries to veins as shown in
Figure 22 below.
Figure 22 Capillary network
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3.
Veins
Like arteries, veins also have an outer layer of connective tissue that
contains elastic fibres, but this muscular outer wall is much thinner than
that of an artery.
Figure 23 Simplified structure of a vein
Figure 24 Cross section of an vein
Veins also have valves as shown in Figure 25 below. The function of a
valve is to prevent the backflow of blood.
Figure 25 Valves in vein
For information only!!!!
Only two arteries in the human body have valves – called semilunar valves.
These valves are in the aorta and the pulmonary artery and their function is
to prevent the blood flowing back into the heart.)
The Exchange of materials (between tissue fluid and body cells)
Blood consists of living cells and a watery yellow fluid called plasma. Plasma
contains many dissolved substances such as glucose, amino acids, respiratory
gases, plasma proteins and useful ions e.g. sodium and potassium.
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Blood arriving at the artery-side of a capillary network is at a higher pressure
than the blood in the capillaries themselves. As the blood is forced into these
narrow vessels, something called pressure filtration occurs. This process
results in most of the plasma (along with small dissolved molecules) being
squeezed out through the capillaries thin walls. This liquid, which bathes our
cells is now called tissue fluid. The only way that it differs from blood plasma is
that it contains little or no protein. Tissue fluid supplies cells with glucose,
oxygen and other substances. Carbon dioxide and other metabolic wastes
diffuse out of the cells and into the tissue fluid to be excreted.
Much of this tissue fluid returns to the blood. Lymphatic vessels absorb excess
tissue fluid and then return this “lymph fluid” back to the blood. (In other
words, there is an exchange of substances between our lymphatic and
circulatory systems.) This is shown in Figure 26 below.
Figure 25 Lymphatic and circulatory systems
Web site
http://www.google.co.uk/url?sa=t&rct=j&q=blood+vessels+site%3Ayoutube.com&source=web&cd=2&cad=rja&uact=8&ved=0CDkQtwIwAQ&url=http%3A%2F
%2Fwww.youtube.com%2Fwatch%3Fv%3DCjNKbL_-cwA&ei=1y6MU8PbJcXFOb_kgNAK&usg=AFQjCNHcgG6jWvYKjv76cHvPJPjILLUu8Q
Structure and function of the heart
The heart is a muscular pump whose function is to pump blood around the body.
It consists of four chambers – the two top chambers that receive blood are
called atria and the two bottom chambers which pump the same volume of blood
out of the heart are called ventricles. See Figure 26 below.
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Figure 26 Structure of the heart
Web site
http://www.bbc.co.uk/learningzone/clips/hearts-and-how-to-keep-them-healthy/1466.html
http://www.bbc.co.uk/learningzone/clips/anatomy-and-physiology-of-the-heart/5367.html
The volume of blood pumped through each ventricle per minute is called the
cardiac output. Cardiac output is determined by two things:
1.
heart rate (the number of times the heart beats per minute)
2.
stroke volume
(the volume of blood pumped out of the heart
by each ventricle when they contract)
Cardiac output is summarised by the following equation:
CO = HR X SV
The table below shows the effect of exercise on cardiac output for an adult
human.
Body state
Heart rate
(beats per min)
Stroke volume
(ml)
Cardiac output by each
ventricle (l/min)
60
120
60
70
3.6
8.4
180
80
14.4
at rest
during exercise
During strenuous
exercise
Web site
http://www.google.co.uk/url?sa=t&rct=j&q=what+is+cardiac+output+site%3Ayoutube.com&source=web&cd=5&cad=rja&uact=8&ved=0CFUQtwIwBA&url=http%3A%2F%2Fwww.youtub
e.com%2Fwatch%3Fv%3DbUW-2GHfX64&ei=p3uMU_qGEMfeOcyegcgD&usg=AFQjCNECNBTFQ0WtIsHQq1unapX26CFjyg
The Cardiac Cycle
The cardiac cycle refers to the pattern of contraction (systole) and relaxation
(diastole) of the heart during one complete heartbeat as shown below in Figure
27. On average the cardiac cycle last for 0.8 seconds.
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Figure 27 Contraction (systole and relaxation (diastole)of the heart
During diastole, blood that has returned to the heart via the atria, flows into
the ventricles. During atrial systole, the atria contract and pump the blood
through the atrioventicular (AV) valves and into the ventricles. Ventricular
systole closes the AV valves and pumps the blood through the semilunar valves
(SL) into the aorta (from the left ventricle) or the pulmonary artery (from the
right ventricle).
In disatole, the higher pressure in these arteries causes the semilunar valves
to close. The opening and closing of the atrioventricular and semilunar valves
are responsible for the “lub dub” heart sounds of the heart that can be heard
with a stethoscope.
Web sites
http://www.bbc.co.uk/learningzone/clips/anatomy-and-physiology-of-the-heart/5367.html
http://www.youtube.com/watch?v=5tUWOF6wEnk
https://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter22/animation__the_cardiac_cycle__quiz_1_.html
The cardiac conducting system
The heart beat originates in the heart itself but it is regulated by both
nervous and hormonal control. The sequence of events that occurs during each
heartbeat is brought about by the activity of the pacemaker and the conducting
system of the heart.
The pacemaker
The pacemaker which is also known as sino-atrial node (SAN), is located in the
muscular wall of the right atrium. It consists of specialised cells called
autorhythmic cells. These cells are responsible for setting the rate at which
the cardiac muscle cells of the heart contract. The timing of cardiac muscle
cells contracting is controlled by a nerve impulse from the SAN (pacemaker)
spreading through the right and left atria, and then travelling to the
atrioventricular node (AVN) and then on through the ventricles. These nerve
impulses generate electrical currents that can be picked up by an
electrocardiogram (ECG). See Figure 28 below.
Web sites
http://www.youtube.com/watch?v=lK32IfBw4hI
http://www.google.co.uk/url?sa=t&rct=j&q=fuction+of+the+pacemaker+site%3Ayoutube.com&source=web
&cd=3&cad=rja&uact=8&ved=0CDoQtwIwAg&url=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3D
IR0nWe47eek&ei=ut6NU-SSHIed0QX_t4CYBQ&usg=AFQjCNGfGlKR2S7jNMBD0tSN2KVGg2VOSg
conducting
fibres
Figure 28 Position of the pacemaker(SAN) and atrioventricular node (AVN)
19
The pacemaker works
automatically, and it would
continue to function even
in the absence of nerve
connections from the rest
of the body.
Our heart is regulated by the antagonistic (opposing parts) action of the
autonomic nervous system (ANS). Control centres in the medulla in the brain
are responsible for controlling our heart rate. This is achieved by sending
nerve impulses to the heart that then regulate the rate of the SAN
(pacemaker).
There are two pathways through which these nerve impulses can travel. The
two pathways are antagonistic to one another in that they have completely
opposite effects on heart rate. The two pathways are:
1.
sympathetic (nerves) – this increases heart rate
2.
parasympathetic (nerves) – this decreases heart rate
Sympathetic accelerator nerves release the hormone adrenaline (also called
epinephrine) which speeds up heart rate. Parasympathetic nerves release a
chemical called acetylcholine which slows down heart rate as shown in Figure 29
below.
medulla
parasympathetic nerves
– decrease heart rate
sympathetic nerve –
increase heart rate
Blood
Figure 29 Effect of sympathetic and parasympathetic nerves on heart rate
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pressure
Blood pressure is the force that is exerted by the blood against the walls of
the blood vessels. It is due to the contraction of the ventricles in the heart.
Blood pressure is at its highest in the large elastic arteries (the aorta and the
pulmonary arteries) that transport blood away from the heart. Blood pressure
is measured using an instrument called a sphygmomanometer, and it is measured
in millimetres of mercury (mm Hg) as shown in Figures 30 and 31 below.
Figure 31 Measuring blood pressure using a sphygmomanometer
Figure 30 A sphygmomanometer
Blood pressure changes in the aorta during the cardiac cycle. An inflatable cuff
is needed to stop the flow of blood. It is then allowed to deflate gradually
which, in turn, allows the blood to flow again and this can be detected by a pulse
which can be heard through a stethoscope. When the pulse is heard, this is
taken to be the systolic pressure. The systolic pressure measures the pressure
inside an artery (in your arm) when the heart beats. The blood then begins to
flow freely through the artery and when the pulse can no longer be heard –
this is the diastolic pressure. Diastolic pressure measure the pressure in the
blood vessels between heartbeats when the heart is resting.
Web site
http://www.youtube.com/watch?v=TkDlofyeWSo
A typical blood pressure reading for a young adult is 120/70 mmHg (120 over
70). 120 is the systolic reading, and 70 is the diastolic reading. Because our
blood pressure changes so often throughout the day, the reliability of the
results is improved by taking several readings and not just one.
Web site
http://www.google.co.uk/url?sa=t&rct=j&q=measurung+blood+ressure+site%3Ayoutube.com&source=web&cd=8&cad=rja&uact=8&ved=0CFUQtwIwBw&url=http%3A%
2F%2Fwww.youtube.com%2Fwatch%3Fv%3D2A_wy8r93os&ei=OCKQU6X7BdSY0AWihYGgDA&usg=AFQjCNGeFzvLyBjORp3oSgMSi83Rwgs1QA
High blood pressure (also called hypertension) is a major risk factor for many
diseases e.g. coronary heart disease and/or strokes, and this is why it is very
important to have your blood pressure checked regularly. (People with systolic
pressure higher than 140 mmHg and diastolic pressure higher than 90 mmHg
are deemed to have high blood pressure).
21
Pathology of cardiovascular disease (CVD)
1.
Atherosclerosis
Atherosclerosis is due to an atheroma or plaque beneath the endothelium
layer in an artery as shown in Figure 32 below.
Figure 32 Formation of an atheroma (or plaque)
An atheroma or plaque is due to an accumulation of fatty material
(consisting mainly of cholesterol), fibrous material and calcium. As the
atheroma grows, the artery wall thickens and causes it to lose it’s
elasticity (often called “hardening of the arteries”). The diameter of the
artery is reduced and the flow of blood becomes restricted. This in turn,
leads to increased blood pressure.
Atherosclerosis is the root cause of various cardiovascular diseases such
as:
o angina
o heart attack
o stroke
o peripheral vascular disease (e.g. deep vein thrombosis – see pages
34 and 24)
2.
Web site
http://www.youtube.com/watch?v=7JihvMpEP4w
Thrombosis
22
Atheromas may rupture and when this happen the endothelium is
damaged. The damage releases clotting factors. These clotting factors
activate a cascade of reactions that results in the conversion of the
enzyme prothrombin (which is inactive) into its active form called
thrombin.
Thrombin then causes molecules of a plasma protein called fibrinogen to
form threads of fibrin. The fibrin thread in turn, form a web that causes
the blood to clot.
prothrombin
(inactive)
thrombin
(active)
fibrinogen
(soluble)
fibrin
(insoluble)
A blood clot seals a wound and also provides a scaffold for the formation
of scar tissue.
Web site
http://www.youtube.com/watch?v=--bZUeb83uU
The formation of a blood clot inside a blood vessel (thrombus) is
referred to as thrombosis.
A thrombosis in a coronary artery may lead to a heart attack (myocardial
infarction). A thrombosis in the brain may lead to a stroke. The reason
why a thrombus is so dangerous is that it deprives cells of oxygen which,
in turn, leads to the death of the tissues.
In some cases, a thrombus may break loose and if it does, it is called an
embolism. An embolism travels through the bloodstream and may
eventually block a blood vessels, which then obstructs the flow of blood.
Web site
http://www.google.co.uk/url?sa=t&rct=j&q=how+does+an+embolism+form%3F+site%3Ayoutube.com&source=web&cd=9&cad=rja&uact=8&ved=0CFsQtwIwCA&url
=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DXgTRS_InDgg&ei=LW6QU6LZB8ec0QXE7YCwAg&usg=AFQjCNHSvcgX441U22udmzAtg4t77L27fg
3.
Peripheral vascular disorders
Peripheral vascular disease is caused by a narrowing of the arteries. This
is due to atherosclerosis of the arteries other than those that transport
blood to the brain or heart. Pain is experienced in leg muscles due to a
limited supply of oxygen.
A deep vein thrombosis (DVT) is a blood clot that forms in a deep vein,
most commonly in the leg – see Figure 33. If the clot breaks off and
23
then travels through the bloodstream to the lungs, it may result in a
pulmonary embolism.
Web site
http://www.youtube.com/watch?v=0PEhvACEROI
Figure 33 Development of deep vein thrombosis (DVT) in the leg
4.
Cholesterol and atherosclerosis
Most cholesterol (which is a lipid) is synthesised in the liver from
saturated fats in the diet. Cholesterol is a basic component of cell
membranes, and it is also the precursor for the synthesis of steroids e.g.
the sex hormones oestrogen and testosterone. Therefore, the presence
of cholesterol in the bloodstream at an appropriate concentration is
essential to the health and well-being of the human body.
Transport of cholesterol
Lipoproteins are molecules that consist of lipids and proteins. They are
present in the blood plasma and they are responsible for transporting
cholesterol (and other lipids) from the liver to other parts of the body.
There are two types of lipoproteins:
1.
high-density lipoproteins (HDLs) and
2.
low-density lipoproteins (LDLs)
A.
HDLs
High-density lipoproteins transport excess cholesterol away from
body cells to the liver to be eliminated. This helps to prevent
cholesterol accumulating in the blood.
B.
LDLs
Low-density lipoproteins transport cholesterol to body cells. Most
cells have LDL receptors that take LDLs into the cell. Once inside
the cell, the LDLs release the cholesterol. Once the cell has a sufficient level
of cholesterol, a negative feedback system inhibits the synthesis of new LDL
24
receptors. This results in LDL molecules circulating in the blood where they
may end up depositing cholesterol in the arteries which then forms an
atheroma.
A higher ratio of HDLs to LDLs will result in lower blood cholesterol. This in
turn reduces that chance of developing atherosclerosis and all the other
condition associated with it.
Web site
http://www.youtube.com/watch?v=hjv5OnbcjE8
Certain life changes can be made to help to raise HDL levels in the blood. These
include:

regular physical activity (i.e. exercise)

reducing the level of total fat in the diet (replacing saturated fats with
unsaturated)

using drugs called statins (these drugs work by inhibiting the synthesis of
cholesterol by liver cells)
Web site
http://www.youtube.com/watch?v=bZ_y6I6fPqc
Unfortunately, some people are genetically predisposed to having high
cholesterol levels. This is called familial hypercholesterolaemia (FH) and it is
due to an autosomal dominant gene. FH genes cause a reduction in the number
of LDL receptors or alter the structure of the receptors. Genetic testing can
be done in order to determine if someone has inherited the FH gene or not. If
they have, they can then be advised how to modify their lifestyle and also be
treated with drugs.
Web site
http://www.google.co.uk/url?sa=t&rct=j&q=familial%20hypercholesterolaemia%20site%3Ayoutube.com&source=web&cd=3&cad=rja&uact=8&ved=0CEIQtwIwAg&url=h
ttp%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DUIjkAPn2CRE&ei=DqWRU_3yLOuW0QXzwoHIBg&usg=AFQjCNGCJW0dTEvqGO5bF66HG55XEwwCZA
5.
Blood glucose levels and vascular disease
Chronically elevation of blood glucose levels leads to the endothelium cells
lining the blood vessels taking in more glucose than normal, and this
damages the blood vessel. As a result, atheroscelrosis may develop
leading to cardiovascular disease, stroke or peripheral vascular disease.
Small blood vessels damaged by elevated glucose levels may result in the
blood vessels in the retina of the eye haemorrhaging (i.e. internal
bleeding – due to blood leaking out of a blood vessel). Elevated glucose
levels can also cause renal (kidney) failure or peripheral nerve
dysfunction.
Regulation of blood glucose
25
The level of glucose in the blood is regulated by the hormones insulin and
glucagon. Both of these hormones are produced by the pancreas and both of
them affect the liver. This is achieved through negative feedback control as
shown in Figure 34 below.
Figure 34 Negative feedback control of glucose
When the level of glucose is too high, receptor cells in the pancreas respond to
this and produces more insulin (and less glucagon). The insulin causes the liver to
remove any excess glucose (which is soluble) from the blood and convert it into
insoluble glycogen. The glycogen is then stored in the liver, and the
concentration of glucose in the blood decreases and returns to its set point.
When the level of glucose is too low, receptor cells in the pancreas respond to
this and produce more glucagon (and less insulin). The glucagon causes the liver
to convert glycogen into glucose which is then secreted into the blood, and the
concentration of glucose in the blood increases and returns to its set point.
Web site
http://www.google.co.uk/url?sa=t&rct=j&q=control+of+glucose+by+insulin+and+glucagon+site%3Ayoutube.com&source=web&cd=4&cad=rja&uact=8&ved=0CDcQtwIwAw&ur
l=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DVL_HM8VmjAM&ei=oMORU4rgIMyY1AWtsoGADA&usg=AFQjCNGqgDfD3cwgXUd-4d3ap7WvI4AIXQ
During exercise, or “fight or flight” responses like -s Figure
stress and fear, glucose levels in the blood are
raised. This is due to another hormone called
adrenaline (also called epinephrine) that is released
from the adrenal glands – see Figure 35 opposite.
Adrenaline stimulates the secretion of glucagon whilst
at the same time, it inhibits the secretion of insulin.
Diabetes
Figure 35 Adrenal glands
26
A diabetic is unable to control their glucose levels. Vascular disease can be a
chronic complication of diabetes. There are two different forms of diabetes:
1.
Type 1 diabetes
This type usually occurs in childhood.
2.
Type 2 diabetes
This is also called “adult onset diabetes” as it typically
develops later in life. It mainly occurs in individuals
who are overweight.
A person with Type 1 diabetes is unable to produce insulin and can be treated
Web site
http://www.google.co.uk/url?sa=t&rct=j&q=type+1+diabetes+site%3Ayoutube.com&source=web&cd=3&cad=rja&uact=8&ved=0CE0QtwIwAg&url=http%3A%2F%2Fwww.yo
utube.com%2Fwatch%3Fv%3D_OOWhuC_9Lw&ei=kGaVU67bD8j70gWau4HoDw&usg=AFQjCNH4wKyH30Ep2XLs_YCp-UutnsE2Hw
http://www.youtube.com/watch?v=nBJN7DH83HA
with regular injections of this hormone. In Type 2 diabetes individuals do
produce insulin, but their cells have become less sensitive to it – this is
referred to as insulin resistance.
Insulin resistance is linked to a decrease in the number of insulin receptors in
the liver. This, in turn, means that excess glucose is not converted into
glycogen. In both types of diabetes this results in a rapid rise in blood glucose
levels after a meal. The kidneys are unable to cope and so glucose ends up being
excreted in a person’s urine. Testing urine for glucose is often used as an
indicator of diabetes.
Glucose tolerance is the body’s ability to deal with ingested glucose, and it is
dependent on the body being able to produce adequate quantities of insulin. The
glucose tolerance test is used to diagnose diabetes by investigating whether (or
not) insulin production is normal.
The blood glucose levels of an individual are measured after fasting (not eating)
for at least eight hours. They are then asked to drink 250-300ml of glucose
solution. Their blood glucose level is then monitored over a period of 2½ hours
and the results are plotted to give a glucose tolerance curve.
Web site
http://www.bbc.co.uk/learningzone/clips/the-effect-of-high-sugar-intake-on-blood-sugar-levels/5371.html
5.
Obesity
Obesity is a major risk factor for cardiovascular disease and type 2
diabetes. Obesity is characterised by excess body fat in relation to lean
body tissue (muscle).
Every person has an ideal body mass (weight). The body mass index
(BMI) for an individual can be calculated by dividing their weight by their
height squared:
27
body mass
BMI =
height2
If a person has a body mass index (BMI) greater than 30, they are deemed to
be obese.
Example:
Weight =
Height =
BMI =
85 kg
1.8m
85
= 26.2 This person is not obese.
2
1.8m
Web site
http://www.youtube.com/watch?v=Vb-nz6op8yg
There are limitations when using the BMI method. Body builders, who have a
relatively low percentage of body fat and an unusually high percentage of muscle
would be wrongly classified as being obese.
In order to measure body fat accurately, a measurement of body density is
needed.
[For information only: Body density is a measurement that expresses your total body mass or weight in
relation to your body volume or the amount of space that your body occupies. Mathematically, it is body
mass divided by body volume. The measurement of body density is commonly used to estimate percentage
of body fat.]
Obesity is linked to:
 high fat diets and
 not taking enough exercise
The amount of energy gained from our diet, should be low in fat and free
sugars. Fats, because they have a high calorific value per gram, and free sugars
because they require no metabolic energy to digest them. Exercise increases
energy expenditure (i.e. the amount of energy we use) and it also helps to
preserve lean tissue. Regular exercise can help to reduce the risk factors for
cardiovascular diseases by helping to keep a person’s weight under control.
Exercise also helps to minimise stress, reduce hypertension (high blood
pressure), and improve HDL blood lipid profiles.
Web site
http://www.bbc.co.uk/learningzone/clips/coronary-heart-disease/5700.html
28