(a) Define digestion. [2 m] (b) Describe the digestion of a piece of ugali in a human digestive system from the mouth until its assimilation. [18mks]

FERTILIZATION.

• This is the fusion of sperm and ovum to form a zygote. It occurs in the oviduct. 
• The human ovum is viable for only 24 hours after which in degenerates. 
• As the sperm approaches the ovum, it undergoes capacitation. 
• This is a process that involves changes in the properties of the acrosome. 
• Once the sperm contacts the ovum, its acrosome bursts spilling the lytic enzymes. 
• The enzymes digest and soften the corona radiata, zona pellucida and vitelline
• membrane. 
• Only one sperm fertlises the ovum.This is because upon the entry of the sperm, the vitelline membrane is transformed into an impenetrable membrane.
• Development starts immediately after fertilization. 
• The zygote undergoes repeated mitotic division in a process known as cleavage resulting in a mass of cells known as the morula.
• The cells then arrange themselves to form a hollow structure known as blastula/blastocyst.
• Cilia and peristaltic action of the oviduct wall propel the developing zygote towards the
• uterus. 
• The blastula then embeds itself into the uterine wall in a process known as implantation. 
• This occurs 7 – 8 days after fertilization. 
• Once implanted the blastula secretes the human chorionic gonadotropin hormone whose functions are:

• i) Prevents the regression of the corpus luteum.
• ii) Inhibits the disintegration of the uterine lining thus preventing menstruation.

• On implantation the blastula develops into embryo and associated membranes. 
• The inner most cells become the embryo,amnion, yolksac and the allantois. The outer most cells become the chorion.
• The chorion forms finger-like projections called chorionic villi. 
• Which grow into the endometrium. The amnion surrounds the embryo forming the amniotic cavity. The membranes of the amnion secrete the amniotic fluid in which the embryo is suspended to allow for proper development. 
• The fluid also acts as a shock absorber to protect the embryo from mechanical injury. 
• The allantois grows outward from the embryo and as it comes into contact with the chorion and endometrium it forms the placenta. 
• Later on the allantois gives rise to umbilical artery and vein. The yolk sac has no significant function in placental mammals such as humans. 
• As the embryo grows and develops it assumes a distinct human form and at this point it is known as foetus.

The placenta

• It is a disc shaped structure that is composed of cells from both the foetus and the mother.
• The foetal cells consist of the chorionic membrane and blood capillaries. 
• The maternal tissues consists of the endometrium. 
• The embryo is attached to the placenta by the umbilical cord which carries the umbilical arteries and umbilical vein.
• 

Functions of the placenta.

1. It is the site of exchange of materials between the foetus and the mother. Substances

Exchanged include:-

• i) Nutrients from the mother’s blood to the foetal blood eg, glucose, amino acids,
• mineral salts and vitamins.
• ii) Waste materials from the foetal blood to the mother’s blood eg, urea and carbon
• (IV) oxide.
• iii) Oxygen from the mother to the foetus. The absorption of oxygen is enhanced by the foetal haemoglobin which has a higher affinity for oxygen than maternal haemoglobin. This ensures continuous extraction of oxygen from the mother to the foetus.

2. It shields the foetus from the high blood pressure of the materials circulation.

3. Being semi-permeable it protects the foetus from bacteria fungi and blood parasitic worms that may be in the mother’s blood. However viruses such as HIV may pass through. 

Harmful substances such as alcohol and heroin if present in the mother’s blood will pass through to the foetus.
4. Secretion of hormones which are oestrogen, progesterone, human chorionic gonadotropin and human placental lactogen.

The exchange of substances is determined by the molecular size and concentration gradient. Movement of substances across is mainly by diffusion.

Adaptations of the placenta to its functions.

1. It has a large surface area presented by the chorionic villi to increase the rate of exchange of substances.
2. It has a thin membrane/barrier separating maternal and foetal blood for faster exchange of substances. The barrier consists of chorionic membrane and chorionic blood capillary walls.
3. It is selectively permeable to prevent blood cells, and pathogens from crossing over.
4. It has a counter current flow system between the foetal and maternal circulation.

GESTATION.

• Refers the carrying of an embryo or foetus inside the body of a viviparous female animal especially mammals.
• Gestation period – refers to the duration of pregnancy from fertilisation and birth. It varies from one mammal to another.

Gestation in humans.

• It lasts 9 months divided into three trimesters each consisting of three months. Within each trimester there are major developments that take place. 
• By the end of the ninth month all the organs and organ systems are fully developed and the foetus has turned with the head directly above the cervix. Birth may then occur.

Ectopic pregnancy.

Occurs when the embryo implants in an area other than the uterus. The commonest site is the

fallopian tube. 

It is a fatal condition especially in the fallopian tube where it must be terminated by a doctor before the tube ruptures.

Multiple births.

These occur when more than one foetus is carried to term in a single pregnancy. There are different types of multiple births depending on the number of offspring. Example twins when two, triplets when three and quadruplets when 4.

Twins.
There are two forms of twins.

i) fraternal/dizygotic twins.

Comes about when two eggs are released into the oviduct at the same time. The two eggs are

fertilised by different sperms to form two zygotes that are genetically different.

ii) Identical/monozygotic twins.

Comes about from one egg fertilised by one sperm which during cleavage stage the resulting

embryo splits into two groups of cells each developing into a separate individual. Such

individuals are genetically identical and have many characteristics in common such as same sex.

 

Role of hormones during pregnancy.

1. Oestrogen.

Produced by the ovary.

i) Growth of the mammary glands in preparation for milk production.

ii) Inhibits follicle stimulating hormone release – to prevent ovulation.

iii) It inhibits release of prolactin hormone which in turn inhibits production of milk during pregnancy.

iv) It increases the size of the uterine muscles in order for them to contract powerfully during birth.

v) It increases sensitivity of myometrium to the hormone oxytocin.

vi) It increases ATP formation especially in the uterine tissues to provide energy for the

uterine muscles.

vii) It prevents infection of the uterus by increasing secretion of the mucus at the cervix.

2. Progesterone.

Produced by the ovary in the first three months then later by the placenta.

Functions.

i) Maintains the thickening of the uterine wall and prevents the endometrium from breaking

down.

ii) Brings about growth of the mammary gland.

iii) Inhibits the release of follicle stimulating hormone and prolactin.

iv) It inhibits the contraction of the myometrium therefore preventing miscarriage.

3. Human chorionic gonadotropin.

Produced by the placenta.

Function.

Maintains the activity of the corpus luteum up to the third month after which the now formed

placenta takes over the function of progesterone secretion.

4. Human placental lactogen.

Produced by the pituitary gland.

Function.

Stimulates growth and development of the mammary gland in preparation for lactation.

BIRTH/PARTURITION.

Before birth the pelvic girdle or hip bone widen due to softening of the cartilage at the pubic

symphysis. 
The onset of birth is signalled by reduction in progesterone levels. 
This allows the maternal pituitary gland to release the hormone oxytocin. 

This hormone removes the inhibitory effect on the uterine wall contraction. 

This result in a wave of contraction of the uterine wall commonly referred to as labour pains.

The labour process occurs in three stages.

1. Contraction of the uterine wall leading to dilation of the cervix and the rupture of the

amnion and chorion membranes. The rupture releases amniotic fluid through the cervix and birth canal in a process known as breaking of the water.

2. The uterine contractions then spread down the uterus pushing the foetus downwards

head first through the cervix and birth canal and thus the baby is born.

3. After the birth the umbilical cord is ligatured and cut to separate the baby from the

placenta. 
Continuous contractions of the uterine wall expel the placenta as an ‘afterbirth’ 10 – 15 minutes later.

While in the womb the pulmonary circulation is not functional as oxygenation of the blood

occurs at the placenta. There is a hole known as foramen ovale in the heart that connects the

right atrium to the left atrium so that most of the blood bypass the pulmonary circulation.

LACTATION AND PARENTAL CARE.

The new-born’s digestive system is not fully developed to process the normal adult diet. In

the early stages the mammalian baby is fed exclusively on the milk secreted by the mother’s

mammary glands. During pregnancy the development of the mammary glands is controlled

by oestrogen, progesterone and human placental lactogen.

The hormone prolactin from the anterior pituitary gland initiates the flow of milk into the

mammary glands in a process called lactation.The release of milk is stimulated by the

hormone oxytocin from the posterior pituitary gland.

The first secretion of the breast after birth is known as colostrum. It is a thick yellow fluid

rich in proteins and low in fat. It also contains antibodies that confer natural passive

immunity to the baby.

The role of sex hormones in reproduction.

Secondary sexual characteristics are physical, physiological and mental changes that occur in

males and females at puberty.

In males.

The changes are controlled by androgens secreted by the interstitial cells in the testis. The

commonest androgen is the testosterone. The changes begin to appear at the puberty when the

pituitary gland secretes the interstitial cell stimulating hormone which then stimulates the leydig cell to secrete testosterone.

The testosterone then stimulates the development of the

secondary male sexual characteristics. These include:

i) Growth of facial hairs.

ii) Growth of pubic, armpit and chest hairs.

iii) Broadening of shoulders.

iv) Deepening of the voice due to enlargement of the larynx.

In the female.

The changes which start at puberty include:-

i) Development of mammary glands.

ii) Enlargement of the pelvic girdle leading to widening of the hips. This is in preparation of

the birth process.

iii) Growth of hairs in pubic and armpit regions.

iv) The ovaries mature and start to release eggs. Externally this appears as menstruation.

HORMONAL CONTROL OF OOGENESIS AND MENSTRUAL CYCLE.

The functioning of the female reproductive system has synchronized events controlled by

four hormones: Follicle stimulating hormone (FSH), Luteinizing hormone (LH),

Progesterone and Oestrogen.

These series of events occur in a regular pattern called Oestrus or Menstrual Cycle. It takes

about 28 days in humans and is marked by discharge of blood and tissue debris called

Menstruation.

Conventionally, the first day of the cycle is taken to be the day menstruation starts.

Menstruation phase continues for 5 – 7 days. During this time the endometrium breaks

down and is passed out through the vagina (menses). At the same time, the pituitary gland

secretes FSH which stimulates the development of a primary follicle.

The development of the primary follicle occurs during the follicular phase. As the primary

follicle develops, it secretes oestrogen. Oestrogen stimulates the proliferation of the uterine

wall and stimulates production of Luteinizing hormone from the pituitary gland. As the levels

of oestrogen in the blood rises, it inhibits the release of FSH through negative feedback. The

oestrogen level reaches its peak on the 11th – 12th day, when it induces the release of

luteinizing hormone from the pituitary gland.

The Luteinizing hormone reaches its peak on 12th – 15th day, at which point it induces

ovulation. During the ovulation phase, the mature primary follicle called Graafian follicle

ruptures to release the ovum (primary oocyte). The luteinizing hormone also stimulates the

remains of the Graafian follicle to become mass of yellow body called Corpus luteum.

The corpus luteum releases the hormones Progesterone and Oestrogen during the luteal

phase. Progesterone causes proliferation of the uterine wall in preparation for implantation,

and maintains it. It also inhibits production of FSH and Luteinizing hormone through

negative feedback.

If fertilization fails, the corpus luteum persists for about 14 days after which it regresses,

leading to decrease in progesterone levels. In the absence of progesterone to maintain it, the

thick uterine wall starts to disintegrate and menstrual flow resumes after the 28th day.

The relationship between:-

(a) Pituitary hormones (Follicle Stimulating Hormone and Luteinizing)

(b) Ovarian hormones (Oestrogen and Progesterone)

(c) Follicle and Corpus luteum development.

(d) Thickness of endometrium.

The above events will continue for as long the woman is in her reproductive years.

Menopause.

This refers to the cessation of the monthly periods in a woman. It marks the end of ovulation

and fertility. On average it occurs at 45 – 50 years of age. It begins with periods becoming

irregular before they finally stop. It is caused by the gradual failure of the ovary due to aging.

Infertility.

This is the inability of an adult to reproduce. This condition affects about 10 % of the both

men and women population. There are various causes of infertility. Depending on the cause of infertility there exist various interventions that are carried out to help the infertile couples

get babies. These include:-

i) In-vitro fertilization.

This is whereby fertilization occurs in a test-tube and the zygote then transferred to the womb

for implantation.

ii) Use of surrogate mothers.

These are women who allow their uteri to be used to artificially carry fertilized ovum from

other persons.

iii) Use of fertility drugs.

These are synthetic hormones that are administered to either stimulate the ovaries to release

ova or prepare the endometrium for implantation.

SEXUALLY TRANSMITTED INFECTIONS.

These are infections and diseases transmitted through sexual intercourse.

1. Syphilis.

Caused by bacterium Treponema pallidum.

Symptoms.

a) In the early stages, a person will develop one or more sores. The sores resemble large round bug bites and are often hard and painless.

b) In later stages they experience a rash typically on the palms of the hands and soles of

the feet.

c) If the infection isn't treated, it may then progress to a stage characterized by severe

problems with the heart, brain, and nerves that can result in paralysis, blindness,

dementia, deafness, impotence, and even death if it's not treated.

Treatment.

Treatment is by use of antibiotics.

Control.

Avoiding indiscriminate sex.Use of condoms during sexual intercourse.

2. Gonorrhoea.

Caused by bacterium Neisseria gonorrhoea.

Symptoms in men

a) Greenish yellow or whitish discharge from the penis

b) Burning when urinating

c) Painful or swollen testicles

Symptoms in women

a) Increased vaginal discharge.

b) Painful urination.

c) Vaginal bleeding between periods, such as after vaginal intercourse.

d) Abdominal pain.

e) Pelvic pain.

Treatment

By use of antibiotics.

Control.

Avoiding indiscriminate sex.Use of condoms during sexual intercourse.

3. AIDS.

Stands for Acquired Immune deficiency syndrome. It is a condition in humans in which the

immune system fails leading to life threatening opportunistic infections. It is caused by the

Human Immunodeficiency Virus – HIV.

In addition to sexual intercourse with an infected person, it is also transmitted through:-

i) use of contaminated needle and other sharp skin piercing instruments.

ii) breast milk from an infected mother to the child.

iii) mixing of baby’s and infected mother’s blood during birth.

iv) transfusion and transplants of infected organs and tissues.

Once in the body the HIV infects and destroys lymphocytes and macrophages. Low levels of

lymphocytes in the blood thus weakening the immune system. With a weakened immune

system opportunistic infections set in. at this point the person is said to have developed to

have developed acquired immune deficiency syndrome – AIDS.

Symptoms.

They are many and varied but are all related to the opportunistic infections. The commonest

include:-

i) chronic diarrhoea.

ii) persistent coughing – lasting for more than four weeks.

iii) skin infections.

Treatment.

There is no cure for AIDS.

Prevention and control.

i) avoid indiscriminate and unsafe sex.

ii) avoid sharing of razors, tooth brushes and skin piercing instruments. Management of AIDS.

i) The HIV positive person is given antiretroviral drugs to reduce the viral load in the blood

thus reducing opportunistic infections.

ii) Prompt treatment of any infection.

4. Trichonomiasis.

Caused by the protozoa Trichomonas vaginalis.

Symptoms.

a) Vaginal itch and yellow, smelly discharge in females.

b) Urethral itch and discharge in males.

Treatment.

By use of the drug metronidazole.

Control.

a) Use condoms correctly every time you have sex.

b) Limit the number of sex partners, and do not go back and forth between partners.

c) Practice sexual abstinence, or limit sexual contact to one uninfected partner.

d) Avoid sharing of inner wears.

5. Candidiasis.

Caused by the fungus Candida albicans.

Symptoms.

a) Itching and irritation in the vagina

b) Redness, swelling, or itching of the vulva (the outside part of the vagina)

c) A white discharge that is thick.

d) Pain during sexual intercourse

e) Burning during and after urination

Treatment.

By use of anti-fungal drugs.

6. Herpes

Herpes is a viral disease caused by Herpes simplex virus. Infection with the herpes virus is

categorized into one of several distinct disorders based on the site of infection. Oral herpes,

the visible symptoms of which are known as cold sores or fever blisters, is an infection of the

face or mouth. Oral herpes is the most common form of infection. Genital herpes, known

simply as herpes, is the second most common form of herpes.

Herpes simplex is most easily transmitted by direct contact with a lesion or the body fluid of

an infected individual. Transmission may also occur through skin-to-skin contact.

Symptoms.

Lesions on the skin around the vagina, penis and mouth.

Treatment.

A cure for herpes has not yet been developed. Once infected, the virus remains in the body

for life.

Control and prevention.

a) Abstain from sexual activity or limit sexual contact to only one person who is

infection-free.

b) Use a condom during every sexual intercourse.

c) Avoid intercourse if either partner has an outbreak of herpes in the genital area or

anywhere else.

7. Hepatitis.

Hepatitis refers to viral infections that cause inflammation of the liver.

It is transmitted by having unprotected (without a condom) penetrative sex

Signs and symptoms of hepatitis B

a) A short, mild, flu-like illness and nausea,

b) Vomiting and diarrhoea

c) Loss of appetite

d) Weight loss;

e) Jaundice (yellow skin and whites of eyes, darker yellow urine and pale faeces);

f) Itchy skin.

Control and prevention.

a) Avoid having indiscriminate and unprotected sex.

b) Use of vaccines to prevent infection.

Advantages of sexual reproduction.

1. There is formation of new genetic combinations especially where there is cross

fertilization. The leads to development of desirable characteristics..

2. Brings about variation which is key to evolution.

Disadvantages of sexual reproduction.

1. There is formation of new genetic combinations which may lead to formation of

undesirable characteristics.

2. Genetic errors/mutations may during meiosis leading to formation of undesirable

characteristics.

3. Many organisms fail to reproduce especially where there are separate male and female

individuals.

Advantages of asexual reproduction.

1. It is a faster means of reproduction as it does not involve some stages such as fertilization

and germination.

2. Since there is no genetic variation, the good qualities of the parents are retained in the

offspring.

3. In plants, the new organisms obtain nutrients from the parent thus increasing the chances

of survival.

4. Only one parent is necessary hence it is faster.

Disadvantages of asexual reproduction.

1. Since there is no genetic variation, the offspring inherit undesirable characteristics from the

parent.

2. The species does not adapt at all or adapts very slowly to changing environmental

conditions thus easily becoming extinct.