Topic 11: Animal physiology

Question 1

Substances on cell surfaces

Answer 1

• All living organisms have proteins and other substances in the plasma membranes on the surface of their cells, especially proteins.
• Viruses are not living organisms, and are not composed of cells, however, they also have unique molecules on their surface. Mostly, a protein coat.
• Unique surface molecules are used in several ways: Viruses recognise and bind to their host using molecules on the surface of the host’s cells, living organisms recognise their own cells and cell types or recognise cells that are not part of the organism- antigens.

Question 2

Antigens on red blood cells

Answer 2

• The ABO blood groups system is based on the presence/absence of a group of glycoproteins in the membranes of red blood cells.
• Glycoproteins in this group cause antibody production if a person does not naturally possess them, known as antigens.
• O, A and B antigens are three different versions of the glycoprotein.
• O antigen is always present, A- by adding an N-acetyl-galactosamine to an O, B- made by adding galactose.

Question 3

Host specificity of pathogens

Answer 3

• Some pathogens are species specific- only infect members of a single species. For ex. polio, measles, syphilis (only humans).
• Others can cross species barriers, so can be transmitted between species. For ex. tuberculosis (both cattle and humans…).

Question 4

Histamine and allergies

Answer 4

Two types of cell in the body secrete histamine:

1. Basophils- a type of white blood cell
2. Mast cells- similar but found in connective tissue
   - Histamine is secreted in response to a local infection and causes the dilation of small blood vessels in the infected area. The vessels become leaky, increasing the flow of fluid containing immune components to the infected area and allowing these components to leave the vessel, resulting in specific and non-specific immune response.
   - Allergies- reactions by the immune system to substances which are normally harmless such as pollen.
   - Substances in these allergens cause over-activation of basophils and mast cells and excessive secretion of histamine.
   - This causes the symptoms associated with allergies.
   - Histamine is also involved in anaphylaxis swelling and to lessen the effects of allergic responses, anti-histamine drugs are used.

Question 5

Antibody production

Answer 5

The production of antibodies by the immune system is one of the most remarkable biological processes.

1. Pathogen enters the body
2. Helper T-cells have antibody-like receptor proteins in their plasma membrane to which one specific antigen can bind. When the antigen binds, the helper T-cell is activated. The antigen is brought to the helper T-cell by a macrophage- a type of phagocytic white blood cell.
3. Activation of B-cells- inactive B-cells have antibodies on their plasma membrane. If these antibodies match an antigen, the antigen binds to the antibody and an activated helper T-cell can bind to the B-cell and activate it.
4. Production of plasma cells- activated B-cells start to divide by mitosis and form a clone of cells- plasma cells (have a lot of rough ER for synthesis of antibodies).
5. Production of memory cells- memory cells are B-cells and T-cells that are formed at the same time as activated T-cells and B-cells. Memory cells persist and allow a rapid response if the disease is encountered again- long-term immunity.

Question 6

The role of antibodies

Answer 6

Antibody- an immunoglobulin.
Tips of the variable region are the antigen binding sites.
The constant region is the part that aids destruction of the pathogen.
Different versions of the constant region.
Antibodies:
1. Make a pathogen more recognisable to phagocytes, easier to be engulfed.
2. Prevent viruses
3. Neutralise toxins
4. Bind to the surface of a pathogen cell and burst it
5. Stick pathogens together so they cannot enter host cells and phagocytes can ingest them more easily.

Question 7

Vaccination

Answer 7

Vaccination- contain antigens that trigger to a disease without actually causing it.
Contain weakened or killed forms of the pathogens. Some vaccines contain the chemical that acts as the antigen.
Injected / swallowed.
Antigens cause antibody production.
Two or more vaccinations sometimes needed.

Question 8

Jenner and smallpox vaccination

Answer 8

• Smallpox was the first infectious disease of humans to have been eradicated by vaccination.
• In 1960s and 70s.
• In 1796, Edward Jenner deliberately injected an 8-year-old boy with cowpox using pus, then tried to infect him with smallpox but found that he was immune.
• Today, it would be considered ethically unacceptable.

Question 9

Epidemiology

Answer 9

The study of distribution, patterns and causes of disease in a population.
Helps plan vaccination programmes.

Question 10

Production of monoclonal antibodies

Answer 10

Large quantities of a single type of antibody can be made using an ingenious technique.

• Antigens that correspond to a desired antibody are injected into an animal like a mouse.
• Plasma cells producing the desired antibody are extracted from the animal.
• Tumour cells grown in a lab are obtained.
• The plasma cells are fused with the tumour cells to make hybridoma cells which divide endlessly.
• The hybridoma cells are cultured and the antibodies that they produce are extracted and purified- monoclonal antibodies.

Question 11

Uses of monoclonal antibodies

Answer 11

In pregnancy tests-
The urine of pregnant women contains hCG secreted by a developing embryo, later by placenta. Tests contain monoclonal antibodies to which hCG binds, causing a coloured band to appear, indicating the presence of the hormone and that the woman is pregnant.

Question 12

Structure of skeletal muscle

Answer 12

Skeletal muscle is attached to bone and causes movement of animal bodies.
Consists of large multinucleate cells called muscle fibres.
Within each muscle fibre are myofibrils.
Around myofibrils is the sarcoplasmic reticulum.
Mitochondria between the myofibrils.
Myofibrils consist of repeating units called sarcomeres which have light and dark bands- give it a striated appearance.
Each sarcomere is able to contract and exert force.

Question 13

Structure of a sarcomere

Answer 13

• At either end is a Z line to which narrow actin filaments are attached.
• The actin filaments stretch inwards towards the centre and between them are thicker myosin filaments which have heads that form cross-bridges by binding to the actin.
• The part of the sarcomere containing myosin is the dark band and the part containing only actin is the light band.

Question 14

Sliding filaments and contraction

Answer 14

The contraction of the skeletal muscle is achieved by the sliding of actin and myosin filaments over each other.
Pulls the ends of the sarcomeres together, making the muscle shorter.
The sliding of the filaments is an active process and requires the use of energy from ATP.
The hydrolysis of one molecule of ATP provides enough energy for a myosin filament to slide a small distance along an actin filament.

Question 15

Control of muscle contraction

Answer 15

• When a motor neuron stimulates a striated muscle fibre, calcium ions are released from the sarcoplasmic reticulum.
• The calcium binds to troponin, causing the shape of troponin to change and the movement of tropomyosin which exposes the binding sites on actin. —– This allows the myosin heads to form cross-bridges by binding to actin.
• Radioactive calcium has been used to investigate the control of muscle contraction. For ex. using autoradiography showed that radioactive calcium is concentrated in the region of overlap between actin and myosin filaments in a contracted muscle but not in relaxed muscle.

Question 16

Mechanism of muscle contraction

Answer 16

The sliding of actin filaments over myosin filaments towards the centre of the sarcomere is achieved by a repeated cycle of stages, in which cross-bridges form and break and energy is released by the hydrolysis of ATP.

1. Myosin filaments have heads which form cross-bridges when attached to binding sites on actin.
2. ATP binds to the myosin heads and causes them to break the cross-bridges.
3. ATP is hydrolysed to ADP and phosphate, causing the myosin heads to change their angle.
4. The head attach to binding sites on actin that are further from the centre of the sarcomere than the previous sites.
5. The ADP is released and the heads push the actin filament inwards towards the centre of the sarcomere- power stroke.

Question 17

Muscles and movement

Answer 17

• Muscles provide the forces needed to move animal bodies.
• Only exert force when they contract and not when they relax and lengthen- can only cause movement in one direction.
• So antagonistic pairs are needed.
• Muscles are elongated structures with tendons forming attachments at both ends.
• One end of the muscle- anchorage (bones in humans and other vertebrates).
• In insects and other arthropods the exoskeleton provides the anchorage.
• Other end of the muscle- insertion.
• Bones and exoskeleton can change the size and direction of the force by the muscle so act as levers.

Question 18

The elbow joint

Answer 18

Biceps- the flexor muscle to bend the arm
Triceps- the extensor muscle to straighten the arm
Humerus bone- provides a firm anchorage for the muscles
Joint capsule- seals the joint
Synovial fluid- lubricates the joint to reduce friction
Cartilage- a layer of smooth and tough tissue to reduce friction
Tendon- attaches muscle to bone
Radius- bone that is the insertion for the biceps and acts as a lever
Ulna- bone that is the insertion for the triceps and acts as a lever

Question 19

Synovial joints

Answer 19

• Junctions between bones are called joints. Some are fixed like joints between the plates of bone in the skull. - - - Others allow movement- synovial joints.
• Synovial joints consist of: cartilage, synovial fluid, joint capsule.
• There are also ligaments which are tough cords of tissue connecting the bones on opposite sides of a joint. They restrict movement and help to prevent dislocation.

Question 20

Antagonistic muscles in an insect leg

Answer 20

Insects have many joints in their legs.
A pair of antagonistic muscles causes these opposite movements.
For ex. in the legs of crickets there are two large muscles inside the femur. The tendons at the distal ends are attached to opposite sides of the exoskeleton of the tibia so one of them is a flexor of the joint between the femur and the other is an extensor.

Question 21

Excretion

Answer 21

• Metabolic pathways are chains and cycles of reactions in living cells used to build up and break down biochemicals.
• In all organisms the metabolic pathways produce waste products that would be toxic if they were allowed to accumulate in cells, so they must be removed.
• The removal from the body of potentially toxic waste products of metabolic pathways is called excretion.

Question 22

Osmoregulation

Answer 22

Water moves in and out of cells by osmosis.
The direction is determined by hydrostatic pressure and solute concentration.
If the pressures are equal, water movement depends on concentrations.
Living organisms can control their movement of water by adjusting their solute concentrations.
Control of internal solute concentration of a living organism- osmoregulation.

Question 23

Nitrogenous waste products

Answer 23

3 nitrogenous waste products excreted by animals:

1. Ammonia- by freshwater fish and larvae. Toxic and has to be excreted as a very dilute solution so a large volume of water required.
2. Urea- by marine mammals, terrestrial mammals, marine fish and adult amphibians. Less toxic, so can be excreted as a more concentrated solution with less water. Conversion of ammonia to urea requires energy but it is worthwhile.
3. Uric acid- by birds and insects. Not toxic even when concentrated so much that forms a semisolid paste. Conversion of ammonia to uric acid requires a lot of energy but worthwhile for animals who need to conserve water or fly (reduces body mass).

Question 24

Osmoconformers and osmoregulators

Answer 24

Osmoconformers- allow their internal solute concentration to fluctuate with that of the water around them, do not attempt to maintain a constant internal solute concentration. For ex. squids.

Osmoregulators- most terrestrial organisms. Maintain a constant internal solute concentration. For ex. humans.

Question 25

Dehydration and over-hydration

Answer 25

• Isotonic- a solute concentration equal to that of normal body fluids.
• Hypotonic- a lower solute concentration than normal body fluids.
• Hypertonic- a higher solute concentration than normal body fluids.
• Dehydration- due to loss of water from the body, but not equal quantity of solutes so body fluids become hypertonic. Produces small quantities of dark concentrated urine and thirst, lethargy, low blood pressure, seizures, death, brain damage.
• Over-hydration- excessive intake of water, body fluids become hypotonic. Produces behaviour changes, confusion, nausea, blurred vision, muscle cramps, coma, seizures, death.

Question 26

Structure and functions of the kidney

Answer 26

Kidney has 2 functions: osmoregulation and excretion.

Cortex - outer, medulla - inner, pelvis - next to medulla, ureter, renal vein, renal artery.

Question 27

Structure of the nephron

Answer 27

Glomerulus is linked to nephron.
A group of nephrons joins up to form one collecting duct.
The glomerulus and Bowman’s capsule produce a filtrate by ultrafiltration.
The proximal convoluted tubule transfers useful substances from the filtrate back into the blood by selective reabsorption.
The loop of Henle establishes high solute concentrations in the medulla.
The distal convoluted tubule adjusts individual solute concentrations and the pH of the blood.
The collecting duct carries out osmoregulation by varying the amount of water reabsorbed.

Question 28

Ultrafiltration in the glomerulus

Answer 28

• Glomerulus- a knot-like ball of blood capillaries.
• All capillaries let some fluid leak out but 20% of the plasma escapes from glomerulus capillaries.

Two reasons:

1. Very high blood pressure
2. Many large pores in the capillary walls.

Two filters beyond the pores that control what passes through:

1. Basement membrane- a gel outside of the capillary, with small gaps through a mesh of protein fibres.
2. Filtration slits- narrow gaps between the foot process of podocyte cells.

Question 29

Selective reabsorption

Answer 29

Most of selective reabsorption occurs in the proximal convoluted tubule.
Large volumes of glomerular filtrate are produced- about one litre every 10 minutes by the two kidneys.
The wall consists of a single layer of cells and has microvilli.
Pumps in the membrane reabsorb useful substances by active transport using ATP produced by mitochondria in the cells.
All of the glucose is reabsorbed, 80% of mineral ions such as sodium. Water moves by osmosis from the filtrate to the cells and then into the blood.

Question 30

Role of the loop of Henle

Answer 30

Filtrate flows through into the medulla into the descending limb of loop of Henle.
Descending and ascending limbs are opposite in terms of their permeability.
Descending limb- permeable to water but impermeable to sodium ions.
Ascending limb- permeable to sodium ions but impermeable to water.
The overall effect of loop of Henle is to increase the solute concentration in the medulla in comparison to normal body fluids.
After the loop of Henle, the filtrate passes through the distal convoluted tubule where the ions can be exchanged between the filtrate and the blood to adjust blood levels.
Then passes into the collecting duct.

Question 31

ADH and osmoregulation

Answer 31

• Osmoregulation- control of solute concentrations in the body fluids, especially the blood plasma.
• The collecting duct has an important role in osmoregulation.

If the water content is too low:

1. Pituitary releases ADH aka vasopressin.
2. This hormone makes the cells of the collecting duct more permeable to water (water channels called aquaporins used).
3. As the filtrate passes down the collecting duct through the medulla, the high solute concentration of the medulla causes much of water in the filtrate to be reabsorbed by osmosis.
4. Causes a small volume of concentrated urine to be produced and blood plasma becomes more dilute.

Question 32

Treatment of kidney failure

Answer 32

Kidney failure- serious condition because toxins build up in the body and solute concentrations are not maintained at the normal level. Untreated kidney failure makes the patient feel increasingly ill and is eventually fatal.

1. Hemodialysis- blood is drawn out of a vein in the arm and passed through a kidney machine for 3-4 hours, 3 times a week. The blood flows next to a semi-permeable dialysis membrane with dialysate on the other side. Pores through the membrane allow small particles to diffuse in either direction but plasma proteins and cells are retained in the blood. Advantages- no urea or waste products, ideal concentrations of glucose and other metabolites, high calcium and low potassium, hydrogencarbonate ions to reduce acidity, a total solute concentration that will cause excess water to be removed by osmosis from blood.
2. Kidney transplants- dialysis can keep the patient alive for years but a better long-term treatment is a transplant. Sometimes a living donor provides one kidney and in other cases the kidneys of a recently died person are used (if the same blood group and their tissue matches).

Question 33

Urine tests

Answer 33

Samples of urine are easily obtained and can be tested for the presence of abnormalities.

• Blood cells- their presence is caused by a variety of diseases like infections and cancers.
• Glucose- almost always indicates diabetes.
• Proteins- small amounts are normal as some are small enough to be filtered out of the blood, but larger amounts are a sign of kidney disease.
• Drugs- many pass out in urine so can show if a drug abuser or an athlete who cheats.

Question 34

Excretion and water conservation

Answer 34

• The max solute concentration of urine varies between species.
• Differences between species depending on their environment.
• A positive correlation between the thickness of the medulla compared to the overall size of the kidney, and the need for water conservation.
• Because a thicker medulla allows the loops of Henle and collecting ducts to be longer so more water can be reabsorbed and the urine can be made more concentrated.

Question 35

The malpighian tubule system

Answer 35

The circulatory system of insects uses hemolymph rather than blood.
Hemolymph is pumped by a vessel that runs from the abdomen forwards through the thorax to the head.
Branches of this vessel carry the hemolymph to different parts of the body and it is then released and is free to flow gradually through tissues until being drawn back for re-pumping.
Body cells are bathed in hemolymph and release their waste into it.
Malpighian tubules- a ring of narrow blind-ended ducts located between the hindgut and midgut of insects.
Cells in the walls extract waste from the hemolymph and pass them into the lumen of the tubule.
Ammonia is extracted and converted by malpighian tubule cells into uric acid.
To create a flow of fluid cells in the tubule walls transfer mineral ions by active transport from the hemolymph to the lumen and water follows passively by osmosis.
The solution produced drains into the lumen of the hindgut where it is mixed with semi-digested food.
This mixture passes out through rectum.
Mineral ions are pumped by cells in the wall of the rectum from the feces to the hemolymph and again water follows by osmosis.
By moving solutes and water into and out of the hemolymph, the Malpighian tubules and rectum prevent dehydration and achieve osmoregulation.

Question 36

Spermatogenesis and oogenesis

Answer 36

Spermatogenesis- production of male gametes in the testes.
Oogenesis- production of female gametes in the ovaries.

Both processes have the same basic stages:

• Mitosis to generate large numbers of diploid cells.
• Cell growth so that the cells have enough resources to undergo two divisions of meiosis.
• Meiosis to produce haploid cells.
• Differentiation so the haploid cells develop into gametes with structures needed for fertilisation.

Question 37

Stages in spermatogenesis

Answer 37

1. An outer layer called germinal epithelium cells divide endlessly by mitosis to produce more diploid cells.
2. Diploid cells grow into larger cells with more cytoplasm and become primary spermatocytes.
3. Each primary spermatocyte carries out the first division of meiosis to produce two secondary spermatocytes.
4. Each secondary spermatocyte carries out second division of meiosis to produce two spermatids.
5. Spermatids become associated with nurse cells, called Sertoli cells, which help the spermatids to develop into spermatozoa. This is an example of cell differentiation.
6. Sperm detach from Sertoli cells and eventually are carried out of the testis by the fluid.

Question 38

Structure of human sperm

Answer 38

• Acrosome- contains enzymes that digest the zona pellucida around the egg.
• Plasma membrane
• Centriole
• Head
• Haploid nucleus- contains the 23 chromosomes that are passed from father to offspring.
• Midpiece
• Tail - provides the propulsion that allows the sperm to swim up the vagina, uterus and oviduct until it reaches the egg.
• Helical mitochondrion- produces ATP by aerobic respiration to supply energy for swimming and other processes in the sperm.
• Microtubules in a 9 + 2 array, make the sperm tail beat from side to side and generate the forces that propel the sperm.
• Protein fibres - strengthen the tail.

Question 39

Stages in oogenesis

Answer 39

1. In the ovaries, germinal epithelium cells divide by mitosis to form more diploid cells.
2. Diploid cells grow into larger cells called primary oocytes.
3. Primary oocytes start the first division of meiosis but stop during prophase I. The primary oocyte and a single layer of follicle cells around it form a primary follicle.
4. When a baby girl is born the ovaries contain about 400,000 primary follicles.
5. Every menstrual cycle a few primary follicles start to develop. The primary oocyte completes the first division of meiosis, forming two haploid nuclei. The cytoplasm of the primary oocyte is divided unequally forming a large secondary oocyte and a small polar cell.
6. The secondary oocyte starts the secondary division of meiosis but stops in prophase II. The follicle cells are proliferating and follicular fluid is forming.
7. When the mature follicle bursts, at the time of ovulation, the egg that is released, is actually still a secondary oocyte.
8. After fertilisation the secondary oocyte completes the second division of meiosis to form an ovum with a haploid nucleus and a second polar cell or body. The first and second polar bodies eventually degenerate.

Question 40

Structure of a mature human egg

Answer 40

• Haploid nucleus that contains the 23 chromosomes that are passed from mother to offspring.
• Two centrioles
• First polar body- not needed to breaks down
• Plasma membrane
• Layer of follicle cells, corona radiata
• Zona pellucida- protects the egg cell and restricts entry of sperm.
• Cortical granules- harden the zona pellucida to prevent multiple fertilisations.
• Cytoplasm - contains droplets of fat and other nutrients needed during early stages of embryo development.

Question 41

Comparing oogenesis and spermatogenesis

Answer 41

• Millions of sperm are produced by men every day from puberty onwards and they can be released by ejaculation. From puberty until menopause women who are not pregnant produce and release just one egg every 28 days.
• Nearly all the cytoplasm is removed during the latter stages of spermatogenesis so sperm contain very little. Egg cells have more cytoplasm than any other human cell. The mitochondria of the zygote are all derived from the cytoplasm of the egg cell. The egg cell destroys the helical mitochondria of the sperm after fertilisation.

Question 42

Internal and external fertilisation

Answer 42

In some species females release unfertilised eggs and males put their sperm over the eggs, so fertilisation takes place outside the body- external fertilisation.
Ex. salmon and other fish, frogs and other amphibians.

In other species the male passes his sperm into the female’s body and fertilisation takes place there- internal fertilisation.
Ex. pythons and other reptiles, albatrosses and other birds, humans and other mammals.

Question 43

Avoiding polyspermy

Answer 43

A diploid zygote is produced when one haploid sperm fuses with a haploid egg- fertilisation.
Fusion of two or more sperm with an egg results in a cell that has three of each chromosome type or more- polyspermy.
Cells produced in this way often die and those that survive are almost always sterile.
There are therefore mechanisms in fertilisation that prevent polyspermy usually.

Question 44

Stages in the fertilisation of human egg

Answer 44

1. Arrival of sperm- attracted by a chemical signal and swim up the oviduct to reach the egg. Fertilisation is only successful if many sperm reach the egg.
2. Binding- the first sperm to break through the layers of follicle cells binds to the zona pellucida- triggers an acrosome reaction.
3. Acrosome reaction- the contents of the acrosome are released by the separation of the acrosomal cap from the sperm. Enzymes from the acrosome digest a route for the sperm through the zona pellucida, allowing it to reach the plasma membrane of the egg.
4. Fusion- the plasma membranes of the sperm and egg fuse and the sperm nucleus enters the egg and joins the egg nucleus- causes a cortical reaction.
5. Cortical reaction- small vesicles called cortical granules move the the plasma membrane of the egg and fuse with it, releasing their contents by exocytosis. Enzymes from the cortical granules cause cross-linking of glycoproteins in the zona pellucida, making it hard and preventing polyspermy.
6. Mitosis- the nuclei from the sperm and the egg do not fuse together. Instead, both nuclei carry out mitosis using the same centrioles and spindle of microtubules- a two–cell embryo is produced.

Question 45

Declining male fertility

Answer 45

During the last 50 years, the average nr of sperm per unit volume of human semen has fallen by 50% and it continues to drop by about 2% per year.
Various factors- female contraceptive pill with oestrogen and progesterone.
Steroids in plastics, food packaging and furniture.

Question 46

Early embryo development and implantation

Answer 46

If a couple wants to have a child- they do not use any contraception.
Semen is ejaculated into the vagina and sperm that it contains swim through the cervix up the uterus and into the oviducts.
If there is an egg in the oviducts, a sperm can fuse with it and produce a zygote.
The zygote produced by fertilisation in the oviduct is a new human individual.
It starts to divide by mitosis to form a 2-cell embryo then a 4 cell embryo and so on until a hollow ball of cells called a blastocyst is formed.
The embryo is transported down the oviduct to the uterus.
When it is about 7 days old, the embryo implants itself into the endometrium where it continues to grow and develop.
If implantation does not occur then the embryo is not supplied with enough food and the pregnancy does not continue.

Question 47

Animal size and duration of gestation

Answer 47

A positive correlation between body mass and duration of gestation (pregnancy).
Some, however, have the same durations but body masses differ by more than two orders of magnitude.
In animals with a long gestation, the offspring are more advanced in their development when they are born with a short gestation time in relation to adult body mass.

Question 48

Hormonal control of pregnancy

Answer 48

Human embryos secrete the hormone hCG from a very early stage. hCG stimulates the ovary to maintain the secretion of progesterone during the first three months of pregnancy.
Progesterone causes the uterus lining to continue to thicken so it can support the embryo after implantation.
By about the 12th week of pregnancy, the ovary stops secreting progesterone but this time, the placenta has developed and takes over the task of secreting progesterone that is needed to sustain the pregnancy until labour/childbirth.
The placenta secretes oestrogen.

Question 49

Hormonal control of childbirth

Answer 49

Through pregnancy, rising levels of progesterone ensure that the uterus develops and supports the fetus.
It also prevents uterine contractions and prevents spontaneous abortions.
Progesterone starts to fall in the last third of the pregnancy and more steeply shortly before the end.
Oestrogen rises, causing an increase in the nr of oxytocin receptors on the muscle in the uterus wall.
When oxytocin binds to these receptors it causes the muscle to contract.
Uterine contractions stimulate the secretion of more oxytocin.
Contractions become stronger and stronger- positive feedback.
While the uterus muscle is contracting, the cervix relaxes and becomes wider.
The amniotic sac bursts and the amniotic fluid is released.
After many hours, the baby is pushed out through the cervix and the vagina.
The umbilical cord is cut and the baby begins its independent life.
Contractions continue for a time until the placenta is expelled.