11 - Physiology of Pregnancy

Question 1

What are the cardiovascular changes that occur during pregnancy?

Answer 1

Increase plasma and total blood volume
Increased cardiac output
Changes in blood pressure
Haemostasis

Question 2

How does Blood volume change throughout pregnancy?

Answer 2

1st trimester - BV increases
2nd trimester - BV rapidly increases
3rd trimester - BV increases slowly

Question 3

How much can the total blood volume increase by?

Answer 3

45%

Question 4

Which increase is smaller - RBC or plasma volume?

Answer 4

RBC

Question 5

Why can you become anaemic in the 3rd trimester?

Answer 5

Haemoglobin concentrations can fall

Question 6

What are the mechanisms that regulate increase in plasma and total blood volume?

Answer 6

 Vasodilation – decreases peripheral resistance
 Decreased renal perfusion
 Activation of RAAS – retention of sodium and increase in total body water
 Increase in erythropoiesis via increased renal EPO production

Question 7

How much can red blood cell mass increase by?

Answer 7

20%

Question 8

How does cardiac output change in a pregnant woman?

Answer 8

o Increased by 35-40% in the first trimester
o Then increase is slower
o Approximately 50% higher at term

Question 9

Why does cardiac output increase?

Answer 9

Increased as a result of increased heart rate (around 25%) and stroke volume (around 25%)

Question 10

How does blood pressure change in a pregnant woman?

Answer 10

o Reaches a low point by around 17-24 weeks before increasing again
o Reaches non-pregnant levels by late second trimester
o Peripheral vascular resistance falls by 50% in early pregnancy
o Oestrogen, progesterone, nitric oxide, relaxin are all implicated
o Systolic and diastolic pressure falls resulting in an increased heart rate

Question 11

When is a woman at risk of pre-eclampsia?

Answer 11

 Arterial blood pressure rises a little towards the end of the 3rd trimester
 Significant increases are a risk for pre-eclampsia
 Characterised by high blood pressure, oedema and proteinuria

Question 12

How is pregnancy and homoeostasis regulated?

Answer 12

• Pregnancy is proposed to induce a hypercoagulable state
• Process of coagulation depends on a complex cascade leading to the formation of s stable vascular plug
• It is proposed that the increased tendency for coagulation is important in maintaining placental function and preventing excessive bleeding during childbirth
• Plasma concentrations of all fibrinogen and all clotting factors (except XI and tissue factor) increase gradually in pregnancy
• There is also a decrease generally in coagulation inhibitors
• There is also increase platelet production and inhibition of fibrinolysis activity
• This increased tendency to clot can lead to thrombosis and thromboembolism

Question 13

How much does the oxygen consumption change during pregnancy?

Answer 13

• Increase oxygen consumption from 250ml/min to 300ml/min

Question 14

Why does the oxygen consumption increase in pregnancy?

Answer 14

• Needed to maintain the addition metabolic requirements of pregnancy

Question 15

How does the increase in oxygen consumption occur?

Answer 15

• increase is alveolar ventilation (the amount of air reaching the alveoli so available for gas exchange)
• increase in minute ventilation (the volume of gas inhaled from the lungs in one minute)
• Large increase in tidal volume
• Small increase is respiratory rate
• Elevation of diaphragm due to expanding uterus means that the residual volume and expiratory reserve volume decrease

Question 16

What are the mechanisms to increase oxygen consumption?

Answer 16

o Progesterone mediated hypersensitivity to carbon dioxide

o Directly stimulated the respiratory centre

Question 17

How does the arterial blood gas change?

Answer 17

o Increased ventilation results in a fall in PaCO2 and a slight rise in PaO2
o Respiratory alkalosis can occur due to the increased loss of carbon dioxide due to hyperventilation
o Renal compensation occurs – bicarbonate loss and H+ retention

Question 18

How much do the kidneys increase in length by during pregnancy?

Answer 18

1cm

Question 19

What other changes occur in the kidneys?

Answer 19

• Dilation of renal calyces, pelvis and ureter due to the action of progesterone (relax smooth muscle)
• Increased cardiac output
• Increased renal plasma flow (from 1.2l/min  1.5l/min)
• Increased glomerular filtration rate (from 120ml/min  140-170ml/min)
• Increase in urea, creatinine, urate and bicarbonate excretion meaning plasma concentrations are slightly

Question 20

What are the mechanisms to create renal changes?

Answer 20

o Increased RAAS activity leads to water retention and decrease is plasma osmolarity
o Angiotensin II levels are important for maintaining blood volume, pressure and uteroplacental flow
o Increased RAAS activity thought to be stimulated by oestrogen which causes increased renin secretion from granular cells

Question 21

What are clinical signs of liver disease that may occur during pregnancy?

Answer 21

spider naevi, palmar erythema

Question 22

What increases in the liver due to placental production?

Answer 22

• Increased concentrations f alkaline phosphatase due to placental production

Question 23

What are common conditions that occurs in the GI system during pregnancy?

Answer 23

• Heart burn/reflux common due to increased intra-abdominal pressure

• • Progesterone mediated reduction in LOS tone (OES not affected as striated muscle)
• • Decrease in tone and motility of small and large bowel
• • Constipation and haemorrhoid formation can occur

Question 24

How much does water absorption increase during pregnancy?

Answer 24

• 60% increase in water absorption

- • Constipation and haemorrhoid formation can occur

Question 25

What is Glycosura?

Answer 25

decrease in reabsorption of glucose probably due to an increased in filtered load which is greatre than the ability of the PCT to reabsorb glucose

Question 26

What are the endocrine changes in pregnant women?

Answer 26

• Hyperplasia of insulin producing beta cells in the pancreatic islets of Langerhans leading to increased insulin production
• Increased insulin sensitivity in early pregnancy so plasma glucose may fall
• In late pregnancy, insulin response blunted by placental hormones so plasma glucose may rise
• Increased placental glucose uptake

Question 27

What are the 3 stages of labour?

Answer 27

o Dilation of cervix/uterine contractions
o Foetal expulsion
o Placental expulsion

Question 28

What are the cardiac output changes prior to parturition?

Answer 28

o Cardiac output increased due to autotransfusion from contracting uterus
o Further increased in blood may be autotranfused as placenta delivers
o Pain or anxiety can activate sympathetic nervous system to increase heart rate and blood pressure

Question 29

What are the cardiovascular post-partum changes?

Answer 29

o 20% in blood volume 72h post partum
o Heart rate and cardiac output return to baseline in 2 weeks
o Proteins and lipids return to baseline in 2-3 weeks

Question 30

What are the urinary changes to a woman post-partum?

Answer 30

o Functional changes retrun to baseline with blood volume decrease
o Structural changes such as dilation of bladder, ureters and renal pelvis may persist for around 3 months or longer

Question 31

How are the mammary glands developed?

Answer 31

o Initiated at the start of puberty by progesterone and oestrogen
o Lactiferous ducts and alveoli (lobes) develop but the breast is not capable of milk production
o In pregnancy, the lobular ductal-alveolar system undergoes hypertrophy
o Ducts proliferate and alveoli mature and adipose tissue is deposited between the lobules of the gland
o This is controlled by placental steroids such as oestradiol, progesterone and placental peptide hormone
o Pituitary growth hormone and prolactin also play a role

Question 32

What is lactogenesis?

Answer 32

o By the middle of pregnancy, the breast is fully developed for milk production
o Prolactin is the primary lactogenic hormone and is present at high levels throughout gestation
o Steroid secretion by the placenta (oestrogen and progesterone) inhibits secretory activity
o Lactogenesis triggered post delivery by fall in steroid secretion as inhibition stops

Question 33

What is the milk ejection reflex?

Answer 33

o Oxytocin is necessary for the milk-ejection reflex
o Hormone is released in response to suckling
o Contraction of myoepitehlial cells causes release of milk from alveoli and small ducts into large ducts and sinuses