Physiology Kanani III

Question 1

What percentage of the CO do the kidneys receive?

Answer 1

20–25%, so that the RBF is 1.0–1.2 Lmin1.

RBF, like many specialised vas- cular beds, is controlled largely by autoregulation. Thus, between mean arterial pressures of 80–180 mmHg, RBF is fairly constant, at about 1.2 Lmin1.

Question 2

There are two main theories to explain how renal autoregulation of blood flow occurs:

Answer 2

Myogenic mechanism: an increase in renal vascular wall tension that occurs following a sudden rise in arterial pressure stimulates mural smooth muscle cells to contract, causing vasoconstriction. This reduces the RBF in the face of rising arterial pressures. Most of this myogenic response occurs in the afferent arteriole

Tubuloglomerular feedback: alterations in the flow of blood that occurs with alterations in the arterial pressure leads to stimulation of the juxtaglomerular apparatus. This leads to a poorly defined feedback loop that results in changes of the RBF to the baseline level

Question 3

Name some other factors that are important for the control of RBF.

Answer 3

SNS: this controls the tone of the afferent and efferent arteriole. By stimulation of ’1- adrenoceptors there is vasoconstriction and reduction of blood flow
Angiotensin II: as part of the control by the renin- angiotensin-aldosterone system. This hormone stimulates vasoconstriction, leading to a reduction of the RBF and GFR
Local mediators: such as PGE2 and PGI2, both of which cause arteriolar vasoconstriction

Question 4

Which agent has traditionally been used to measure the RBF?

Answer 4

The organic acid, para-aminohippuric acid (PAH).

Question 5

Spirometry tracing: Which of the volumes and capacities may be measured directly?

Answer 5

Note that the ‘capacities’ are derived by adding ‘volumes’ together. The following can be measured directly:
 Tidal volume (TV)
 Inspiratory reserve volume (IRV)
 Expiratory reserve volume (ERV)
 Inspiratory capacity (IC)  (TV  IRV)
 Vital capacity (VC)  (IRV  TV  ERV)

Question 6

Then, which must be calculated by other sources?

Answer 6

Residual volume (RV)
FRC(RVERV)
Total lung volume (TLV) (VC RV)

Question 7

Give some typical values for the TV, IRV and ERV.

Answer 7

TV: 500 ml, or 7 mlkg1
IRV: defined as the volume that can be inspired
above the TV. Typically 3.0 L
ERV: the volume of gas that can be expired after a quiet expiration. Typically 1.3 L

Question 8

Define RV.

Answer 8

This is the volume that remains in the lung following maximal expiration, and may only be measured using the same method as the FRC (see below). The normal value is around 1.2–1.5 L.

Question 9

Define FRC. How may it be measured?

Answer 9

This is defined as the sum of the RV and the ERV. It represents the volume of gas left in the lung at the end of a quiet expiration.
There are three main methods for its measurement:
Gas dilution method: using helium placed within the spirometer. The subject breathes through the system starting at the end of a quiet expiration. Helium is not absorbed by the blood but distributed throughout the lungs. The concentration of helium expired at the end of equilibration can be used to calculate the FRC
Total amount of helium Volume of gas in spirometer initial concentration of helium Helium concentration at equilibration (volume of spirometer FRC)
Nitrogen washout: subject breathes pure oxygen from the end point of a quiet expiration. By analysing the changes in the concentration of nitrogen, the FRC may be calculated
Plethysmography: uses an airtight chamber to measure the total volume of gas in the lungs

Question 10

What is the normal range for the FRC? What factors may cause it to increase or decrease?

Answer 10

The normal range is 2.5–3.0 L.
It may be decreased by:
 Supine position
 Any restrictive lung disease
 Pregnancy
 Following abdominal surgery
 Following anaesthesia
It may be increased by continuous positive airway pres- sure (CPAP) and gaseous retention of obstructive lung diseases.

Question 11

What is the ‘effective’ TV?

Answer 11

This is defined as the TVanatomic dead space, and represents the volume of inspired air that reaches the alveoli.

Question 12

What is the definition of ‘dead space’?

Answer 12

This is the volume of inspired air that is not involved in gas exchange.

Question 13

What types of dead space volume do you know?

Answer 13

There are three types of dead space:
Anatomic dead space: formed by the gas conduction parts of the airway that are not involved in gas exchange, such as the mouth, nasal cavity, pharynx, trachea and upper bronchial airways. Measured using Fowler’s method
Alveolar dead space: composed of those alveoli that are being ventilated but not perfused. They are therefore, in effect, not contributing to gas exchange
Physiologic dead space: this is the sum of the two volumes above. The normal value is 2–3 mlkg1 measured using Bohr’s method

Question 14

What is the main function of the small intestine?

Answer 14

This is the principle site for the absorption of carbohy- drate, lipid, proteins, water, electrolytes, vitamins and essential minerals.

Question 15

What is the transit time for chyme to pass through the small bowel?

Answer 15

2–4 h.

Question 16

What are the three main types of small bowel motion seen after a meal?

Answer 16

Peristalsis: in common with the rest of the gut
Segmentation: more frequent than the above, occurring about 8 times per minute in the ileum, lasting for several seconds. Involves localised contraction of 1–2 cm of bowel that leads to the propulsion of chyme in both directions. Important for mixing chyme with the digestive juices
Pendular movements: longitudinal muscle contractions lead to movement of the bowel wall over luminal contents. Also important for mixing

Question 17

How does the motility differ when the small bowel is empty of contents?

Answer 17

During fasting, a migrating motor complex spreads from the duodenum to the ileocaecal junction. This contractile wave helps to clear the small bowel of any remaining contents.

Question 18

What is the composition of small bowel secretions?

Answer 18

This is made up of mucous, water and NaCl, predom- inantly.

Question 19

What is the output of this daily?

Answer 19

1,500 mL per day.

Question 20

How does this compare to the rest of the gut?

Answer 20

The daily volume of gut secretions in (mL per day) may be summarised:
 Saliva: 1,500
 Gastric: 2,000
 Bile: 500
 Pancreatic: 1,500
 Small intestine: 1,500

Question 21

How much water does the small bowel absorb per day?

Answer 21

Assuming that oral intake is 2,000mL daily, the small bowel absorbs about 8,500 mL of water daily. The colon, about 400 mL daily. This leaves around 100 mL excreted in the faeces per day.

Question 22

What are the effects of terminal ilectomy?

Answer 22

Loss of bile salt re-uptake: this alters the colonic flora and changes the consistency of stools. There is also increased bile salt manufacture by the liver in response to reduced uptake, increasing the incidence of gallstones
Decreased Vitamin B12 uptake: producing macrocytic anaemia
Reduced water absorption: this is one of the important functions of the terminal ileum. This can lead to loose and frequent stools
Reduced uptake of g-globulin: the terminal ileum is full of lymphatic tissue, and there is some re-uptake of immunoglobulin. Loss of this tissue may affect local gut immune surveillance

Question 23

Which conditions may trigger the SIADH?

Answer 23

Lung pathology: pneumonia, lung abscess and TB
Malignancy: small cell carcinoma of the lung, brain
tumours, prostatic carcinoma
Other intra-cranial pathology: head injury, meningitis
Alcohol withdrawal

Question 24

What are the causes of hypernatraemia?

Answer 24

Water loss
 Diabetes insipidus
 Insufficient intake or administration
 Osmotic diuresis, e.g. hyperglycaemia
 Excess sodium over water
 Conn’s or Cushing’s syndrome
 Excess hypertonic saline

Question 25

How does the body monitor the ECF volume? Where in the body are these located?

Answer 25

Through a series of receptors that monitor the intravascular volumes and pressures.

Low-pressure receptors: baroreceptors are located in the walls of the cardiac atria and pulmonary vessels, and they respond to distension that occurs with an increase in the circulating volume
High-pressure volume receptors: these are baroreceptors located in the aortic arch, carotid sinus and afferent arteriole of the kidney. Also there is the juxtaglomerular apparatus of the kidney

Question 26

What is the juxtaglomerular apparatus composed of?

Answer 26

This is formed from three components:
The macula densa: of the thick ascending limb
Granular cells of the afferent and efferent arterioles
Mesangial cells: these act as antigen-presenting cells

Question 27

Why is this structure so important to the control of the circulating volume and sodium balance?

Answer 27

The granular cells of this apparatus produce renin, which goes on to initiate the R-A-A cascade.

Question 28

Under what conditions is the R-A-A system stimulated?

Answer 28

The trigger to the release of renin by the juxtaglomeru- lar apparatus is three fold:
Fall of renal perfusion pressure: this is principally detected by the baroreceptors of the afferent arterioles
Activation of the SNS: this occurs when there is a fall in the arterial pressure
Reduced sodium delivery to the macula densa: this also occurs when there is a fall in the renal perfusion pressure

Question 29

What is the difference between starvation, fasting and cachexia?

Answer 29

Starvation is a chronic state resulting from inadequate intake of energy
Fasting is a state of energy deprivation lasting no more than several days
Cachexia is the state resulting from a chronic deprivation of energy and nutrients irrespective of the adequacy of intake, e.g. in malignant cachexia, there is protein and energy malnutrition even when there is adequate intake of food

Question 30

What is the basic difference between marasmus and kwashiorkor?

Answer 30

Marasmus is characterised by inadequacy of all nutrients and energy sources

Kwashiokor is characterised by a lack of protein, but there is some intake of energy sources

Question 31

During a period of fasting, from which source does the body obtain glucose?

Answer 31

From glycogen, found most abundantly in the liver and skeletal muscle.

How long does this supply last?
About 24 h.

Question 32

When this supply is exhausted, why doesn’t the body become hypoglycaemic?

Answer 32

Because, through gluconeogenesis, the liver is able to convert some molecules into glucose

Question 33

What are the substrates for gluconeogenesis?

Answer 33

Glycerol: which is released from the breakdown of triglycerides
Amino acids: from the so-called glucogenic amino acids, such as alanine
Lactate: production of which is increased following fasting and starvation

Question 34

What is the result of fasting on the body’s store of protein and fat?

Answer 34

In the early stages of fasting, because of the requirements for gluconeogenesis, there is a rapid breakdown of muscle to release amino acid, which is transported to the liver for conversion to glucose. This breakdown of muscle slows down the longer starvation proceeds
Adipose tissue is continually broken down into free fatty acids and glycerol. This mobilisation of the adipose tissue becomes relatively more important the longer the period of starvation goes on

Question 35

Summarise briefly the differences between fasting and prolonged starvation, in terms of the biochemical adaptation.

Answer 35

During starvation, relatively more adipose tissue is being mobilised
Also, there is relatively less muscle protein mobilisation
There is increased generation of ketone bodies as a source of energy during starvation

Name the ketone bodies.
Acetone
Acetoacetate
’-hydroxybutyrate

Question 36

Which organ is particularly reliant on ketones during starvation?

Answer 36

The brain. This organ is usually heavily dependant on glucose as its energy source, but during starvation, adapts to using ketones.

Question 37

How else does the body adapt to starvation?

Answer 37

There is a general reduction of energy requirement, which is partly due to a reduction in the BMR brought on through a fall in the secretion of tri- iodothyronine by the thyroid, and reduced peripheral conversion of T4.

Question 38

Which hormones are the most important for the mediation of the body’s adaptation to starvation?

Answer 38

Glucocorticoids: these act to increase serum [glucose]
Catacholamines: these cause a temporary increase in
the serum [glucose]
Glucagon
Thyroid hormone: as mentioned above
Insulin: a lack of the effects of this hormone triggers the adaptive response