1. What are the major functions of the colon?

• Absorption of water: the most important

• Absorption of minerals: predominantly sodium. There is, however, net secretion of potassium and bicarbonate

• Expulsion of faeces

• Indirect role: bacterial flora in the colon are able to synthesise vitamin K and some of the B vitamins. They also produce some important fatty acids

2. What types of contraction does the colon have in common with the small bowel?

• Segmentation: this mixes the contents of the colon, facilitating absorption

• Peristalsis: propelling the contents distally

3. What type of contraction is peculiar to the colon?

Mass action contraction. There is simultaneous contraction of the smooth muscle over a very long length. This moves material from one portion of the colon to another in one movement. It occurs between 1–3 times per day.

4. Identify one way in which the basic electric rhythm of the colon differs from that of the small bowel.

Unlike in the small bowel, the frequency of the wave of contraction increases along the colon. At the ileocaecal valve it is 2 per minute, and in the sigmoid colon, up to 6 per minute.

5. What is the gastro-colic reflex?

This occurs after a meal enters the stomach, leading to an increase in the motility of the proximal and distal colon, together with an increase in the frequency of mass movements.

6. Outline the events that occur during defecation.

1. What is the basic histologic structure of the thyroid gland?

• The thyroid is composed of numerous follicles that have a central fluid-filled cavity. They are lined with follicular cells that secrete the main hormones

• Interspersed among the follicles are the parafollicular cells

2. Which hormones does the thyroid produce?

• Tetra-iodothyronine (T4, thyroxine): the principle hormone of the thyroid gland

• Tri-iodothyronine (T3): measure for measure, this is more potent than T4, however, has a shorter duration of action

• Calcitonin: produced by the para-follicular cells. This is important in the regulation of serum calcium (see ‘Calcium balance’)

3. Name another source of T3 other than the thyroid.

This may also be produced by the conversion of T4 in the peripheral tissues. In fact, the thyroid accounts for only 20% of the extrathyroid pool of T3.

4. Which other hormone may be produced following the peripheral conversion of T4?

Reversed-T3 (r-T3). This is an inactive hormone acts as a point of peripheral thyroid hormone control.

5. Outline the steps involved in the production of T3 and T4.

• Iodide trapping: dietary iodine is concentrated into the follicular cells by an active pump mechanism

• Oxidation: of iodide to a reactive form by the enzyme peroxidase. This is located on the apical membrane

• Organification: through binding with amino acids – mainly tyrosine. These form tyrosyl units

• Thyroglobulin formation: tyrosyl units combine with a protein core to form thyroglobulin

• Internal coupling: tyrosyl units combine on the thyroglobulin molecule to form T3 or T4 molecules still bound to the protein core

• […]

1. What is the definition of the CO?

This is defined as the product of the heart rate and the stroke volume.

2. Give a normal resting value for the CO.

5–6 Lmin−1.

3. How do the outputs of the two ventricles compare to one another?

In the normal state, the outputs of both ventricles are essentially equal – both being 5–6 Lmin−1.

4. Which factors influence the stroke volume?

• Preload

• Afterload

• Myocardial contractility

• Heart rate

5. What determines the preload?

This is determined by the venous return to the heart. The amount of venous return itself depends on the difference between the systemic filling pressure (driving blood back to the heart) and the central venous pressure (CVP) (working against the venous return).

6. Define the afterload. What is this analogous to, in simple terms?

This is the ventricular wall tension that has to be generated in order to eject blood out of the ventricle. It is analogous to the arterial pressure. An increase in the CO causes a rise in the arterial pressure (afterload). This in turn, has a negative feedback effect on the output. Since more energy is consumed generating a high enough pressure to overcome a high arterial pressure, the resulting stroke volume is less – reducing the output in subsequent beats.

7. What happens to the Frank-Starling curve of the heart when the myocardial contractility is increased?

There is an upwards shift of the curve, so that the stroke volume is higher for any given end diastolic volume.

1. What are the body's sources of energy? How much energy does each supply?

• Glucose: provides 4.1kcalg−1

• Fat: 9.3kcalg−1

• Protein: 4.1kcalg−1

2. What is meant by the respiratory quotient?

This is defined as the ratio of the volume of CO2 produced to the volume of oxygen consumed from the oxidation of a given amount of nutrient. Values for the different energy sources are:

• Carbohydrayte: 1.0

• Fat: 0.7

• Protein: 0.8

3 What is the recommended daily intake for protein and nitrogen?

• Protein: 0.80 gkg−1

• Nitrogen: 0.15 gkg−1

4. What is an essential amino acid? How many are there, and give some examples?

These are amino acids that cannot be synthesised by the body and need to be ingested. There are 9 of them; leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan, histidine and valine.

5. What are the main carbohydrates in the diet?

Direct carbohydrate is composed mainly of the polysaccharide starch, some disaccharides such as sucrose and fructose and a small amount of lactose. Other important polysaccharides include cellulose, pectins and gums. These are not digested, and make up the roughage in the diet.

6. In what form is fat stored in the body?

As triglycerides.

7. What are these composed of?

These consist largely of long chain saturated and unsaturated fatty acids (predominantly palmitic, stearic and oleic acids) that have been esterified to glycerol.

8. What is an essential fatty acid? Which ones are there, and why are they particularly important?

These are fatty acids that cannot be synthesised in the body.

What types of hyperaldosteronism are there, and what basic features characterise each?

• Primary hyperaldosteronism: or Conn's syndrome, due to autonomous secretion by the adrenal cortex

• Secondary: associated with increased levels of hormones of the renin-angiotensin-aldosterone (R-A-A) system, e.g. following dehydration, blood loss, cardiac and liver failure with third-spacing of fluid. Also occurs with renal artery stenosis

2. What causes Conn's syndrome?

It is most often due to a single (rarely more) adenoma of the zona glomerulosa of the adrenal cortex. Also following idiopathic bilateral hyperplasia of the zona glomerulosa.

3. What is the dominant clinical feature?

Hypertension due to chronic salt and water retention. Peripheral odema is not, however, usually present despite water excess – the mechanism for this is not understood fully.

4. What biochemical abnormalities might you detect?

• Hypernatraemia: following salt retention. However, water retention may lead to a normal plasma [Na+]

• Hypokalaemia: patients may have associated muscle weakness and cardiac arrhythmias

• Metabolic alkalosis: associated with hypokalaemia and loss of H+ following increased exchange with Na+ at the kidney

5. What will the urine show?

• Increased potassium

• Low sodium

• High aldosterone concentrations

6. Apart from the important features mentioned above, what other clinical feature commonly occurs with Conn's syndrome?

Ployuria. This is due to tubular nephropathy, leading to a reversible diabetes insipidus.

7. Which aldosterone antagonist has been used in the medical management of this disorder?

The diuretic spironalactone.

8. What are the most common causes of Cushing's syndrome of cortisol excess?

In their order of frequency:

• Iatrogenic steroid administration

• […]

1. Which systems of the body show physiologic changes following prolonged immobilization?

• The musculoskeletal system

• Cardiovascular system

• Autonomic nervous system

• The extra-cellular fluid compartment

• There are also changes in overall body composition of fat and protein

2. What are these changes in the overall body composition that you have mentioned?

• Reduction in the lean body mass: this is seen as an increase in the excretion of nitrogen after the 5th day of bed rest. The level of protein catabolism falls after several weeks, but is still higher than normal

• Increase of adipose tissue deposition: as a replacement for loss of muscle mass

• Increased potassium excretion: since this is the major intracellular cation and especially rich in muscle, loss of potassium is an indicator of loss of total body lean tissue mass

3. How long after continued bed rest are cardiovascular changes seen?

About three weeks.

4. What are these changes?

• Increase in heart rate: after three weeks, the rate increases about half a beat per minute per day of immobilization

• Reduction of stroke volume: this is associated with a measure of cardiac atrophy

• CO and arterial pressure are maintained: owing to the conflicting changes above

• Adaptations to postural changes are impaired: this is because of impairment of the inotropic and CO response to a fall in the arterial pressure, despite an exaggerated peripheral vascular response. There is also a reduction in the overall activity of the ANS, leading to a blunting of cardiovascular responses

5. What happens to the musculoskeletal system following three weeks of bed rest?

1. What are the main functions of the lung?

• Oxygenation

• Ventilation: elimination of carbon dioxide

• Acid-base balance: forms the respiratory component to acid-base homeostasis

• Endocrine: production of angiotensin converting enzyme

2. Broadly speaking, which parts of the brain are responsible for controlling the rate and depth of ventilation?

• The brainstem: pons and medulla involved mainly. These give ventilation its automacity and rhythmical nature

• Cerebral cortex: this gives some voluntary control

3. Which parts of the brainstem have been identified as being particularly important? Outline the role that each plays in control.

Note that these areas of the brainstem have collectively been termed the respiratory centre. They consist of:

• Medullary respiratory centre: found in the reticular formation. Composed of a dorsal group (involved in inspiration) and a ventral group (involved in expiration). The expiratory area in the ventral group is not normally active during quiet respiration, since expiration is predominantly a passive process

• Apneustic area: located in the pons. This area is thought to prolong the inspiratory phase of the respiratory cycle

• Pneumotaxic area: also located in the pons. This inhibits the activity of the inspiratory area of the medulla. It may be involved in ‘fine tuning’ of respiratory rate, depth and rhythm

4. Which physiologic variables form the basis for control of ventilation? Place them in order of importance.

• PaCO2: the most important regulatory factor

• PaO2

• pH of the blood and CSF: has some influence above and beyond the PaCO2

5. How are changes in these parameters detected?

1. What is the main function of the small intestine?

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

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

2–4 h.

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

• 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

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

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.

5. What is the composition of small bowel secretions?

This is made up of mucous, water and NaCl, predominantly.

6. What is the output of this daily?

1, 500 mL per day.

7. How does this compare to the rest of the gut?

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

8. How much water does the small bowel absorb per day?

1. On which factors does adequate blood oxygenation depend on?

• Normal ventilation of the lung: this is determined by a normal respiratory drive and a functionally normal respiratory apparatus (includes the brain, chest wall, airways and lung parenchyma)

• Adequate diffusion of respiratory gasses across the alveolar wall

• Matching of ventilation and perfusion

• 2. By what process do the respiratory gases pass through the various anatomic barriers to pass into the blood?

• Through diffusion.

• 3. Which physical law determines diffusion across membranes?

• Fick's law of diffusion: This states that the amount of gas diffusing per unit time (i.e. the rate of diffusion) is inversely proportional to the thickness of the barrier and directly proportional to the surface area of the barrier.

• 4. What are the anatomic layers that respiratory gases have to pass through to reach the haemoglobin molecule in the red cells?

• The fluid lining the alveoli: gases initially dissolve in this before proceeding to the next layer

• Alveolar epithelium and through its basement membrane

• Interstitial space: which also contains fluid

• Basement membrane of capillary endothelium

• Capillary endothelium

• Plasma

• Red cell membrane

5. What is shunt?

This refers to venous blood that passes to the systemic circulation without first being oxygenated in the lungs.

6. Is this always pathological?

No, under normal circumstances, 1–2% of the CO bypasses the alveoli. This is called the anatomic shunt.

7. Where are the sites for normal anatomic shunt?

• The bronchial circulation

• Cardiac Thebesian veins: that drain coronary venous blood directly into the left side of the heart

• […]

1. What are the components of the ANS?

The sympathetic and parasympathetic nervous systems (SNS and PNS).

2. Where are the locations of the cell bodies of the neurones that make up the SNS?

• Preganglionic cells: cell bodies are located at the lateral horns of the spinal grey matter. This extends from T1 to L2 (thoraco-lumbar outflow)

• Postganglionic cells: cell bodies are located in the sympathetic chain

3. Other than their location, how else do pre and postganglionic cells of the SNS differ?

• Preganglionic fibres are myelinated: therefore, their diameter is larger with more rapid transmission of the action potential

• Differences in the neurotransmitter: the transmitter substance secreted by preganglionic cells is ACh (acting on nicotinic receptors), and by postganglionic cells is noradrenaline

4. What is special about the mode of sympathetic supply to the adrenal medulla?

The chromaffin cells of the adrenal medulla receive direct innervation from preganglionic sympathetic fibres arising from the lateral horn of the spinal cord. Stimulation leads to secretion of catacholamine from the chromaffin cells, so these cells are akin to postganglionic cells of the SNS.

5. How does the origin of the PNS differ from the SNS?

• Preganglionic parasympathetic: these neurones take origin from specific cranial nerve nuclei and from sacral segments 2–4 of the spinal cord (cranio-sacral outflow vs. thoraco-lumbar origin of the SNS)

• Parasympathetic ganglia: unlike the sympathetic chain, the PNS ganglia are located at discrete points close to their respective target organs

6. Which cranial nerves have a parasympathetic outflow?

A well-known doctor once told me that “learning is the noblest form of begging”. This is certainly what it feels like just before the MRCS exam when the brain labours with the weight of temporary information. Physiology is not an inherently difficult subject – only made so by the unholy trinity of a bad night on-call, dwindling time and a thick textbook. I hope that this book is the remedy to this unfortunate combination, and helps a little to play the game.