Flashcards in Physiology Kanani III Deck (38):
What percentage of the CO do the kidneys receive?
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.
There are two main theories to explain how renal autoregulation of blood flow occurs:
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
Name some other factors that are important for the control of RBF.
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
Which agent has traditionally been used to measure the RBF?
The organic acid, para-aminohippuric acid (PAH).
Spirometry tracing: Which of the volumes and capacities may be measured directly?
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)
Then, which must be calculated by other sources?
Residual volume (RV)
Total lung volume (TLV) (VC RV)
Give some typical values for the TV, IRV and ERV.
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
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.
Define FRC. How may it be measured?
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
What is the normal range for the FRC? What factors may cause it to increase or decrease?
The normal range is 2.5–3.0 L.
It may be decreased by:
Any restrictive lung disease
Following abdominal surgery
It may be increased by continuous positive airway pres- sure (CPAP) and gaseous retention of obstructive lung diseases.
What is the ‘effective’ TV?
This is defined as the TVanatomic dead space, and represents the volume of inspired air that reaches the alveoli.
What is the definition of ‘dead space’?
This is the volume of inspired air that is not involved in gas exchange.
What types of dead space volume do you know?
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
What is the main function of the small intestine?
This is the principle site for the absorption of carbohy- drate, lipid, proteins, water, electrolytes, vitamins and essential minerals.
What is the transit time for chyme to pass through the small bowel?
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
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.
What is the composition of small bowel secretions?
This is made up of mucous, water and NaCl, predom- inantly.
What is the output of this daily?
1,500 mL per day.
How does this compare to the rest of the gut?
The daily volume of gut secretions in (mL per day) may be summarised:
Small intestine: 1,500
How much water does the small bowel absorb per day?
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.
What are the effects of terminal ilectomy?
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
Which conditions may trigger the SIADH?
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
What are the causes of hypernatraemia?
Insufficient intake or administration
Osmotic diuresis, e.g. hyperglycaemia
Excess sodium over water
Conn’s or Cushing’s syndrome
Excess hypertonic saline
How does the body monitor the ECF volume? Where in the body are these located?
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
What is the juxtaglomerular apparatus composed of?
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
Why is this structure so important to the control of the circulating volume and sodium balance?
The granular cells of this apparatus produce renin, which goes on to initiate the R-A-A cascade.
Under what conditions is the R-A-A system stimulated?
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
What is the difference between starvation, fasting and cachexia?
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
What is the basic difference between marasmus and kwashiorkor?
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
During a period of fasting, from which source does the body obtain glucose?
From glycogen, found most abundantly in the liver and skeletal muscle.
How long does this supply last?
About 24 h.
When this supply is exhausted, why doesn’t the body become hypoglycaemic?
Because, through gluconeogenesis, the liver is able to convert some molecules into glucose
What are the substrates for gluconeogenesis?
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
What is the result of fasting on the body’s store of protein and fat?
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
Summarise briefly the differences between fasting and prolonged starvation, in terms of the biochemical adaptation.
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.
Which organ is particularly reliant on ketones during starvation?
The brain. This organ is usually heavily dependant on glucose as its energy source, but during starvation, adapts to using ketones.
How else does the body adapt to starvation?
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.