Physiology 20 Flashcards
How are oral and rectal body temperature related?
rectal is usually 0.5°C higher than oral
what is a normal scrotal temperature?
32°C
What is the normal circadian fluctuation in temperature?
0.5-0.7°C
What processes contribute to heat production in the body during exercise?
BMR (60%)
Thermogenic effect of food (digestion/absorption/storage) (8%)
Non-exercise activity thermogenesis (NEAT) (7%)
Exercise (25%)
What is ‘countercurrent heat exchange?’
Transfer of heat from arteries to adjacent veins warms blood returning to the core
To what extent is the skin able to control heat loss?
Up to eightfold increase in temperature conductance between full contriction and dilation of skin vessels
What are the main mechanisms of body heat loss?
At room temperature:
Radiation (60%)
Evaporation (22%)
Conduction/convection via air (15%)
Conduction via contact eg. chair (3%)
Outline the thermoregulatory mechanism
Posterior hypothalamus receives afferents from peripheral (via lateral spinothalamic tracts) and central cold thermoreceptors
Anterior hypothalamus (preoptic nuclei) receives afferents from peripheral heat thermoreceptors via lateral spinothalamic tracts. This centre can exert an inhibitory effect on the shivering / cold centre in the posterior hypothalamus
Efferents act to:
- Modify vascular tone
- Modify behaviour
- Stimulate/inhibit shivering/sweating
- Stimulate thermogenesis
Discuss peripheral thermoreceptors
Two types:
Skin:
- Cold receptors - most numerous. in dermis. Discharge regularly and periodicallym increasing frequency below 25°C. via Aδ fibres.
- Warm receptors - maximal discharge rate at 45-50°C. Type C fibres
Deep tissue:
-Mainly in spinal cord, abdo viscera and great veins. Can induce shivering when core temperature falls by 0.5°C
Summarise the structure and function of sweat glands
Two components - Gland and duct
Gland secretes ‘primary secretion’ similar to plasma and duct reabsorbs Na, Cl and H2O.
Reabsorption almost total at low levels of sweating. At high levels of sweating, significant loss of NaCl and H2O can occur.
How does acclimatisation to heat occur?
Release of aldosterone limits sodium loss
Takes 4-6 weeks
What is chemical thermogenesis?
- Production of heat that is not related to ATP production
- Happens as a result of uncoupling proteins (which cause mitochondrial H+ ions to bypass ATP synthase) so produce heat without ATP
Occurs with sympathetic excitation, raised thyroxine levels and in brown adipose tissue found in infants
What is the function of the ‘set point’ for temperature regulation?
- The temperature below which the posterior hypothalamus will exhibit an effector response eg. shivering.
- It is modified by skin temperature ie. set point is lower if skin temperature is higher
How may epidurals during labour affect body temperature?
Basal temperature increases following epidural/CSE analgesia - ‘epidural fever’
Thought to be due to disturbance in central thermoregulation
What is the classical description of an endocrine gland?
Why is this concept of limited use?
Ductless and specalised
Limited as hormones are also produced and released by other tissues eg. heart or liver tissue
When is the term ‘neurocrine’ most appropriate?
The hypothalamus and posterior pituitary release hormones directly from nerve tissue and are thus ‘neurocrine’ glands
Contrast the terms endocrine, paracrine and autocrine
Endocrine: chemical release affects distant tissue
Paracrine: chemical release affects nearby cells in the same organ/tissue
Autocrine: chemical released influences the releasing cell
How are hormones classified chemically?
Proteins/peptides:
-Diverse in origin, structure and function
Amines:
- Derived from tyrosine
- All have an alpha amino group on a benzene ring
- Includes catecholamines and thyroid hormones
Steroids:
- Derived from cholesterol
- Includes adrenal hormones, sex hormones and vitamin D
What is the difference between a peptide and a protein?
Peptide: 3-100 amino acid chain
Protein: >100 aa chain
How are peptide/protein hormones produced?
RER -> prehormone
SER / Golgi apparatus -> completed hormone, stored in vesicles
How are peptide hormones released?
Secretory vesicles are exocytosed in response to increased IC [Ca2+]
Where do peptide hormones exert their effects?
On membrane-bound receptors of target cells due to water solubility and inability to cross membranes
How does the adrenal medulla produce adrenaline?
-It possesses the enzyme phenylethanolamine N-methyltransferase, which produces adrenaline from NA
[this enzyme is usually lost in phaeochromocytoma]
How are amine hormones stored and released?
Catecholamines:
- Stored as secretory vesicles
- When released are 50% albumin bound
Thyroid hormones:
- Stored complexed with thyroglobulin
- When complex is split, T3/4 diffuses out of the cell and binds to thyroid binding protein, leaving little free hormone with a large, stable bound reserve
What is the common structure of steroid hormones?
- 4-ring structure: 3 hexamic; 1 pentamic
- Physiological differences relate to side chain modifications