MasterPass PHYSIOLOGY 6

Via the umbilical vein.

Causes of raised IOP include increased extrinsic pressure (e.g., retrobulbar haematoma, orbital compression), increased aqueous humour (e.g., open-angle glaucoma, closed-angle glaucoma), sulphur hexafluoride injection during retinal surgery, and increased choroidal blood volume (e.g., gravitational effects, hypoxia, hypercarbia, hypertension).

Via the umbilical arteries, which arise from the common iliac arteries.

Reduced circulating volume is detected by the reduction in renal afferent arteriolar pressure, causing renin secretion from the juxtaglomerular cells. Renin cleaves angiotensinogen to produce angiotensin I, which is then converted to angiotensin II in the pulmonary vasculature by angiotensin-converting enzyme (ACE). Angiotensin II promotes aldosterone secretion.

Corticosteroid hormones (glucocorticoids and mineralocorticoids) and androgens.

A pre-operative ECG is essential to look for any underlying cardiovascular pathology, such as arrhythmias or ischemic heart disease, which are more common in elderly patients.

Aging leads to decreased pulmonary elasticity, chest wall and lung compliance, TLC, FEV1, FVC, and IRV, while RV increases. The closing capacity gradually encroaches on FRC and exceeds it when supine from around 65 years old. Upper airway tone and cough reflex decrease, increasing the risk of obstruction and airway soiling.

Endothelium refers to the simple squamous epithelium that lines organs, blood vessels, and body cavities. This single layer of cells lies on top of a basement membrane and is in proximity to smooth muscle in small arteries, arterioles, and glands.

T4 has a half-life of around 1 week, while T3’s is much shorter at around 1 day.

Renal blood flow and GFR increase by up to 50%.

Blood pressure, sodium, and water.

It is under the control of the hypothalamus, where hypothalamic hormones either stimulate or inhibit the release of hormones from the anterior pituitary.

A rise in plasma ACTH exerts a direct effect in increasing aldosterone secretion from the zona glomerulosa.

Increased protein catabolism, increased hepatic gluconeogenesis, increased plasma glucose levels, and increased lipolysis.

Cortisol suppresses the immune system, impairs wound healing, and has anti-inflammatory effects.

Mineralocorticoids (aldosterone).

To prevent them from reverting to fetal circulation, which can be disastrous as they no longer have the ability to oxygenate themselves via a placenta.

Symptoms: Irritable, change in behaviour, anxiety, eye changes, goitre. Signs: Tremor, restless, irritable, frank psychosis in severe cases, goitre, hyper-reflexia, exophthalmos (Graves’), lid lag, ophthalmoplegia.

A modest rise in creatinine level may reflect a significant decline in renal function.

Yes, it crosses easily.

The renin–angiotensin–aldosterone system.

It travels from the left atrium to the left ventricle and is then pumped out into the aorta.

10% flows through the pulmonary bed, and 90% flows through the ductus arteriosus to the descending aorta.

Examples include the hepatic portal, placental, hypothalamo-hypophyseal, and renal circulations.

The renin–angiotensin–aldosterone (RAA) system.

It allows hormones synthesized by hypothalamic neurons to be transported rapidly and directly to the anterior pituitary, avoiding dilution or destruction in the systemic circulation.

The first breath generates negative intrathoracic pressure, reducing pulmonary vascular resistance, increasing blood flow through the lungs, and causing mechanical closure of the foramen ovale.

The ductus arteriosus closes due to a reduction in blood shunted across it, a reversed gradient between pulmonary vessels and the aorta, and higher pO2 levels in the baby's blood. It closes physiologically around 15 hours after birth and anatomically by 15 days.

Aldosterone increases the reabsorption of sodium from the distal convoluted tubules of the kidney, resulting in sodium retention and plasma expansion. It also increases urinary potassium excretion.

Symptoms: Weakness, tremor, fatigue. Signs: Proximal muscle wasting, tremor, palmar erythema, pretibial myxoedema.

At the placental bed.

Induction agents (except ketamine) and volatile agents lower IOP. Nitrous oxide has no effect on IOP unless used with sulphur hexafluoride during vitreo-retinal surgery. Non-depolarising neuromuscular blocking agents slightly lower IOP, while suxamethonium causes a transient rise in IOP.

The Eustachian valve causes preferential streaming of more highly oxygenated blood from the inferior vena cava across the foramen ovale to the left atrium, supplying the brain and heart with the most oxygen-rich blood.

Gas exchange, nutrient and waste exchange, transfer of immune complexes, hormone synthesis (e.g., HCG, oestrogens, progesterone, TSH, prostaglandins).

Lipid solubility, degree of ionisation, degree of protein binding, pH, molecular weight, concentration gradient across the placenta.

The more ionised a substance is, the less easily it diffuses across the placenta.

Elderly patients have decreased upper airway tone and cough reflex, making them more prone to obstruction and airway soiling. Additionally, the incidence of diseases such as COPD increases with age, contributing to a higher risk of infections and pulmonary emboli.

Glucocorticoids (cortisol).

Continuous endothelium consists of a continuous basement membrane with endothelial cells anchored together via tight junctions. It has low permeability and is found in the blood-brain barrier and the lung.

Bupivacaine has fewer motor effects than other drugs and has a relatively long duration of action.

About 6 minutes.

Endothelial damage exposing blood to tissue factor.

Approximately 60%.

Reduced serum sodium is detected by the macula densa in the distal convoluted tubule of the kidney, stimulating aldosterone secretion to increase sodium retention.

Cortisol binds to glucocorticoid receptors and promotes specific enzyme synthesis.

Cortisol is essential in maintaining vascular reactivity to noradrenaline.

The kidney’s ability to both preserve and excrete water and electrolytes decreases with age.

pH affects the degree of ionisation of the drug, as dictated by its pKa. Acidosis decreases protein binding.

Substances with a molecular weight <600 Daltons cross readily.

The hypothalamus secretes corticotrophin-releasing hormone (CRH), which stimulates the release of adrenocorticotropic hormone (ACTH) from the anterior pituitary. ACTH stimulates cortisol secretion from the zona fasciculata of the adrenal cortex. Cortisol exerts negative feedback on both CRH and ACTH release.

The most active thyroid hormone is T3, which is five times more potent than T4. T4 is converted to T3 peripherally to cause its biological effect.

Gastric emptying is reduced during labor due to the effects of pain and opiates.

The endothelium lining the smaller blood vessels forms a semi-permeable membrane, allowing the formation of interstitial fluid based on Starling’s forces.

Urea and creatinine levels reduce, meaning that a 'normal' creatinine level in pregnancy is abnormal.

Symptoms: Menorrhagia. Signs: Infertility.

The sixth cranial nerve (abducens nerve) supplies the lateral rectus muscle, the fourth cranial nerve (trochlear nerve) supplies the superior oblique muscle, and the third cranial nerve (oculomotor nerve) supplies all other extraocular muscles (medial rectus, superior rectus, inferior rectus, and inferior oblique).

40%.

The oculocardiac reflex is bradycardia or asystole resulting from traction on the extraocular muscles or extrinsic compression of the eye. It involves the trigeminal and vagus nerves and is more pronounced in children. It can be prevented by administering anticholinergics such as atropine.

On top of the upper poles of the kidneys, at the level of the twelfth thoracic vertebra.

The Eustachian valve directs more highly oxygenated blood from the inferior vena cava across the foramen ovale to the left atrium, ensuring the brain and heart receive the most oxygen-rich blood.

Synthesis of adrenaline and noradrenaline.

Stimulation by pre-ganglionic (cholinergic) nerve fibers from the sympathetic nervous system (via splanchnic nerves).

Prostaglandin E1 (e.g., alprostadil) and conditions causing raised pulmonary vascular resistance such as cold and acidosis help keep the ductus arteriosus open.

The major types of endothelium include Continuous endothelium, Fenestrated endothelium, and Discontinuous endothelium.

Avoid NSAIDs in those with pre-existing renal failure, and limit their use to 3 days in those with normal renal function.

Pethidine.

The thyroid gland takes in iodine by active transport and concentrates it. This iodine is oxidized to atomic iodine by peroxidase. The atomic iodine iodinates tyrosine residues on the thyroglobulin molecule to form mono- or di-iodotyrosine. These iodinated tyrosine residues then couple to form either T3 or T4, which are stored as part of the thyroglobulin molecule.

Confusion and dementia increase with age and can be exacerbated by anesthesia.

Symptoms: Ankle swelling. Signs: Bradycardia, ischaemic heart disease, peripheral oedema, low-output cardiac failure, pericardial effusion (rare), hypertension, vasoconstricted and cold peripheries.

Glycosuria and proteinuria are common during pregnancy.

Reduced volumes of local anesthetic agents are required, with a dose reduction by one-third.

No, they are fully ionised, bulky, and poorly lipid-soluble.

Intraocular pressure is normally less than 15 mmHg.

A portal circulation is one in which blood from the capillary bed of one organ structure drains into the capillary bed of another organ structure through a larger vessel, usually a vein or venule.

The pO2 in the umbilical vein is ~4.7 kPa, representing saturations of 80–90%, making the fetus hypoxaemic compared to the mother.

The posterior pituitary contains axons and axon terminals of neurosecretory cells from the hypothalamus, which produce oxytocin and antidiuretic hormone (ADH). These hormones are transported to the axon terminals in the posterior pituitary and released into nearby capillaries upon nerve impulse stimulation.

Clamping the cord removes the low-pressure placental circulation, increasing systemic vascular resistance and reducing venous return to the heart, which helps keep the foramen ovale closed.

Oxygen, bradykinins, and prostaglandin antagonists (e.g., indomethacin) accelerate ductus arteriosus closure.

The hypothalamus produces thyroid-releasing hormone, which stimulates the release of TSH from the anterior pituitary. This causes the release of T4/T3 from the thyroid gland. The T4/T3 released exerts a negative feedback effect on the hypothalamus and pituitary, reducing further release of stimulating hormones.

The thyroid gland is a highly vascular structure made up of two lobes, joined together by the thyroid isthmus. The lobes are found on either side of the trachea, anterolaterally, below the larynx. The isthmus passes in front of the trachea overlying the second to fourth tracheal rings in the adult.

The concentration gradient affects the speed of transfer.

Discontinuous endothelium has large gaps between the endothelial cells and basement membrane, making it the most permeable type. It is found in the liver and spleen.

'Ion trapping' occurs when the drug crosses into the fetus, becomes more ionised, and cannot move out again, which is exacerbated if the fetus becomes more acidotic.

The risk of aspiration increases during pregnancy but returns to normal levels 48 hours postpartum.

Thyroid hormones increase the basal metabolic rate by increasing the rate of oxidative metabolism, increase sensitivity to catecholamines, increase protein breakdown causing muscle wasting if unchecked, and increase turnover of calcium from bone.

Endothelium releases vasoactive substances like nitric oxide (NO) and endothelin-1. NO causes vasodilatation by activating guanylate cyclase to produce cGMP, while endothelin-1 causes vasoconstriction.

It crosses rapidly but does not cause excessive sedation unless doses exceed 8 mg/kg.

The conversion of angiotensin I to angiotensin II.

Intraocular pressure is determined by the effects of external pressure and the pressure exerted by intraocular contents such as aqueous humour, choroidal blood volume, and vitreous humour.

The pituitary gland is a pea-shaped structure within the sella turcica of the sphenoid bone, approximately 1–1.5 cm in diameter, and attached to the hypothalamus via the infundibulum. It consists of the anterior pituitary (adenohypophysis) and the posterior pituitary (neurohypophysis).

It is more deoxygenated with saturations of ~25% and is directed to the right ventricle to be distributed to the lungs and through the ductus arteriosus.

Zona glomerulosa, zona fasciculata, and zona reticularis.

Chromaffin cells.

Both respiratory rate and minute ventilation increase during pregnancy.

Catecholamines are synthesized in the chromaffin cells of the adrenal medulla.

L-Tyrosine → Tyrosine hydroxylase → L-DOPA → DOPA decarboxylase → Dopamine → Dopamine hydroxylase → Noradrenaline → Phenylethanolamine N-methyltransferase → Adrenaline.

Symptoms: Palpitations, racing heart. Signs: Tachycardia, atrial fibrillation, hypertension, high-output cardiac failure, warm and dilated peripheries.

Bupivacaine can cross the placenta but less readily than lignocaine because its pKa is higher, making it more ionised at physiological pH.

Norpethidine, which causes sedation, respiratory depression, and is pro-convulsant.

Symptoms: Fatigue, slowness of thought. Signs: Flat affect, deafness, frank psychosis in severe cases, slow relaxing reflexes, ataxia.

As the fetus offloads CO2 to the mother, the fetal oxyhaemoglobin dissociation curve shifts left, and the maternal curve shifts right, aiding oxygen uptake by the fetus.

Care should be taken to avoid causing pain in affected joints, with special attention to neck mobility and the risk of atlanto-axial subluxation.

Aging can alter pharmacokinetics due to decreased body fat, body water content, renal and liver blood flow, and protein levels, as well as increased sensitivity to central depressants and polypharmacy.

Glucose crosses the placenta by facilitated diffusion to protect the fetus from fluctuating maternal levels.

Angiotensin-converting enzyme (ACE).

They reach the anterior pituitary via portal blood vessels, specifically the hypothalamo-hypophyseal portal circulation.

Simple diffusion (e.g., O2 and CO2), facilitated transport (e.g., glucose), secondary active transport (e.g., amino acids), active transport (e.g., iron and calcium), pinocytosis (e.g., IgG), bulk transport.

The more lipid-soluble a substance is, the more readily it will diffuse across the placenta.

It is needed for the development of the nervous system and normal myelination.

A reduction in muscle bulk is reflected in a decreased baseline creatinine.

Because the kidney’s ability to manage water and electrolytes decreases with age.

Oxygen consumption can increase by up to 60%, increasing the risk of developing hypoxia during induction of anesthesia.

Symptoms: Weight loss despite increased intake, vomiting, diarrhoea. Signs: Weight loss.

It can lead to difficulties in communicating and can increase confusion.

Up to 20 hours in the mother and up to 62 hours in the neonate.

A quiet, calm, well-lit environment.

Fetal hemoglobin has a P50 of ~2.5 kPa, and maternal hemoglobin has a P50 of ~3.5 kPa, supporting oxygen uptake by the fetus.

It flows up the inferior vena cava to the right atrium.

Because of the high resistance afforded by the lungs.

There are two umbilical arteries and one vein.

The outer cortex and the inner medulla.

Aging leads to fewer pacemaker cells, making atrial fibrillation and other arrhythmias more common. There is reduced compliance of the vascular system and left ventricle, which hypertrophies due to increased afterload. Baroreceptor reflexes become less efficient, leading to reduced compensation for posture changes and a tendency towards bradycardia during general anesthesia.

Anesthesia should be induced slowly with intravenous agents like propofol, titrating the dose to effect, and ensuring adequate but not overhydration prior to induction to avoid rapid swings in blood pressure.

Only the unbound (‘free’) substance is available to cross the placenta, so a highly bound substance will not cross readily.

Dead space increases due to bronchodilation.

Chest wall compliance reduces, but lung compliance remains unchanged.

Fenestrated endothelium has pores (fenestrae) within the endothelium, making it very permeable. It is found lining renal glomeruli.

The acuity of the special senses (sight, hearing) decreases with age.

T3, the major active hormone, combines with a receptor in the cell nucleus and modulates protein synthesis at the level of the DNA.

In the blood-brain barrier, tightly anchored endothelial cells form a relatively impermeable barrier that protects the central nervous system.

The epidural space reduces due to the engorged extradural venous plexus, and CSF volume is also reduced.

MAC (Minimum Alveolar Concentration) decreases, and inhalational induction is faster.

For elective surgery, it may be sensible to refer elderly patients to respiratory physicians for pre-optimization of any chest pathology to minimize risks during anesthesia and surgery.

Androgens (dehydroepiandrosterone).

The three different thyroid hormones are thyroxine (T4), 3,5,3-triiodothyronine (T3), and 3,3,5-triiodothyronine (reverse T3).

Barrier pressure reduces due to increased intragastric pressure.

The endothelial lining at the alveolar-capillary interface plays an important role in the diffusion of gases (e.g., O2 and CO2) and lipid-soluble agents (e.g., anesthetic drugs), following Fick’s law.

Avoid intra-operative hypotension and centrally acting drugs that increase confusion, such as atropine.

Symptoms: Weight gain. Signs: Weight gain, constipation.

Vascular endothelium synthesizes prostaglandins, with prostacyclin (PGI2) being the major derivative. PGI2 promotes vasodilatation and inhibits platelet adhesion, reducing the risk of vasospasm and thrombosis when the endothelium is damaged.

If the mother suffers from pseudocholinesterase deficiency.

The thyroid gland is made up of thousands of follicles, each consisting of a single layer of cells surrounding a cavity. These epithelial cells make thyroid hormones and secrete them into the cavity, where they are stored bound to thyroglobulin.

pH increases to approximately 7.5, P_o2 increases to approximately 14 kPa, P_co2 reduces to approximately 3.5 kPa due to hyperventilation, and HCO3- reduces to approximately 18 mmol/L.

Symptoms: Loss of libido, gynaecomastia. Signs: Oligo/amenorrhoea, gynaecomastia.

TSH drives the process of thyroid hormone synthesis via cAMP. It stimulates the release of hormones by driving endothelial cells to take in colloidal thyroglobulin by pinocytosis. Once in the cell, proteolysis of the molecule releases T3 and T4, which diffuse into the blood supply and are transported out of the gland bound mainly to T4-binding globulin, but also to albumin and transthyretin.

The movement of the oxyhaemoglobin dissociation curve depending on CO2 tension and pH, facilitating oxygen offloading to tissues.

Because their skin may be fragile, and attention to pressure areas is needed to avoid the development of pressure sores.

Symptoms: Thinning of hair, loss of eyebrows, dry skin. Signs: Proximal myopathy, muscular hypertrophy, myotonia.

Levels of T3 and T4 increase, which may suppress TSH.

Insulin secretion increases from hypertrophied beta cells, but there is also increased production of 'anti-insulin' hormones like cortisol, which can result in gestational diabetes.

Endothelium plays essential roles in coagulation, inflammation, and vasomotor tone. Its functions include diffusion, osmosis, filtration, acting as a barrier, regulating vasomotor tone, and participating in inflammation.

The pores within the endothelium of the Bowman's capsule permit the passage of fluid and electrolytes via bulk flow, enabling blood to be filtered.

Because balance, strength, and postural reflexes worsen with age, and early mobilization helps prevent significant loss of function.

Thyroid hormones are needed for normal growth of tissues. They exert a direct effect and also have a permissive effect on growth hormone.

The increased affinity of deoxygenated hemoglobin for CO2 and vice versa, facilitating gas exchange between the fetus and mother.

It may be necessary to decrease the dose or increase the time between consecutive doses.