Felicita Jusof

Alveolar-Capillary Exchange

The fractional pressure contributed by a gas to the total pressure exerted by a mixture of gases. The partial pressure, P X is simply calculated by multiplying the fractional concentration of the gas, N X in the mixture with the total pressure exerted by the gas mixture, P Total . In atmospheric air, the gases of concern are nitrogen, oxygen and carbon dioxide. The air partial pressure of oxygen at its fractional concentration of 21% is 0.21 × 760 mmHg, giving 160 mmHg.

Control of Respiration

The central medullary chemoreceptors in the brain stem responds to arterial PCO2. The respiratory control neurons also receives afferent inputs from chemoreceptors at the carotid/aortic bodies. After denervation of the carotid chemoreceptors, the response to PO2 and pH in stimulating ventilation is abolished. However, the response to arterial PCO2 is reduced only about 35%.

Sodium and Potassium Balance

Sodium balance can also be termed sodium homeostasis. This is different from the control of sodium concentration. The concentration of sodium is related instead to water balance and osmoregulation.

Heart as an Electrical Organ

Definition: The pre-potential where a (gradual) spontaneous depolarization of the sinoatrial nodal cells leads to the generation of action potential that determines the heart rate ().

Physiological Adaptations of the Cardiovascular System

Definition: The percentage of the red cell volume to the total blood volume. The blood cellular volume are largely those of erythrocytes. PTC: The gender-dependent normal range is on average 40–45%. Normal erythropoiesis will give a normal hematocrit value.

Pathways from the Heart

Definition: Blood flow rate has the unit volume per time while the unit for velocity of blood flow is distance per time. PTC: It is common to talk loosely of ‘blood flow faster’ when what is meant is not the velocity but the rate of blood flow in volume/min. An increase in cardiac output above 5 L/min in a 70 kg, male adult is an increase in flow rate not the velocity. The blood flow along every sector of the cardiovascular arterial tree to the capillaries is the same. Similarly, the venous return has the same value as the cardiac output. There is no vascular volume congestion along the blood traffic from the arterial to the venous side and into the right heart.

Acid-Base Balance

Definition The total rate of excretion of non-carbonic acid in the urine. PTC: The excreted rate of a solute is given by Urine solute concentration (UX) multiply by the urine flow rate (V). The two major form of excreted acid in urine are urinary phosphate (H2PO4) and ammonium (NH4). Therefore the net acid excretion will include the addition of (UX.V) for both the phosphate and the ammonium. Metabolism of foods can generate strong acids e.g. from sulphate-containing substrate.

Carbon Dioxide Transport

Definition The total amount of carbon dioxide carried in blood, expressed as volume of CO2 in volume of blood. PTC The transport capacity of blood for CO2 is higher than that for oxygen. This is also true in the arterial blood where the CO2 content is more than twice that of oxygen. Unlike for oxygen, there is no specific carrier for CO2 in blood. The total load of CO2 in blood is carried in three forms of ‘baggages’. Carbon dioxide dissolves in the plasma from the alveolar air in line with Henry’s law. The dissolved CO2 gives the partial pressure of CO2 in blood (PCO2).

Glomerular Filtration and Renal Blood Flow

Definition The regulatory maintenance of the constancy of the extracellular fluid environment of cells by controlled, feedback mechanisms. PTC When asked, ‘What are your kidneys for?’, the most obvious answer is for excretion by the production of urine. Certainly, that is an essential function of the kidneys where metabolites from both endogenous and exogenous substances are excreted.

Heart as a Pressure Producer

Definition The maximum aortic blood pressure that is due to the pressure exerted on the vascular wall by the ejected stroke volume. PTC Since the left cardiac ventricular pump is not a continuous pump, the pressure in the aorta fluctuates during a cardiac cycle. The maximum pressure exerted on the aortic wall during systolic contraction is called the systolic blood pressure.