Page 86 - Cardiac Nursing

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62 PA R T I / Anatomy and Physiology
oxygen is approximately 100 mm Hg; thus, oxygen diffuses Oxygen diffuses from the capillary to the mitochondria along a
into the blood. The partial pressure of systemic arterial oxygen concentration gradient. In 1919, Krogh 182 defined a model of oxy-
is slightly less than 100 mm Hg because of the admixture of gen delivery that is characterized by a tissue cylinder surrounding
oxygenated and deoxygenated blood. Blood from pulmonary each capillary. The Krogh model was based on the idea that oxy-
veins is mixed with some deoxygenated blood from bronchial gen consumption takes place along the capillary, with a progressive
veins, and in the left heart it is mixed with deoxygenated blood decrease in oxygen from the artery to the vein. This longitudinal
from thebesian veins draining cardiac muscle tissue (physiolog- oxygen gradient suggests that cells receiving oxygen from the ve-
ical shunt). nous end of the capillary would be at increased risk for decreased
Carbon dioxide is removed in the pulmonary capillaries. The oxygen delivery under conditions of decreased flow or arterial oxy-
partial pressure of carbon dioxide in pulmonary arterial blood gen. Current research indicates that while there is a longitudinal
(systemic venous blood) is 46 mm Hg and that of blood leaving delivery gradient there is less heterogeneity than originally con-
the lung (which becomes systemic arterial blood) is 40 mm Hg. ceived. 183 In addition, oxygen delivery occurs not only along the
The release of carbon dioxide is aided by the conversion of hemo- capillaries, but also from the arterioles. The current models also
globin (Hgb) to oxyhemoglobin. suggest that oxygen consumption occurs not only within the tis-
Oxygen reaching the tissues must dissociate from Hgb and sue, but also in the arteriolar endothelium. 184,185 The implication
pass out of the red blood cells and move to the mitochondria. Just of these findings is the need to redefine the radial gradient for oxy-
as in the lungs, once the oxygen molecule leaves the cell, passive gen delivery to include endothelial oxygen consumption and ac-
diffusion becomes the driving force in the movement of oxygen. count for the relatively homogeneous delivery gradients. 184,185
Unlike the relatively short distances encountered in the lung, the
oxygen diffusion from the blood to the mitochondria in the tar- Mitochondrial Respiration
get cell is much greater. The partial pressure of oxygen at the ar- Ninety percent of the body’s oxygen consumption occurs in the mi-
terial end of the capillary, approximately 90 mm Hg, quickly tochondria. Inside the mitochondria the hydrogen ions produced
drops to approximately 30 mm Hg in the tissues and to approxi- during glycolysis are passed to the electron transport chain and
mately 1 to 3 mm Hg in the mitochondria. 156 Factors other than through the step-wise process of oxidative phosphorylation they
diffusion that influence oxygen delivery include the rate of oxygen combine with molecular oxygen (dioxygen) to form water. The fi-
delivery, the position of the P 50 (right or left shift), and the rate of nal step in the process is the reduction of oxygen by cytochrome a 3 .
cellular oxygen consumption. 180 Under aerobic conditions, the electron transport chain produces
Transport of carbon dioxide in the blood begins with the dif- three ATP molecules during this process. Of clinical importance,
fusion of carbon dioxide out of the tissue cells. The PCO 2 of the mitochondrial respiration may be disrupted in sepsis by NO, which
tissues (50 mm Hg) is greater than the Pa CO2 in systemic capil- inhibits cytochrome a,a 3 and pyruvate dehydrogenase, which is re-
laries (46 mm Hg). Thus, carbon dioxide diffuses from the tissues sponsible for the conversion of pyruvate. 57,186,187
into the blood. The P CO2 in the tissues is proportional to the
amount of energy expended. Once carbon dioxide has diffused Oxygen Delivery, Consumption,
into the capillaries, a series of chemical reactions can occur. Car- Extraction
bon dioxide is carried in the blood by three mechanisms. Approx-
imately 6% of carbon dioxide is carried in the dissolved state, Oxygen Delivery
20% to 25% combines with Hgb, and the remainder (approxi- The delivery of adequate oxygen for normal cellular function de-
mately 70%) of it combines with hydrogen to form bicarbonate. pends not only on the total amount of oxygen in the arterial blood
In the normal physiological state, an average of 4 mL of carbon (arterial oxygen content) but also on the ability of the heart to
dioxide is transported from the tissues to the lungs in each 100 provide adequate blood flow (cardiac output). Oxygen delivery
˙
mL of blood. The amount of carbon dioxide carried in the blood (DO) is defined as the transport of oxygen to the tissues per
can greatly alter the acid–base balance and must be carefully mon- minute. Oxygen delivery is determined by the combined
itored in the critically ill patient. The diffusion of carbon dioxide processes of ventilation and diffusion (pulmonary function),
into the blood is determined by two factors, the Pco 2 of the tis- Hgb-binding capacity, convective movement of blood (cardiac
sues and the oxygen content of the blood; both factors are in turn function), microvascular distribution, and delivery of oxygen to
determined by the environment of the tissues. Thus, the physio- the mitochondria (passive diffusion).
chemical state that results from this exchange of gases is controlled Cardiac Output. Cardiac output is a main determinant of
by the metabolic demands of the tissues.
oxygen delivery. Decrease in blood flow decreases the supply of
oxygen to the cells, thereby initiating a series of compensatory
Oxygen Cascade mechanisms to increase oxygen transport and extraction. Careful
The oxygen cascade describes the partial pressure gradient for oxy- monitoring of the determinants of cardiac output (preload, after-
gen as it moves from air (PI O2 149 mm Hg at 37 C at sea level) load, and contractility) and heart rate are necessary to optimize
through the respiratory tract where it is humidified to the alveo- oxygen delivery. Arterial oxygen content and cardiac output are
lus (PA O 100 mm Hg), across the alveolar–capillary membrane
˙
2 combined in the oxygen delivery (DO2) equation to measure the
90 to 100 mm Hg) and the capillaries
to arterial blood (PA O 2 amount of oxygen delivered to the tissues in a given unit of time.
(P O 30 to 40 mm Hg) and then into the tissues and finally to
2 Further discussion of the clinical implications of the oxygen con-
the cytoplasm and the mitochondria. The oxygen concentration sumption–delivery relationship is presented in Chapter 21.
at the tissue level varies based on the organ, by regional variations
in perfusion, oxygen consumption, and the distance from the cap- Hemoglobin. In the red blood cell, the Hgb molecule acts
illary. Mitochondrial function is generally not impaired until cel- as an oxygen-binding site responsible for carrying 97% of
lular oxygen drops below 1 to 2 mm Hg. 181 the oxygen in the blood. Hgb is a protein of four subunits of