The passage details the passage of substances through the liver. The portal triad consists of the portal vein, hepatic artery, and bile duct. Blood from the intestines enters the liver through the portal vein and is filtered by hepatocytes on its way to the central vein that connects to the inferior vena cava and onto the rest of the venous circulation. The liver is thus able to filter out toxic metabolites before they reach systemic circulation.

The pulmonary veins have the greatest concentration of oxygenation, because they bring oxygenated blood from the lungs to the left atrium. They are the only veins that carry oxygenated blood.

Blood in the pulmonary arteries is deoxygenated and travels from the right ventricle to the lungs for gas exchange. Blood in the vena cavae is returning to the heart after systemic circulation, and is thus deoxygenated. Blood in the capillaries is a mixture of oxygenated and deoxygenated, but is always less oxygenated than the blood of the pulmonary veins.

Keep in mind that the saturation percentage of hemoglobin depends on the pressure of oxygen. Before returning to the lungs, hemoglobin has given up the majority of attached oxygen to the body's tissues. As a result, the hemoglobin will be least saturated just before entering the lungs to become oxygenated once again. This is why the pulmonary artery will contain blood with the lowest saturation of hemoglobin.

The low partial pressure of oxygen in the pulmonary artery, compared to the high partial pressure of oxygen in the alveoli, generates the gradient for oxygen to enter the blood stream.

The arterioles feed into the capillaries, and control which tissues and parts of the body get more oxygenated blood. During cold weather conditions, the arterioles are activated in vasoconstriction of oxygenated blood to the capillary beds in the skin.

Increased blood vessel wall permeability can lead to edema. Edema is the result of abnormal fluid homeostasis; proper fluid homeostasis is achieved by balancing hydrostatic pressure and oncotic pressure in blood vessels. If hydrostatic pressure is greater than oncotic pressure in a blood vessel, fluid will filter out of the blood vessel and into the interstitium. The Starling Equation describes fluid movement across capillary membranes in relation to hydrostatic pressure and oncotic pressure within the blood vessel. 

In the general circulation, the highest blood pressure is found in the aorta and the lowest blood pressure is in the vena cava. As this suggests, blood pressure drops in the general circulation as it goes from the aorta to the rest of the body. Pressure drops form the aorta to the arteries, the arteries to the arterioles, and the arterioles to the capillaries. Flow rate reaches a minimum in the capillaries before blood begins to pool in the venules. Pressure continues to drop from the venules to the veins to the vena cavae.

Systolic blood pressure is generated by the contraction of the left ventricle and the ejection of blood into the aorta. As blood is ejected into the aorta, the aorta expands to accommodate the large volume of blood. The wall of the aorta then begins to recoil and pushes blood through the arteries. This is how blood pressure continues to stay elevated even when the heart is not contracted.

Right ventricular contraction causes blood to go into the pulmonary circulation, which is not measured with blood pressure testing. The viscosity of blood does not impact the pressure that is exerted on the wall of a blood vessel. Venous pressure is not typically measured, as it is much lower than arterial pressure and is relatively unaffected by heart contractions due to the narrowness of the preceding capillaries.

Blood pressure tends to be the greatest near the heart, and decreases as blood flows to the capillaries. The pressure is greatest at the aorta and gradually decreases as blood moves from the aorta to large arteries, smaller arteries, and capillaries. The pressure is lowest in the venous system, which is why blood can pool in the veins and act as a "blood reservoir". Veins contain valves that allow them to pump blood back to the heart. 

The heart is incapable of selectively pumping blood to certain areas, nor can it manipulate the speed of blood flow through certain areas. Instead, the sympathetic nervous system can stimulate the smooth muscle surrounding arterioles to constrict. This constriction of arterioles allows blood to be redirected to other areas of the body where blood is needed. Venule shunts are important for counteracting gravity, but the lack of thick smooth muscle lining these vessels makes them ineffective at redirecting blood flow.

Some—but not all—capillary beds experience transudation of fluid from the vessels. Recall that capillaries can be discontinuous (spaces between adjacent cells), completely closed with tight junctions between cells (as in the brain), or fenestrated (pores through their cytoplasmic membranes (as in the kidney). Closure of arteriolar pre-capillary sphincters can reduce or eliminate the blood flow to a region of tissue. This is why your fingers blanche in very cold weather. Although the arterioles are the major resistance vessels in a circuit, the capillary beds have some contribution. Transudation of fluids, but not large molecules such as protein, from inside to outside a capillary indeed raises the osmotic pressure of the remaining blood; this is Starling's Law, not to be confused with the Frank-Starling law of the heart.

Choice V is a true statement regarding the hypothalmo-hypophyseal portal system.