Hemoglobin is the principal oxygen-carrying protein in humans. It exists within erythrocytes, and binds up to four diatomic oxygen molecules simultaneously. Hemoglobin functions to maximize oxygen delivery to tissues, while simultaneously maximizing oxygen absorption in the lungs. Hemoglobin thus has a fundamentally contradictory set of goals. It must at once be opitimized to absorb oxygen, and to offload oxygen. Natural selection has overcome this apparent contradiction by making hemoglobin exquisitely sensitive to conditions in its microenvironment.

One way in which hemoglobin accomplishes its goals is through the phenomenon of cooperativity. Cooperativity refers to the ability of hemoglobin to change its oxygen binding behavior as a function of how many other oxygen atoms are bound to the molecule. 

Fetal hemoglobin shows a similar pattern of cooperativity, but has unique binding characteristics relative to adult hemoglobin. Fetal hemoglobin reaches higher saturation at lower oxygen partial pressure.

Because of cooperativity, adult and fetal oxygen-hemoglobin dissociation curves appear as follows.

Beyond its ability to carry oxygen, hemoglobin is also effective as a blood buffer. The general reaction for the blood buffer system of hemoglobin is given below.

H⁺ + HbO₂ ←→ H⁺Hb + O₂

The hemoglobin gene can be the site of catastrophic genetic changes, one of which is the change seen in sickle cell anemia. In this disorder, hemoglobin mutations cause red blood cells to take on a sickled appearance. These cells are less able to flow freely in the blood through tight spaces. Which of the following vessels is most likely to be the site of accumulation of these misshapen cells?

Hemoglobin is the principal oxygen-carrying protein in humans. It exists within erythrocytes, and binds up to four diatomic oxygen molecules simultaneously. Hemoglobin functions to maximize oxygen delivery to tissues, while simultaneously maximizing oxygen absorption in the lungs. Hemoglobin thus has a fundamentally contradictory set of goals. It must at once be opitimized to absorb oxygen, and to offload oxygen. Natural selection has overcome this apparent contradiction by making hemoglobin exquisitely sensitive to conditions in its microenvironment.

One way in which hemoglobin accomplishes its goals is through the phenomenon of cooperativity. Cooperativity refers to the ability of hemoglobin to change its oxygen binding behavior as a function of how many other oxygen atoms are bound to the molecule. 

Fetal hemoglobin shows a similar pattern of cooperativity, but has unique binding characteristics relative to adult hemoglobin. Fetal hemoglobin reaches higher saturation at lower oxygen partial pressure.

Because of cooperativity, adult and fetal oxygen-hemoglobin dissociation curves appear as follows.

Beyond its ability to carry oxygen, hemoglobin is also effective as a blood buffer. The general reaction for the blood buffer system of hemoglobin is given below.

H⁺ + HbO₂ ←→ H⁺Hb + O₂

During exercise the flow of blood is changed, and blood flow is preferentially directed toward working muscles. These muscles are then able to utilize the oxygen carried by hemoglobin. What vessels are most likely to be directly mediating changes in blood flow?

Hemoglobin is the principal oxygen-carrying protein in humans. It exists within erythrocytes, and binds up to four diatomic oxygen molecules simultaneously. Hemoglobin functions to maximize oxygen delivery to tissues, while simultaneously maximizing oxygen absorption in the lungs. Hemoglobin thus has a fundamentally contradictory set of goals. It must at once be opitimized to absorb oxygen, and to offload oxygen. Natural selection has overcome this apparent contradiction by making hemoglobin exquisitely sensitive to conditions in its microenvironment.

One way in which hemoglobin accomplishes its goals is through the phenomenon of cooperativity. Cooperativity refers to the ability of hemoglobin to change its oxygen binding behavior as a function of how many other oxygen atoms are bound to the molecule. 

Fetal hemoglobin shows a similar pattern of cooperativity, but has unique binding characteristics relative to adult hemoglobin. Fetal hemoglobin reaches higher saturation at lower oxygen partial pressure.

Because of cooperativity, adult and fetal oxygen-hemoglobin dissociation curves appear as follows.

Beyond its ability to carry oxygen, hemoglobin is also effective as a blood buffer. The general reaction for the blood buffer system of hemoglobin is given below.

H⁺ + HbO₂ ←→ H⁺Hb + O₂

Hemoglobin takes time to unload oxygen at tissues that need it, as well as time to pick up carbon dioxide from working cells. Blood flow must slow down during the most active periods when hemoglobin is absorbing and releasing atoms. Since cross-sectional area is inversely proportional to flow velocity, which of the following vessel beds has the greatest total cross sectional area?

One component of the immune system is the neutrophil, a professional phagocyte that consumes invading cells. The neutrophil is ferried to the site of infection via the blood as pre-neutrophils, or monocytes, ready to differentiate as needed to defend their host.

In order to leave the blood and migrate to the tissues, where infection is active, the monocyte undergoes a process called diapedesis. Diapedesis is a process of extravasation, where the monocyte leaves the circulation by moving in between endothelial cells, enters the tissue, and matures into a neutrophil.

Diapedesis is mediated by a class of proteins called selectins, present on the monocyte membrane and the endothelium. These selectins interact, attract the monocyte to the endothelium, and allow the monocytes to roll along the endothelium until they are able to complete diapedesis by leaving the vasculature and entering the tissues.

The image below shows monocytes moving in the blood vessel, "rolling" along the vessel wall, and eventually leaving the vessel to migrate to the site of infection.

Capillaries, such as the one depicted in the passage, not only exchange immune cells, but also gas and nutrients with neighboring tissues. Which of the following is true of capillaries? 

I.  Hydrostatic pressure is greater at the arteriolar end.

II.  Osmotic pressure is greater at the arteriolar end.

III.  There is a net loss of fluid to the extravascular tissue from the arteriolar to the venule end of the capillary.

All of the following are examples of human capillary classifications except __________.

Fluid from the blood often leaks from between the cells lining capillary walls, entering into the interstitium. If this fluid is not returned to circulation it can result in edema, or swelling of the region due to the excess fluid. What happens to this excess fluid in a healthy individual to prevent edema?

Which of the following vessel types will have the slowest velocity of blood flow?

What characteristics of arteries and veins allow the heart to pump blood strong enough to travel through the body against gravity without backing up?

Which of the following incorrectly matches the type of blood with the vessel or structure carrying it?

Pressure throughout the body is lowest in the __________.