A closed (1) circulatory system is needed to sustain fast movements because it allows oxygen to be rapidly delivered, via the blood, to meet the increased tissue oxygen demand (0.5) that occurs in response to exercise. A closed circulatory system is able to do this because it enables high blood pressures to be attained, and enables the animal to regulate the speed of the flow and to direct blood flow to meet tissue oxygen demand. In contrast, open circulatory systems do not allow for high blood pressures to be attained, and allow only limited control over the direction and speed of blood flow. Thus, the important features of a closed system are high pressure (0.5), and the ability to tightly regulate blood flow rate (0.5) and direction (0.5).

A closed circulatory system does not necessarily mean that oxygenated and deoxygenated blood are physically separated. Recall frog hearts, which are part of a closed system, in which oxygenated and deoxygenated blood enters a single ventricle.

Hearts, be they part of an open or a closed circulatory system, always contain gap junctions.

Whether a circulatory system is open or closed has no effect on the blood’s affinity for oxygen, or the rate of oxygen absorption by the tissues. The tissues of invertebrates with open circulatory systems absorb oxygen just as efficiently as the tissues of invertebrates with closed circulatory systems. Rather, it is the speed with which the blood can be delivered to those tissues that makes the difference in the ability of the circulatory system to meet tissue oxygen demand.

A drop in blood pressure triggers both hormonal (0.5) and neuronal (0.5) responses in an effort to bring BP back up to some set-point range. The main homeostatic responses include stimulation of the autonomic nervous system (ANS), specifically the sympathetic (1) ANS, to release the catecholamines (0.5), epinephrine (0.5) and norepinephrine (0.5). Epinephrine is released by the adrenal cortex, norepinephrine is released by symthpathetic fibers. E and NE bind to adrenergic receptors in the heart (to increase pacemaker firing at the SA node), and on blood vessels (to cause vasoconstriction (0.5)). Heart rate (0.5), stroke volume (0.5) and hence cardiac output (0.5) all increase, all of which increase BP. Additional changes that can also occur in response to a drop in BP include release of anti-diuretic hormone (ADH) (0.5), which increases blood volume by increasing water resorption in the kidneys (which reduces urine production).

Nitric oxide stimulates vasodilation, which leads to a drop in BP. Adenosine reduces heart rate and inhibits release of catecholamines, both of which reduce BP. Therefore, increased release of NO and adenosine would NOT occur in response to a drop in BP since these chemicals would only serve to drop BP further.

Hypertension is defined as high blood pressure.

Homeostatic responses are those which will bring conditions back to a certain set range or setpoint. These responses occur mainly through negative feedback systems. In this case, a drop in BP stimulates responses that increase BP to bring BP back to the setpoint.

A drop in blood pressure can be due to several things: for example, a drop in blood volume (e.g. due to dehydration, hemorrhaging), lower HR, or vasodilation, to name a few. You cannot tell from BP alone which of these is responsible.

Vasoconstriction refers to the contraction of smooth muscles in the wall of ANY vessel, either arterial or venous.

The sympathetic system stimulates the release of hormones and neurotransmitters rather than the other way around.