23: Circulation

What are the three essential components of any circulatory system?

1. Fluid (blood or hemolymph)

2. Vessels

3. A pump (heart)

Compare Open vs. Closed circulatory systems regarding fluid type and pressure.

Open: Fluid is hemolymph (interstitial fluid); operates at low pressure; low efficiency.

Closed: Fluid is blood (confined to vessels); operates at higher pressure; high efficiency.

Which vertebrate groups exhibit "Double Circulation"?

Amphibians, Reptiles, Birds, and Mammals. (Fish have single circulation).

Contrast the heart chambers of a fish, an amphibian, and a mammal.

Fish: 2 chambers (1 atrium, 1 ventricle).

Amphibian: 3 chambers (2 atria, 1 ventricle).

Mammal/Bird: 4 chambers (2 atria, 2 ventricles).

What is the function of the Atrioventricular (AV) and Semilunar valves?

They are flaps of connective tissue that prevent the backflow of blood.

• AV: Between atria and ventricles.

• Semilunar: Between ventricles and major arteries (Pulmonary/Aorta).

Sequence the electrical impulse of a heartbeat (5 steps).

1. SA Node (Pacemaker) initiates AP.

2. Impulse spreads through Atria (contraction).

3. AV Node (0.1-second delay).

4. Bundle Branches (passes to apex).

5. Purkinje Fibers (spreads through ventricles for contraction).

Why is the SA node considered "autorhythmic"?

It generates its own action potentials internally via leaky $Ca^{2+}$ channels, rather than requiring a signal from the central nervous system.

How do the Sympathetic and Parasympathetic nervous systems affect the SA node?

Sympathetic: Increases heart rate and blood pressure.

• Parasympathetic: Decreases heart rate and blood pressure.

What are the three layers of a blood vessel?

Endothelium (lumen lining), Smooth Muscle, and Connective Tissue (elastic/collagen).

Compare the walls of Arteries vs. Capillaries.

Arteries have thick walls (muscle/connective) to handle pressure; Capillaries have only the endothelium and basal lamina to facilitate rapid exchange.

What is the primary difference between a Vein and an Artery?

Direction of flow. Arteries move blood away from the heart; Veins move blood toward the heart. (Oxygen content is not the defining factor).

Define the "Antagonistic Forces" at work during capillary exchange.

Blood Pressure: Pushes fluid out of the capillary.

Osmotic Pressure: Pulls fluid into the capillary (due to blood proteins).

What happens to the 15% of fluid that is not reabsorbed by the venous end of the capillary?

It remains in the interstitial fluid and must be recovered by the Lymphatic System to be returned to the blood.

What is "Lymph" and how does it move through the body?

Lymph is watery fluid derived from interstitial fluid. It flows via skeletal muscle contractions (it does not have its own pump).

List three primary functions of the Lymphatic System.

1. Return interstitial fluid to the blood. 2. Absorb lipids from the small intestine. 3. Defend against disease (filtering at lymph nodes).

Where does lymph eventually go?

It returns to the circulatory system via lymph ducts that empty into the large veins.