Circulation

The blood flows through the body under pressure, which is produced by

  • The heart (mainly)

  • Gravity

The pressure is highest during contraction (systolic pressure), while it is at its lowest during dilation (diastolic pressure).

Gravity also has an effect due to the hydrostatic pressure that any liquid would have due to gravity in a vertical plane. In other words, the liquid/blood at the lowest point (feet) has the highest pressure, while the pressure decreases with higher altitudes.

The flow of blood is determined by the total fluid energy, which comes from 3 sources

  • Potential energy produced by the heart’s contractions

  • The kinetic energy of the moving blood

  • Potential energy due to gravity

The potential energy of the heart is the factor with the highest effect (especially in a horizontal plane), meaning that blood flows from the heart to the [capillaries].

The radius of the blood vessels has the greatest effect on the blood flow. Only a small increase in the radius gives a tremendous increase in blood flow. This is used frequently by blood vessels, as they can constrict and dilate, meaning they have good control of the blood flow.

Note that also a small constriction of a blood vessel increases the resistance dramatically. (they are inversely proportional.)

Note that also viscosity has a large effect on flow. The internal resistance degrades the kinetic energy of blood flow into heat. In other words, pressure ultimately converts to heat, which can be seen as a drop in blood pressure in regions relatively far away from the heart.

Mammalian and avian circulation

  • Evolved independently from one another, but are similar

  • Closed circulatory system

    • = Blood is always separated from the tissues by blood vessels

      • In contrast to open circulatory systems, where the blood leaves the blood vessels to directly bathe in the tissues.

  • The systemic circuits are in series with the pulmonary circuits, ensuring that the systemic tissues are always supplied with freshly oxygenated blood. Nice for mammals and birds, who typically have high demands of oxygen.

    • Systemic circuits= supplies the thorax, abdomen, body wall, limbs)

    • Pulmonary circuits= supply lungs for gas exchange

All blood vessels have a single-layered vascular epithelium, with cells that can synthesize blood-borne hormones and secrete substances that constrict/dilate the blood vessels.

There are different classes of blood vessels:

  • Arteries

    • Thick walls

    • Smooth muscle with elastic and collagenous connective tissues

    • Transport blood under large pressure from the heart to the periphery

      • When receiving blood, they stretch to absorb the potential energy in the artery walls. When the heart dilates, the arteries rebound to their unstretched state, thus increasing blood pressure

        • In other words, the elastic properties of the artery walls dampen the oscillation in arterial pressures

    • As they branch outward toward the periphery of the circulatory system, the arteries become narrower and their walls become thinner.

  • Microcirculatory beds

    • Three types of vessels

      • Arterioles

        • Thick walls

        • Smooth muscle and connective tissues

        • Can dilate and constrict, thus controlling blood distribution (eg when cold)

      • Capillaries

        • Extremely thin walls (1 cell layer, the vascular endothelium) with pores

          • Pores allow diffusion of oxygen and nutrients from the blood to the tissues

        • Low pressure and thin walls make blood flow extremely slow (almost static)

          • Allows maximal chance of exchange of oxygen and nutrients

          • Downside: loss of blood pressure (from ca 90 mmHg to 10-15 mmHg)

      • Venules

        • Small vessels

        • Thin walls (not as thin as capillaries)

        • Connective tissue and muscle cells

  • Veins

    • Transport the blood back to the heart

    • Thin walls due to low pressure of the blood

      • Due to such low pressure, the blood cannot get back to the heart on its own. One-way valves are used instead, that prevent the blood from flowing back

    • The diameter increases through a series of veins

Ultrafiltration (hypothesis)

Each capillary has fluid on both sides of the walls:

  • Blood plasma on the inside

  • Extracellular tissue fluid on the outside

Two forces tend to oppose each other along the capillary

  • Hydrostatic pressure of the blood, greater than the external tissue fluid

    • This drives out blood plasma from the pores into the tissues. This process is called ultrafiltration.

  • Presence of large protein molecules in the blood plasma

    • Cannot easily pass through the capillary walls

      • Create an osmotic pressure along the entire length of the capillary

        • Drives tissue fluid into the plasma

Since the hydrostatic pressure is greater, fluid moves into the tissues. However, since the blood loses pressure along the capillary, the osmotic pressure eventually exceeds and drive the fluid back into the blood plasma. Nevertheless, there tends to be a net loss of fluids from the capillaries, but this is restored by the lymphatic system.

Circulation in fishes

  • Closed circulatory system

  • Systemic tissues and breathing organs are in series, like in birds and mammals

  • Heart deals with only deoxygenated blood. Pumps it through the gills and is then immediately transported to the systemic tissues.

    • Note that fish have lower metabolic rates than mammals and birds, so they require less oxygen. Blood flows and arterial pressures are thus also lower in fish.

      • In active fish like tuna, their hearts pump blood rapidly, and often warm up their eyes and brain for maximum awareness.

Circulation in invertebrates

  • Closed circulatory systems

    • Annelid worms

    • Cephalopod molluscs

      • 3 hearts, one systemic heart and two weaker auxiliary branchial hearts

  • Open circulatory systems

    • All other invertebrates

      • Well-developed heart that discharges blood in vessels that are “cut off”, causing the blood to bathe directly in the tissues.

        • Perfuses tissues through a network of

          • Lacunae = small spaces among tissue cells

          • Sinuses = larger spaces

        • No clear distinction between the blood and the tissue fluid

          • Blood is thus referred to as haemolymph (hemolymfa)

          • Is eventually channelled back into discrete vessels

    • Typically low-moderate pressure systems

      • Bivalves usually 1-4 mmHg, while spiny lobsters have 10-15 mmHg (considered high-pressure system)

      • Since resistance is also low, it doesn’t have a significant effect on the blood flow rate, meaning it’s on par with closed systems

    • Insects:

      • Long elongated dorsal heart, pushes blood forward.

      • No capillaries

        • Have trachea though so it’s cool