lecture 11 - circulatory systems

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Circulatory Systems

38

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Concept 38.1 Circulatory Systems Can Be Open or Closed

All animals must exchange substances with the external environment.

Nutrients and oxygen must reach the cells, and carbon dioxide and wastes must exit.

Small organisms can accomplish this using gastrovascular systems to bring the external environment inside the animal.

Gastrovascular cavity

Food

Epidermis

Mouth

Tentacles

Gastrodermis


Larger organisms require a circulatory system to transport substances around the body.

It consists of:

• Circulatory fluid (blood)

• Set of interconnecting tubes (blood vessels)

• Muscular pump (heart)

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Open circulatory system:

•  Arthropods, molluscs, some other invertebrates

•  Heart pumps fluid (hemolymph) through circulatory vessels into sinuses (cavities) around organs, allowing direct material exchange.

•  Body movements help circulation by squeezing the sinuses.


Closed circulatory system—blood vessels keep circulatory fluid (blood) separate from the fluid around cells (interstitial fluid).

Plasma and small molecules (e.g. oxygen) leak out of vessels, into interstitial fluid, where material can be exchanged with cells.

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Advantages of open systems:

•  Lower pressure, less energy required

•  Hydrostatic pressure can be used for movement

Advantages of closed circulatory systems:

• Higher pressure = more efficient delivery of nutrients and O2 = higher metabolism

• Blood flow to specific tissues can be controlled by varying vessel diameter

Concept 38.4 Blood Consists of Cells Suspended in Plasma

Blood is a connective tissue made of:

•  Fluid plasma with dissolved ions and proteins, such as clotting factors.

•  Erythrocytes – red blood cells

•  Leukocytes – white blood cells

•  Platelets – pinched- off fragments of cells that function in clotting.

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Red blood cells are the most numerous of the blood cells. Produced in the bone marrow. Life span ~120 days.

Function: Transport oxygen. One RBC can hold ~250 million molecules of hemoglobin

Heme is the reason blood is red.


Erythropoietin, a hormone released in the kidney in response to insufficient oxygen, or hypoxia, controls red blood cell production.

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Quick Poll

What is the difference between arteries and veins?

a)  Arteries carry oxygenated blood; veins carry deoxygenated blood.

b)  Arteries carry blood away from the heart; veins carry blood towards the heart.

c)  Both of the above.


Vessels of the closed circulatory system:

• Arteries—carry blood away from the heart

• Capillaries—the site of exchange between blood and interstitial fluid

• Veins—deliver blood back to the heart

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Concept 38.2 Circulatory Systems May Have Separate Pulmonary and Systemic Circuits

Fish/shark circulatory systems:

•  Single circuit (blood passes through the heart only once)

•  Heart has two chambers: • Atrium—receives blood

from the body • Ventricle—receives pumped

blood from the atrium and sends it to the gills


Lungfish circulatory systems:

A lung formed from the gut functions in air.

A divided atrium separates blood into pulmonary and systemic circuits.

Valves can direct blood flow to the lungs OR the gills depending on the medium.

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Amphibian circulatory systems:

Amphibians have three-chambered hearts.

Left atrium receives oxygenated blood from the lungs, right atrium receives blood from the body.

The incompletely-divided ventricle directs the flow to the pulmonary or systemic circuit.


Turtle/lizard/snake circulatory systems:

Reptiles have three-chambered hearts and two aortas.

The ventricle is partially but not completely divided; bloodstreams are separate.

Right aorta can receive blood from either side of the ventricle and allows them to bypass lungs when diving or during low activity.

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Crocodilian/bird/mammal circulatory systems:

Four-chambered hearts and completely separate pulmonary and systemic circuits.

Evolutionarily convergent between mammals and birds/crocodiles.

Key adaptation for endothermy – allows them to meet high energy demands.


1.  Right atrium 2.  Right ventricle 3.  Pulmonary arteries 4.  Lungs 5.  Pulmonary veins 6.  Left atrium 7.  Left ventricle 8.  Aorta 9.  Body (arteries, arterioles, capillaries, venules,

veins) 10. Vena cavae

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Atria collect incoming blood and transfer it to the ventricles. Ventricles pump blood out to lungs or body.

Valves prevent backwards flow. •  Atrioventricular (AV) valves •  Pulmonary valve and aortic

valve at base of arteries.

Concept 38.3 A Beating Heart Propels the Blood

The cardiac cycle of contraction and relaxation has two phases:

1.  Systole—when ventricles contract (pumps) 2.  Diastole—when ventricles relax (fills)

The atria contract just before the ventricles, to add blood volume to the ventricles.

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The pacemaker cells in the sinoatrial node initiate contractions without input from nervous system.

The spread of action potentials through gap junctions between cells stimulates contraction in unison.


An electrocardiogram (ECG or EKG) uses electrodes to record events in the cardiac cycle.

P = depolarization of atrial muscle

Q, R, S = depolarization of ventricles

T = relaxation and repolarization of ventricles

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Concept 38.5 Blood Circulates through Arteries, Capillaries, and Veins

Blood pressure and flow is lower through the capillaries—each artery supplies many capillaries, which have an enormous surface area.

What does this mean for taxa with a single circuit?


Arterial walls have elastin that allows them to stretch and recoil to move blood forward.

Blood movement in veins is assisted by skeletal muscle contractions that squeeze the veins.

One-way valves in the veins prevent backflow.

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Blood pressure can be controlled at the level of the blood vessels.

Smooth muscles contract = vasoconstriction = increased blood pressure.

Smooth muscles relax, increase in diameter = vasodilation = reduced blood pressure.


Blood pressure is monitored by the medulla, which integrates information and activates sympathetic/parasympathetic response.

Blood pressure is also under endocrine regulation, primarily in conjunction with kidneys (regulating water retention).

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Osmolarity of blood (amount of solutes in solution) affects movement of materials in and out of bloodstream.

Blood pressure pushes fluid and small molecules OUT of vessels and into tissues.

High concentration of solutes in blood pulls fluids back IN from tissues.


The lymphatic system returns interstitial fluid to the blood.

Lymph capillaries continually take up excess fluid, and merge into two thoracic ducts that empty into veins in the neck.

Lymph nodes produce lymphocytes that screen lymph fluid for pathogens.

lecture 11 - circulatory systems

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