General Biology/Tissues and Systems/Lymphatic System

Lymphatic system: part of the immune system, a one-way, or open, system. Takes up interstitial fluid not taken up by venules.

Lymphatic structures:
1. lymphatic capillaries
2. lymphatic vesicles
3. lymph nodes
4. lymphatic organs (spleen and thymus)

Lymph: movement in mammals through one-way valves, similar to blood movement in veins. (Some non-mammals have lymphatic hearts of unknown embryonic origin. Frogs and salamanders have several.) Lymph rejoins cardiovascular system into a large vein near the heart via single large thoracic duct.

As lymph passes through system, passes lymphocytes, second part of immune system.

Heart: pumps blood, design varies between animals. In adult mammal,four chambers form two separate circulations
1. pulmonary circulation to and from lungs and
2. systemic circulation to and from tissues of body.

Everything in the heart comes in pairs: 2 atria, 2 ventricles (left and right).

Diagrams usually drawn as though animal were on its back.

Pattern of blood flow through heart: blood returning from major veins (vena cava) enters right atrium, contraction there delivers blood to right ventricle through a tricuspid valve, one of atrial ventricular valves (AV valve). Contraction of right ventricle drives blood through semi lunar valve into pulmonary circuit and to lungs.Blood return to heart in pulmonary veins, is oxygenated. Goes to left atrium, which contracts and delivers blood to left ventricleby way of aortic semi-lunar valve, then goes to systemic circulation.

Both atria and ventricles contract in unison, left is more powerful than right (to all system vs. just lungs).

Systole: heart contraction, diastole: heart relaxed

Timing of heart contraction: ventricles rebound to relaxed shape (diastole), and semi-lunar valves close. Both atria(singular: atrium) fill with blood coming from pulmonary and systemic circulations.Pressure rises in the atria and blood begins to move into the ventricles.The atria then contract, forcing more blood into the ventricles. There is a pause, then ventricles contract. This raises ventricle pressure, atrio-ventricular(AV) valves shut and semi-lunar valves open, forcing blood from the left ventricle into the major arteries and from the right ventricle into the aorta.

Control for this action doesn’t rely on nervous stimulation, has intrinsic rhythmicity, called myogenic. This is the case in mammal as well as in mollusk hearts. Other animals have neurogenic hearts that rely on nervous stimulation for heart action, originating in the cardiac ganglion.

The rhythmicity of mammalian heart relies on the sino-atrial (SA)node, or pacemaker. This is a phylogenic (based on evolutionary history) remnant of an early vertebrate heart that had one more chamber than modern hearts.

How the heart contracts: waves of depolarization start in SA node and spread through atria. Connectile tissue pauses the spread of depolarization at the atrial ventricular node. Signal continued by bundle branches to lower ventricle, begins to stimulate heart to contract. Contraction starts at bottom of heart at heart apex,then signals spread through heart.

Medulla (in the brain) controls autonomic nervous system. (The medulla is part of the brain, is continuous with the spinal cord, and controls involuntary actions of the body). Sympathetic cardiac acceleratorconnects to spinal cord, uses norepinephrine to signal. Parasympathetic cardio-inhibitory center reaches heart through Vagus nerve, usesacetylcholine to signal. Hyperpolarizes membrane to inhibit heart contraction. (Autonomic nervous system: two parts working in contra to control from both sides.) Dominant effect here is inhibitory. If we cut Vagus nerve, heart rate promptly rises about 25 bpm.