Question

1. What are the three important steps in blood coagulation? Explain.
2.   Trace the path of a red blood cell starting from the right leg and back to the leg as oxygenated red blood cell.
3. Trace the pathway of the electrical signal through the heart.
4. Compare and contrast action potential in cardiac muscle vs skeletal muscle.
5. List and define the various components of an electrocardiogram.

Answer 1

Answer to the above question:

The three important steps in blood coagulation are:

Coagulation (blood clotting) is a complicated series of physical reactions that transform liquid blood into a gel that provides a secure patch to the injured blood vessel.

The coagulation process can be described in three major steps.

• Formation of factor X and prothrombinase. Prothrombinase can form either inside the blood vessels (intrinsically) or outside the blood vessels (extrinsically). In the intrinsic pathway, the collagen of the damaged blood vessel initiates a string of reactions that activates factor X. In the extrinsic pathway, damaged tissues release thromboplastin, which starts up a shorter, more rapid sequence of reactions to activate factor X. In both pathways, activated factor X combines with factor V (with vitamin K present) to form prothrombinase.
• Prothrombin is converted to thrombin. In this common pathway that follows both the intrinsic and extrinsic pathways, prothrombinase (with vitamin K) converts prothrombin to thrombin.
• Fibrinogen is converted to fibrin. The common pathway continues as thrombin (with vitamin K) converts fibrinogen to fibrin. Fibrin forms long strands that bind platelets together to form a dense web. Thrombin also activates factor XIII, which helps fibrin strands stick to one another. The result is a clot.

The journey starts with the red cell being created inside the bone. In the bone marrow, it develops in several stages starting as a hemocytoblast, then becoming an erythroblast after 2 to 5 days of development. After filling with hemoglobin it becomes a reticulocyte, which then becomes a fully matured red blood cell.After creation, the red blood cell starts travelling to the heart via capillaries. The blood cell is currently deoxygenated.

The deoxygenated red blood cell now makes its way to the vena cava within the heart, and is then pushed into the right atrium.

The right atrium then contracts, pushing the blood cell through the tricuspid into the right ventricle.

The right ventricle then contracts, pushing the red blood cell out of the heart through the semi lunar.After leaving the heart, the red blood cell travels through the pulmonary artery to the lungs. There it picks up oxygen making the deoxygenated red blood cell now an oxygenated blood cell. The blood cell then makes it way back to the heart via the pulmonary vein into the left atrium.

After entering the left atrium, which then contracts and pushes the blood cell through the bicuspid, the red blood cell then enters the left ventricle.

The left ventricle then contracts, pushing the red blood cell through the semi lunar, and out of the heart into the aorta.

Travelling through the aorta, the red blood cell goes into the kidneys trunk and other lower limbs, delivering oxygenated blood around the body. They typically last for 120 days before they die.

And that’s the whole process! Although this seems like a lengthy process, the whole thing takes less than a minute from start to finish, depending on the individual’s heart rate.

Electrical signals arising in the SA node (located in the right atrium) stimulate the atria to contract. Then the signals travel to the atrioventricular node (AV node), which is located in the interatrial septum. After a delay, the electrical signal diverges and is conducted through the left and right bundle of His to the respective Purkinje fibers for each side of the heart, as well as to the endocardium at the apex of the heart, then finally to the ventricular epicardium; causing its contraction.^[1] These signals are generated rhythmically, which in turn results in the coordinated rhythmic contraction and relaxation of the heart.

On the microscopic level, the wave of depolarization propagates to adjacent cells via gap junctions located on the intercalated disc. The heart is a functional syncytium (not to be confused with a true "syncytium" in which all the cells are fused together, sharing the same plasma membrane as in skeletal muscle). In a functional syncytium, electrical impulses propagate freely between cells in every direction, so that the myocardium functions as a single contractile unit. This property allows rapid, synchronous depolarization of the myocardium. While advantageous under normal circumstances, this property can be detrimental, as it has potential to allow the propagation of incorrect electrical signals. These gap junctions can close to isolate damaged or dying tissue, as in a myocardial infarction (heart attack).

Cardiac action potentials in the heart differ considerably from action potentials found in the skeletal muscle cells.

One major difference is in the duration of the action potentials. In skeletal muscle cells, the action potential duration is approximately 2-5 ms. In contrast, the duration of cardiac action potentials ranges from 200 to 400 ms.

Another difference between cardiac and nerve and muscle action potentials is the role of calcium ions in depolarization. In nerve and muscle cells, the depolarization phase of the action potential is caused by an opening of fast sodium channels. This also occurs in non-pacemaker cardiac cells; however, in cardiac pacemaker cells, calcium ions are involved in the initial depolarization phase of the action potential. In non-pacemaker cells, calcium influx prolongs the duration of the action potential and produces a characteristic plateau phase.

An electrocardiogram (EKG, ECG) is a test that measures the electrical signals that control heart rhythm. The test measures how electrical impulses move through the heart muscle as it contracts and relaxes.

The electrocardiogram translates the heart's electrical activity into line tracings on paper. The spikes and dips in the line tracings are called waves.

• The P wave is a record of the electrical activity through the upper heart chambers (atria).
• The QRS complex is a record of the movement of electrical impulses through the lower heart chambers (ventricles).
• The ST segment shows when the ventricle is contracting but no electricity is flowing through it. The ST segment usually appears as a straight, level line between the QRS complex and the T wave.
• The T wave shows when the lower heart chambers are resetting electrically and preparing for their next muscle contraction.

Left atrium Aorta Right atrium P-R Q-R-S Electrocardiogram waves Right ventricle Left ventricle