Human Physiology - The Blood System

Arteries convey blood at high pressure from the ventricles to the tissues of the body. Their walls contain thick layers of muscle and elastic fibers to maintain the pulse flow, represented by the relationship $P = \frac{F}{A}$.

Capillaries allow for the exchange of materials between blood and interstitial fluid. They have very thin walls (one cell thick) and a diameter of approximately $5-10 \mu m$ to maximize the surface area to volume ratio for diffusion.

Veins collect blood at low pressure from the tissues of the body and return it to the atria of the heart. They contain valves to prevent backflow and ensure blood flows in one direction toward the heart.

The heart beat is myogenic, meaning the signal for cardiac compression arises within the heart muscle itself. The sinoatrial ($SA$) node acts as the primary pacemaker, sending electrical impulses across the atria.

The cardiac cycle consists of systole (contraction) and diastole (relaxation). Pressure changes in the left ventricle can range from approximately $0 \text{ mmHg}$ during diastole to $120 \text{ mmHg}$ during systole.

Blood is composed of plasma, erythrocytes (red blood cells for $O_2$ transport), leucocytes (white blood cells for immune defense), and platelets (for blood clotting).

Oxygen is transported primarily by binding to hemoglobin ($Hb$) to form oxyhemoglobin ($HbO_8$), while carbon dioxide ($CO_2$) is transported as dissolved $CO_2$, bound to hemoglobin, or as bicarbonate ions ($HCO_3^-$).

The control of the heart rate is influenced by the medulla oblongata via the autonomic nervous system and by the hormone epinephrine (adrenaline), which increases the rate of impulse generation at the $SA$ node.