Blood Components, Functions, and Heart Mechanics

Posted on Dec 17, 2024 in Biology

Blood Composition and Function

Blood is composed of liquid and solid elements. The liquid portion, making up 55-60% of blood, is primarily water (90%) and other substances such as glucose, proteins, enzymes, gamma globulins, minerals, gases, lipids, hormones, and vitamins. The solid portion, 40-45%, consists of:

  • Red Blood Cells: Disc-shaped cells without a nucleus, numbering around 4.5 million in women and 5 million in men. They are produced in the red bone marrow and destroyed by the liver and spleen. Red blood cells contain hemoglobin, which binds to oxygen and carbon dioxide for gas transport. A deficiency in iron or hemoglobin can lead to anemia.
  • Platelets: Fragments of cytoplasm from large cells in the red bone marrow, with a count of 250,000-300,000. They play a crucial role in blood coagulation, forming clots at damaged vessel walls.
  • Leukocytes (White Blood Cells): With a count of 5,000-10,000, these cells have a nucleus and are produced in the bone marrow, spleen, lymph nodes, and thymus. They are essential for defense, crossing vessel walls to fight infection. A deficiency in leukocytes is called leucopenia. Leukocytes are further divided into:
    • Agranulocytes: Lack cytoplasmic granules and have large nuclei. Monocytes are large phagocytic cells with a horseshoe-shaped nucleus. Lymphocytes are smaller cells formed in the lymph, including B cells (antibodies) and T cells (T4 helpers and T8 suppressors).
    • Granulocytes: Contain cytoplasmic granules, including neutrophils (70%), acidophils (2-1%), and basophils.

Blood Volume and Properties

Blood volume is approximately 5.5 liters. Its temperature is slightly higher than body temperature due to metabolism and muscle activity, regulated by the nervous system. Blood density is slightly greater than water.

Functions of Blood

Blood performs several vital functions:

  • Transporting waste products from cells
  • Delivering nutrients
  • Transporting gases
  • Regulating body temperature
  • Defensive role against pathogens
  • Blood coagulation
  • Transporting hormones

The Heart: Structure and Function

The heart is a muscular organ located in the chest, supported by the myocardium and the left lung. It is about the size of a fist and has a hollow wall divided into two parts, each with two chambers: the upper atria and the lower ventricles. The atria receive blood entering the heart, while the ventricles eject blood out of the heart. The right ventricle pumps blood to the lungs, and the left ventricle pumps blood to the rest of the body. Each atrium is separated from its ventricle by a valve: the tricuspid valve on the right and the bicuspid (mitral) valve on the left.

Blood Flow Through the Heart

  • Right Atrium: Receives deoxygenated blood from the superior vena cava (head, arms, neck) and the inferior vena cava (body).
  • Right Ventricle: Pumps deoxygenated blood to the lungs via the pulmonary arteries.
  • Left Atrium: Receives oxygenated blood from the lungs via the pulmonary veins.
  • Left Ventricle: Pumps oxygenated blood to the body via the aorta.

Semilunar valves at the entrance of the arteries prevent backflow of blood.

Heartbeat Mechanism

The heart muscle can contract without nervous impulses. Special cells near the tricuspid valve initiate contraction, creating a wave that spreads through the atria, causing atrial contraction (atrial systole). This wave then reaches the ventricles, causing ventricular contraction (ventricular systole). During ventricular systole, blood is pumped into the pulmonary arteries and the aorta. After the wave passes, the ventricles relax (diastole), and blood flows into the atria. The cardiac cycle lasts approximately 0.8 seconds.

Heart Rate

Heart rate is the number of beats per minute, typically around 70. The autonomic nervous system controls heart rate, with sympathetic nerves increasing it and parasympathetic nerves decreasing it. Tachycardia is an increased heart rate, bradycardia is a decreased heart rate, and arrhythmia is an irregular heart rhythm.

Blood Vessels

Arteries

Arteries carry blood away from the heart.

Veins

Veins carry blood back to the heart. They carry deoxygenated blood, except for the pulmonary veins. Veins have thin walls and are often superficial. The vena cava returns blood to the right atrium. Blood flow in veins is aided by:

  • Heartbeat
  • Valves in veins
  • Muscle activity
  • Breathing movements