Blood Composition and Functions

• Blood makes up about 8% of human body weight.
• Blood contains erythrocytes, leucocytes, thrombocytes, and plasma.
• The volume percentage of blood cells in whole blood is about 45% in adults, this is the hematocrit.
• The remaining volume is liquid plasma, which consists of water, plasma proteins, and electrolytes.
• Blood is composed of cells, cell fragments, and aqueous solution (plasma).

Functions of Blood

• Transports gases such as oxygen, carbon dioxide, and nitrogen, as well as nutrients and end products of cell metabolism.
• Tissues are supplied with blood gases and nutrients from the blood stream.
• In exchange, the bood stream transports end products like carbon dioxide, urea, uric acid, and creatinine to the lungs, liver, and kidneys.
• Carries chemical messengers or hormones to their target organs.
• Acid-base homeostasis is regulated through gas diffusion between alveoli and blood in the lung, known as alveolar diffusion.
• Blood transports oxygen from the alveoli to body cells.
• Oxygen diffuses to tissues, while carbon dioxide diffuses from cells into the blood.
• Blood balances water levels between capillaries and intracellular and extracellular spaces.
• Blood maintains a constant body temperature.
• Blood contains coagulation factors (proteins).
• It halts bleeding by activating coagulation factors in response to blood vessel damage, ultimately leading to thrombus formation.

Blood Cellular Components

• Erythrocytes transport oxygen from the lung to tissues.
• Oxygen binds to the heme group of hemoglobin in erythrocytes.
• Erythrocytes are round and have a biconcave shape without a nucleus with a diameter of 8 to 10 µm.
• Leukocytes have a nucleus.
• The types of leukocytes include neutrophils, lymphocytes, monocytes, eosinophils, and basophils.
• Leukocytes function in immune defense.
• Platelets are derived from megakaryocytes.
• They enable hemostasis when damage to blood vessels occurs.

Hormones

• Endocrine glands secrete hormones into the bloodstream.

Classification of Hormones and Receptors

• There are steroids, non-steroids and amino-acid based hormones

Steroids

• Steroid hormones are made from cholesterol.
• Steroid hormones come from the adrenal glands and the gonads.
• Being hydrophobic or non-polar, steroid hormones do not like watery environments.
• They travel through the bloodstream bound to transport proteins to reach their target cells.
• Steroids can diffuse across the phospholipid membrane of target cells.
• Once inside, they bind to receptors that activate certain genes in the nucleus.

Non-steroids

• Non-steroid hormones are either peptides or proteins.
• They are hydrophilic and do not require transport proteins.
• Since they can't pass through the phospholipid bilayer, these hormones bind to cell surface receptor proteins.
• Once the receptors bind, they change shape activating proteins and enzymes that alter gene expression.
• The non-steroid hormone never enters the cell, but there is a change inside of it.

Amino-acid hormones

• These hormones derive from the amino acid tyrosine.
• Thyroid hormones, epinephrine, and norepinephrine fall into this category.