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A haematocele is a collection of blood within a body cavity or sac; in scrotal anatomy it commonly refers to blood in the tunica vaginalis space. It differs from a solid testicular lesion and from a non-bloody fluid collection.

The branch of medicine concerned with blood cells, blood formation and disorders of coagulation. Cell counts, morphology and molecular tests reveal different parts of a blood disorder.

Formation of the cellular constituents of blood; normal adult production is chiefly associated with bone marrow.

Blood within the pericardium; a sufficiently rapid collection can cause cardiac tamponade. ^[ref]

Expectoration of blood from the lower respiratory tract, distinguished from haematemesis. The immediate danger can be airway obstruction.

Escape of blood from the vascular system. It can occur into tissues, body cavities or externally, so absence of visible external blood does not exclude substantial hemorrhage.

An audible vibration associated with cardiac motion and blood-flow events. The principal sounds relate to valve closure and resulting vibrations; a sound’s timing can connect auscultation with stages of the cardiac cycle.

Administration of blood-forming stem cells intended to re-establish hematopoiesis. Donor relationship, preparatory treatment and immune effects differ across forms, so it is more than replacement of mature blood cells by transfusion.

Hemoglobin-concentration and saturation separation is the distinction between how much hemoglobin a volume of blood contains and what fraction of its available oxygen-binding capacity is occupied. Oxygen saturation is a ratio; hemoglobin concentration is an amount per volume. Neither quantity alone specifies arterial oxygen content.

Let $H$ be total hemoglobin concentration, $F$ the fractional oxyhemoglobin and $\beta$ a chosen oxygen capacity per unit hemoglobin mass. A simple content model iswhere the final term represents dissolved oxygen using a solubility coefficient $\alpha$ and oxygen partial pressure $p_{{\mathrm{O}}_2}$. The model separates carrier amount, carrier occupancy and dissolved gas. Functional oxygen saturation can replace $F$ only when the excluded hemoglobin species are negligible or separately accounted for.^[ref][ref]

For an original comparison, neglect dissolved oxygen and stipulate $\beta =1.34$ mL/g and $F=0.98$. Samples with $H=15$ and $H=8$ g/dL then carry about $19.70$ and $10.51$ mL of bound oxygen per dL, respectively. The equal oxygen saturation does not imply equal oxygen content. Conversely, halving $H$ and doubling $F$ preserves the product $HF$ only while the new fraction remains physically attainable.

A primary instrument illustrates how the distinction can be measured rather than merely defined. In 1989, a capillary-tube oximeter used red and infrared light, a detector and digitized processing to derive hemoglobin concentration and oxygen saturation separately.^[ref] Measurements sensitive to different optical features supply information that one saturation number cannot contain. At fixed blood flow, oxygen delivery scales with content; equal saturation readings alone cannot establish equal delivery.

Escape of blood from blood vessels.

The hepatic artery supplies arterial blood to the liver and associated territories through its branches. Hepatic tissue also receives portal venous blood. The dual input means flow volume and oxygen delivery cannot be inferred from the arterial route alone.

A hepatic sinusoid is a specialized microvascular channel through which blood passes between hepatocyte plates. Its exchange interface permits close interaction between circulating material and liver cells. It is distinct from a bile canaliculus despite their close spatial relationship.

A hepatocyte is the major functional epithelial cell of the liver. Its polarized organization supports exchange with sinusoidal blood and secretion into bile canaliculi. Metabolic processing and directional secretion therefore occur at different surfaces of the same cell.

A historical explanatory system in which health and disease depend on the mixture and balance of bodily fluids. The four-fluid scheme includes blood, humoral phlegm, yellow bile and black bile. These categories are not a modern analysis of blood composition. ^[ref]

H. Vink investigated the functional properties of the endothelial glycocalyx. In work with B. R. Duling, the researcher compared the domains occupied by different components of blood within mammalian capillaries.^[ref]

The 1996 experiments combined bright-field observations of blood cells with fluorescence microscopy of circulating macromolecules in hamster muscle. The study also included a light-and-dye perturbation, testing the relationship between the observed surface layer and the space accessible to flowing components.^[ref]

An illuminating measurement question is “accessible to what?” A boundary inferred from a fluorescent tracer need not coincide with the vessel wall or with the path occupied by a blood cell. Comparing components makes the probe's physical interaction with the surface part of the interpretation, rather than assuming that every measured width denotes the same anatomical opening.

Excess carbon dioxide in the blood; it must not be equated simply with lack of oxygen.

An abnormally low concentration of calcium in blood. Total and ionized calcium are different measurements; changes in protein binding can alter total calcium without an equivalent change in its physiologically active fraction.

An abnormally low potassium concentration in blood. Loss from the body and redistribution into cells are different causes, while effects on membrane excitability connect the chemical change with muscle or cardiac dysfunction.

The hypophyseal portal system is the vascular route carrying regulatory hormones from the hypothalamus directly to the anterior pituitary. Hypothalamic neurons release their chemical signals into a first network of capillaries. Hypophyseal portal veins then carry that blood to a second capillary network alongside the secretory biological cells of the anterior pituitary. This is a portal system: two capillary beds connected in sequence.^[ref]

The route makes a small local secretion an effective input to another gland. In an ideal steady mass balance, let a hormone enter blood plasma at molar rate $r$, with plasma volumetric flow rate $Q$. Ignoring removal and assuming complete mixing, the downstream molar concentration isThis is a delivery model, not a universal physiological constant: changing secretion, flow, extraction or mixing changes the resulting signal. It explains the benefit of reaching the target before whole-body dilution.

A 1982 experiment by Iain J. Clarke and James T. Cummins sampled this upstream communication directly in conscious ovariectomized sheep, using a procedure that maintained pituitary function. They measured gonadotropin-releasing hormone (GnRH) in portal blood plasma and luteinizing hormone (LH) in peripheral jugular blood plasma. The secretory pulses were synchronous: each observed LH episode was linked to a pulsatile GnRH discharge through the portal network.^[ref]

Sampling the two locations matters. The portal measurement detects the hypothalamic message before it reaches the anterior pituitary; the peripheral measurement detects the pituitary's output. A single downstream hormone trace would not reveal both sides of that timing relationship.

A hypophyseal portal vein is a vein connecting the first and second capillary networks of the hypophyseal portal system. The main route carries blood from the median eminence region of the hypothalamus, down the pituitary stalk, to the anterior pituitary. It delivers hypothalamic hormones before their dilution in the wider circulation.^[ref]

The word “portal” describes the route: blood leaving one capillary bed reaches another capillary bed before returning to the heart. Here the first bed receives a regulatory message and the second delivers it to responding biological cells. A hypophyseal portal vein is therefore an interconnecting segment, rather than the entire hypophyseal portal system.

A useful example is a pulse of gonadotropin-releasing hormone. In conscious sheep, simultaneous sampling of portal and peripheral blood plasma linked portal GnRH pulses with peripheral luteinizing hormone episodes. Sampling the hypophyseal portal vein route accesses the incoming hypothalamic signal; peripheral sampling detects the resulting pituitary output. That distinction is why the sampling location is part of the experiment, not just a convenient place to draw blood.^[ref]